Improving the Processibility and Mechanical Properties of Poly(lactic acid)/Linear Low-Density Polyethylene/Paraffin Wax Blends by Subcritical Gas-Assisted Processing.
Over the coming decades, bioplastic materials are expected to complement and gradually replace some of the petroleum-based polymers. Most bioplastic materials are made by renewable resources such as starch, cellulose, roots, or sugar . As one of the most commonly used biobased, biodegradable, and biocompatible aliphatic polyesters, poly(lactic acid) (PLA) can be used for various end-use applications, including food packaging, medical devices, transportation, structural components, and disposable utensils [2-4]. However, PLA is inherently brittle, exhibits low impact strength, has a slow crystallization rate, is very viscous as a melt, and has low thermal resistance, thus limiting its applications. To overcome this, PLA has been modified in several ways to increase its processability, mechanical properties, and crystallization kinetics for improved thermal stability [5, 6]. Such modifications include blending it with other fillers and polymers such as organic montmorillonite (nanoclay) , graphite , bamboo fibers , talc , natural rubber , polytetrafluoroethylene (PTFE) , polystyrene (PS) , polypropylene (PP) , poly([epsilon]-caprolactone) (PCL) , poly (ethylene glycol) (PEG) , thermoplastic polyurethane (TPU)  and introducing foaming agent [18-21]. Previous studies have shown that paraffin wax (PW) is capable of improving the ductility and fluidity of PLA matrices . However, PLA and PW are immiscible, thus the low melting temperature of PW (at around 55[degrees]C) poses processing difficulties and/ or practical application limitations on PLA/PW blends. For example, once the temperature of the mixture is higher than 55[degrees]C, the PLA/PW blends will start softening. Therefore, there is a need to improve the processability and thermal stability of PLA/PW blends.
The most common matrix used for blending paraffin waxes is polyethylene (PE) [23-30]. This is because PW and PE both belong to the ethylene family ; thus, they have similar chemical structures which promotes good compatibility and miscibility. Some prior studies have employed different grades of PE to examine the miscibility of PW and PE. These studies have found that the miscibility between linear low-density polyethylene (LLDPE) and PW were much better than with lowdensity polyethylene (LDPE) or high-density polyethylene (HDPE) [25, 32]. They also reported that 90% LLDPE/10% PW blends were miscible and that the melting temperature of the LLDPE/PW blends was much higher than that of PW. Hence, LLDPE was incorporated into the PLA/PW blends in this study to improve its processability, thermal stability, and elongation-at-break.
A twin-screw extruder was employed to melt compound PLA, PW, and LLDPE. In addition, a special process called "sub-critical gas-assisted processing" (SGAP) extrusion was employed in an attempt to achieve a better dispersion of LLDPE and PW in the PLA matrix. This SGAP method introduced atmospheric gases such as nitrogen ([N.sub.2]) or carbon dioxide (C[O.sub.2]) at a subcritical state as the physical blowing agent into the extruder barrel to facilitate the dispersion of PW and LLDPE by adding elongational stresses through bubble expansion during extrusion . Based on our previous studies [22, 34], the SGAP process facilitated better and more uniform dispersion of fillers in the polymer matrix, resulting in improved morphology, crystallinity, thermal resistance, and mechanical properties.
In this study, neat PLA, neat LLDPE, LLDPE/PW, PLA/ LLDPE, and PLA/LLDPE/PW blends with different compositions were compounded as listed in Table 1. Since it has been reported that 90%LLDPE/10%PW was miscible, this 9:1 ratio of LLDPE to PW was maintained for the blends. Note that for both of the PLA/LLDPE and PLA/LLDPE/PW blends, the weight ratio of PLA to LLDPE was 5:3. In addition to SGAP extrusion, the conventