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Injection Molded Glass Fiber Reinforced Poly(trimethylene terephthalate) Composites: Fabrication and Properties Evaluation Wanjun Liu,† Amar K. Mohanty,*,‡ Lawrence T. Drzal,† Manjusri Misra,† Joseph V. Kurian,§ Ray W. Miller,§ and Nick Strickland§ Composite Materials and Structures Center, 2100 Engineering Building, Michigan State University, East Lansing, Michigan 48824, The School of Packaging, 130 Packaging Building, Michigan State University, East Lansing, Michigan 48824, and Sorona R&D, Bio-Based Materials, E. I. du Pont de Nemours and Company, Inc., Wilmington, Delaware 19880
Injection-molded composite materials as fabricated from chopped glass fiber and poly(trimethylene terephthalate), PTT, are evaluated through physicomechanical and thermomechanical analysis. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to measure the thermal properties of the above composites. The mechanical properties including impact properties, tensile properties, and flexure properties of these composites were tested with a United Testing System SFM-20. Results showed that impact strength, tensile strength and modulus, flexure strength, and modulus of glass fiber reinforced PTT composites were improved significantly with increasing fiber content. The simultaneous improvement of both stiffness and toughness of composite materials is one of the important findings of this investigation. It was found that the crystallization temperature of PTT increased and heat deflection temperature doubled through reinforcement with glass fiber. The tensile fractured surfaces are evaluated through environmental scanning electron microscopy (ESEM) for studying the interaction between fiber and the matrix. Introduction
Scheme 1
There is a growing urgency to develop biobased materials as replacements/substitutes of fossil fuel based materials. Although fully renewable resource based materials are more ecofriendly, such materials may not satisfy performance attributes for certain industrial applications. The polymers and materials derived from mixed renewable and fossil fuel sources not only show strong promise in alleviating the fossil fuel dependency but also have an added advantage of delivering the desired performance from a more sustainable stock material. The DuPont SORONA polymer, e.g., poly(trimethylene terephthalate), PTT, a threecarbon glycol terephthalate (3GT), is an example of a condensation polymer (Scheme 1) that can be made from 1,3-propanediol (derived from renewable corn sugar) and fossil fuel derived terephthalic acid (TPA). PTT belongs to the thermoplastic aromatic polyester family, which includes poly(ethylene terephthalate) (PET) and poly(butylenes terephthalate) (PBT). Even though PTT was patented five decades ago,1 it has not been used as a commercial product because of its high cost. The manufacture of 1,3-propanediol from renewable resources such as corn has sped up the commercialization of PTT. PTT polymers have drawn attention for their applications in the textile industry. Owing to its high elasticity and recovery, the PTT fibers can be used widely in garments requiring good resilience and may also be a substitute for nylons in carpets and * To whom correspondence should be addressed. Phone: (517) 355-3603. Fax: (517) 353-8999. † Composite Materials and Structures Center, Michigan State University. ‡ The School of Packaging, Michigan State University. § E. I. du Pont de Nemours and Company, Inc.
other coverings.2-4 PTT can also be used as an engineering thermoplastic because it possesses good thermal and mechanical properties. Some of research work about structure and properties of multicomponent polymeric material, like poly(ethylene naphthalate), poly(ether imide), polystyrene, poly(ethylene terephthalate), and polycarbonate blending, with PTT were reported.5-9 Additionally, some reports focus on the crystal structure, morphology, and chemical structure of PTT, but only a few pay attention to the mechanical properties and processing of fiber-reinforced PTT composites.10,11 Recently, PTT has been investigated as engineering thermoplastic for fiber-reinforced composites. Glass fiber reinforced PTT composites significantly improve the mechanical properties of PTT.12 Additional improvements may be obtained by modifying the fiber surface, which enhances the adhesion between fiber and matrix. However, since little work has focused on this area, the present paper will describe fabrication and property evaluations of glass fiber-PTT based composite materials. Experiments Materials. Poly(trimethylene terephthalate) (PTT), product name of SORONA, was supplied by Dupont and
10.1021/ie049112f CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005
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Table 1. Detailed Parameters of PTT and Its Composites in Injection Molding Processing and Weight of Injection Molding Bar glass fiber content (%)
shot size (in.)
fill pressure (psi)
back pressure (psi)
hold pressure (psi)
wt of bar (g)
0 15 30 40
0.93 0.96 1.00 1.03
400 900 900 900
400 700 700 700
300 500 500 500
10.55 11.78 12.87 13.29
is derived completely from petroleum resources. The biobased PTT as would be marketed by DuPont in the near future is supposed to impart nearly similar properties as that of the presently petroleum-derived PTT. Glass fiber (treated with E43 (polypropylene grafted maleic anhydride/PP-g-MA) from Eastman Chemical Co.) was supplied by Johns Manville Corp. PTT and glass fiber were dried in a vacuum oven overnight at temperature of 100 °C for extrusion. Fabrication of Glass Fiber Reinforced PTT Composites. A ZSK-30 Werner and Pflider twin-screw extruder with a six-zone barrel was used, with processing temperatures of 235, 240, 240, 240, 245, and 245 °C from zone 1 to zone 6 of the barrel and a screw speed of 100 rpm. PTT was fed at a speed of 35 g/min. The feeding speed of glass fibers was 6.3, 15, and 23.5 g/min for 15%, 30%, and 40% fiber reinforced composites, respectively. The PTT and glass fiber reinforced pelletized composites were formed into testing specimens using a Cincinnati Milacron injection molder operating with a barrel temperature of 245 °C and a mold temperature of 35 °C. The detailed processing parameter is shown in Table 1. Thermal Properties. (1) DMA. A dynamic mechanical analyzer (2980 DMA, TA Instruments) was used to measure the heat deflection temperature (HDT) of composites with a load of 66 psi according to ASTM D648 under DMA control force and three-point bending mode. The specimen with a length of 55 mm was cut from the center part of injection molding bar. The heating rate was 2 °C/min for HDT. For dynamic mechanical properties of PTT and glass fiber reinforced composites, DMA multifrequency and three-point bending mode were used. The frequency was 1 Hz. The heating rate was 4 °C/min. Tests were performed from room temperature to 220 °C. (2) DSC. Differential scanning calorimetry (2920 DSC, TA Instruments) was used to measure the melting and crystallization temperature of PTT. Sample weights ranged from 8 to 12 mg. All samples were heated from room temperature to 250 °C, isothermal for 10 min, cooled to -60 °C, and then reheated to 250 °C at a rate of 10 °C/min. First cooling and second heating curves were recorded. Mechanical Properties. The tensile and flexural properties of injection mold specimens were measured with a United Testing System SFM-20 according to ASTM D638 and ASTM D790, respectively. System control and data analysis were performed using Datum software. The notch impact properties were measured with a Testing Machines, Inc., 43-02-01 monitor/impact machine according to ASTM D256. Impact specimens with dimensions of 2.5 × 0.5 × 0.125 in. were cut from injection mold tensile specimens. Notches 0.1 in. deep were cut into sample beams using a TMI notch cutter. A 5 ft-lb pendulum was used to impact the samples. In all mechanical properties measurements, five specimens were measured for each sample.
Figure 1. DSC curves of glass fiber reinforced PTT composites. Table 2. Thermal Properties of PTT/Glass Composites Obtained from Figure 1 sample
crystallization temp (°C)
melting temp I (°C)
melting temp II (°C)
PTT 15% glass/PTT 30% glass/PTT 40% glass/PTT
195.51 199.01 197.55 198.18
217.50 218.11 216.98 217.19
229.83 229.03 228.94 228.77
Morphology Observation. The tensile fracture surface of glass fiber reinforced PTT composites was observed with environmental scanning electron microscopy (ESEM) (Phillips Electroscan 2020) with an accelerating voltage of 20 kV. The fracture surfaces were sputter coated with gold prior to examination. Results and Discussion Crystallization and Melting Behavior of Composites. The DSC curves of PTT and glass fiber reinforced composites are shown in Figure 1. Detailed information is shown in Table 2. It was found that the crystallization temperature of PTT increased after adding glass fiber, indicating that glass fibers have a nucleating effect on PTT. This is significant as nucleation will provide contribution to processing of PTT and reduce solidification time of PTT during processing. Additionally, the crystallization time is reduced after addition of glass fiber. The effect of glass fiber on crystallization time and temperature indicates that glass fiber increases the crystallization rate of PTT from the melt state. However, the crystallization temperature of glass fiber reinforced PTT composites decreases with increasing content of glass fiber. Actually, there are two factors controlling crystallization of polymeric composite
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systems. One is that additives have a nucleating effect, which results in an increase of crystallization temperature, a positive effect on crystallization. The other is that the additives hinder the migration and diffusion of polymer molecular chains to the surface of the nucleus in the composites, which resulted in decreases the crystallization temperature, a negative effect on crystallization. In this case, the effect of glass fiber on the crystallization temperature of PTT is a competition of above two factors. Thus, it is easy to understand why the crystallization temperature decreases after glass fiber content is raised above 15 wt %. PTT has two melting peaks, one at 230 °C and another at 217.5 °C. These two melting peak corresponds to a lamellar with two different thicknesses formed during PTT crystallization at the given condition because the melting peak temperature is related to the lamellar thickness of the polymer. Wu’s viewpoint 13 supports this result. Additionally, melting temperature depends on the heating rate, crystallization conditions, and thermal history. The 230 °C melting temperature of PTT decreased following the addition of glass fiber because the glass fiber acts as an impurity and depresses the melting point.14 The melting point at 217.5 °C was also affected by glass fiber content. With 15% adding glass fiber, it increased. In contrast, with adding more glass fiber, it decreased. This should be related with the unstable crystallization during the cooling of PTT. Detailed discussion about this will be given in another paper. Dynamic Mechanical Properties of Composites. The storage modulus versus temperature curves is shown in Figure 2. These results show that the storage modulus of PTT increased upon addition of glass fiber and also increased with increasing glass fiber content. This result is consistent with the tensile and flexure modulus. As the temperature increases, there is a sharp decline in the modulus corresponding to the glass transition temperature of the PTT. Following this the modulus then increases with temperature as the more mobile molecules may reorganize and crystallize. It is believed that crystallization has the largest contribuition to the increase in modulus. A similar phenomenon was found in Tan ∆ curves and the DSC first heating curves (shown in Figure 3). On the first heating curves, there is a cold crystallization peak, which is crystallization from the amorphous state (at temperatures greater than Tg) where PTT molecules are able to repack and crystallize. This means that the crystallization of PTT is still lower and resulted a secondary crystallization due to the mold temperature under glass transition temperature of PTT though glass fiber has a nucleating effect on the crystallization of PTT. Through controlling and adjusting the mold temperature, the cold crystallization of PTT can be avoided, which is interesting and will be given in another study. It was also found that the peak value of Tan ∆ in glass transition region decreased upon addition of glass fiber. It is well-known that the damping in the transition zone measures the imperfection in the elasticity and that much of the energy used to deform a material during DMA testing is dissipated directly into heat.15 This result indicates that after addition of glass fiber, the molecular mobility of the composites decreased and mechanical loss to overcome inter-friction between molecular chains reduced. Generally, the damping of the polymer is much greater than that of the fibers. Thus,
Figure 2. Dynamic mechanical properties of glass fiber reinforced PTT composites.
Figure 3. DSC first heating curves of glass fiber reinforced PTT composites.
addition of fibers to polymeric materials will increase the elasticity and decrease in the viscosity and less energy will be used to overcome the friction forces between molecular chains as to decrease mechanical loss. Mechanical Properties of PTT and Composites. (1) Tensile Properties. The importance of fiberreinforced composites mainly comes from the significant improvement in strength and modulus, which supply a good chance for composites application. The tensile properties of glass fiber reinforced PTT composites with different content of glass fiber are shown in Figure 4. The tensile strength and modulus of composites increased with glass fiber content. When the weight
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Figure 4. Tensile properties of glass fiber reinforced PTT composites.
Figure 6. Flexural properties of glass fiber reinforced PTT composites.
Figure 5. Plot of tensile modulus of PTT composites vs glass fiber volume fraction.
percentage of glass fiber reached 40%, the strength and modulus of composites was 2 and 5.5 times than those of PTT, respectively. The rule of mixture was used to predict the modulus of fiber-reinforced composites. In this case, the simple eq 1 was used to depict the modulus results of fiber reinforced PTT composites
Ec ) ξEVfEf + VmEm
(1)
where Ec is the modulus of composites, ξE is the fiber efficiency factor of composite modulus considering the contribution of fiber length and orientation, Ef is the modulus of glass fibers, Vf is the volume fraction of glass fiber, Em is the modulus of matrix, and Vm is the volume fraction of matrix. In the experiments, weight fraction was used to incorporate the glass fiber. The weight fractions of glass fiber were converted to volume fraction by using eq 2
Vi )
Wi/Fi
∑Wi/Fi
Figure 7. Impact strength of glass fiber reinforced PTT composites.
(2)
where Vi, Wi, and Fi are the volume fraction, weight fraction, and density of component i in the composites. Densities of 2.5 g/cm3 for glass fiber and 1.26 g/cm3 for PTT were used for calculation of volume fraction. The fitting curve that is shown in Figure 5 has good agreement with the experiment data. From the curves, the fiber efficiency factor of ξ was calculated with a value of around 0.65. This is due to higher extent of fiber orientation along the flow direction during injection molding, indicating that the modulus of glass reinforced PTT composites is adoptable with rule of mixture. (2) Flexural Properties. The flexural properties of glass fiber reinforced PTT composites are shown in Figure 6. It was found that the bend strength and
Figure 8. HDT behavior of glass fiber reinforced PTT composites.
modulus of glass fiber reinforced composites increased with increasing glass fiber content. When the fiber content reached 40 wt %, the bend strength and modulus improved 2 and 5 times. This indicates that the flexural properties of glass fiber reinforced PTT composites follow similar trends with tensile properties of the composites. (3) Impact Strength. The impact strength of a material describes the energy required to break the specimen. The magnitude of impact strength reflects the ability of material to resist impact. Notch Izod impact strength emphasizes the energy to propagate a crack under impact load. Impact strength of fiber reinforced polymeric composites is complex because of the role of the fiber and the fiber/matrix interface in addition to the polymer. The notch Izod impact strength of glass fiber reinforced PTT composites with different glass fiber contents are shown in Figure 7. The impact strength of glass fiber reinforced composites increased with increasing fiber content, indicating that glass fibers
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Figure 9. ESEM micrographs of fracture surface of glass fiber reinforced PTT composites for (a) PTT, (b) 15% glass reinforced PTT composites, (c) 30% glass fiber reinforced PTT composites, and (d) 40% glass fiber reinforced PTT composites.
have a positive contribution to impact strength of PTT. When adding 40% glass fiber, the impact strength improved by more than three times, which indicates that this glass fiber reinforced composite has excellent potential to be used as an engineering materials. Generally, there are two factors controlling the impact strength of fiber-reinforced composites.15 The first is a diminishing effect of fibers on impact strength due to a drastic decrease in the break elongation and hence a reduction of the area under the stress-strain curves. In addition, new stress concentrations will be formed around the fiber ends, area of poor adhesion, and region of fiber aggregation. The second is fiber increasing the impact strength due to the fact that fibers reduce the crack propagation rate by forcing cracks around the fibers. 16 The practical effect of fiber on impact strength of composites depends on the competition of these two factors. In this system, it is obvious that the reducing effect of glass fiber on crack propagation rate of composites dominates crack initiation through forming new stress concentration. In this case, the reason for glass
fiber increasing the impact strength of PTT should be the glass fiber restricted the crack propagation rate so as to alleviate the fracture of the composites. The energy required to produce a new fractures surface increased with fiber volume fraction leading to an increase in the impact strength. Heat Deflection Temperature. Heat deflection temperature (HDT) was denoted as the maximum temperature at which polymer can be used as a rigid material. Here, HDT is defined as the temperature at which the deflection of the sample reaches 250 µm under an applied load of 66 psi according to ASTM D648. Fiber-reinforced composites will have enhanced HDT. HDT behavior of glass fiber reinforced PTT composites are shown in Figure 8. It was found that HDT temperature doubled after adding glass fiber even though the detailed value was not obtained (the absolute value is more than 220 °C) due to the significant improvement in modulus of composites. With increasing fiber content, the modulus of composites increased based on above mechanical properties.
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Morphology. The morphology of the tensile fracture surfaces show phase information and fracture characteristics reflecting the reasons why the mechanical properties have been changed and in turn decide the mechanical properties of polymeric composites.17 The tensile fracture surface morphology of glass fiber reinforced PTT composites are shown in Figure 9. It was found that the fibers had good dispersion in the matrix and that some fibers were pulled out from matrix. In addition, almost all the fibers on the fracture surface were orientated in the flowing direction of injection molding. This indicates that composites have higher degree of fiber orientation, which results in higher fiber efficiency factor and hence higher mechanical strength. Closer observation found that although some of the fibers were pulled out, however, some of the materials still could be found on the surface of the fiber. This suggests that the fibers have some interaction with the matrix. The glass fibers used in this study are treated with polypropylene grafted maleic anhydride (PP-g-MA). PP-g-MA on the glass fiber possibly reacts with PTT so as to improve interaction between fiber and matrix. It is possible that the anhydride group had some physical or chemical interaction with hydroxyl groups in the end chain of PTT molecule during processing. The outcome from these interactions is also an improvement in the impact strength and tensile strength. Conclusion Glass fiber increases the crystallization temperature of PTT and hence glass fiber has nucleation effect to PTT. Glass fiber lowers the melting temperature due to the role of impurity. The cold crystallization phenomenon appear after the glass transition temperature of PTT, for PTT as well as glass fiber reinforced PTT composites. Impact strength, tensile strength and modulus, and flexure properties of glass fiber reinforced PTT composites improved with increasing fiber content. Glass fiber reinforced PTT composites improves the storage modulus and lowers the loss factor (damping). HDT temperature of glass fiber reinforced PTT composites was improved more than twice than that of PTT. This result indicates that glass fibers reinforced composites posses’ excellent potential for application for structural materials in wide range of temperature. Glass fiber had some interaction with PTT matrix because of presence of PP-g-MA on glass surfaces. This glass fiber reinforced PTT composite has stronger potential to be used as a new engineering material in various structural applications, such as electronic housing and automotive parts. Acknowledgment Authors are thankful to E. I. du Pont de Nemours and Company, Inc. Wilmington, Delaware, DE 19880, USA for providing SORONATM polymers. We are also thankful to Johns Manville Corp. for supplying the glass fibers.
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Received for review September 13, 2004 Revised manuscript received November 19, 2004 Accepted December 6, 2004 IE049112F
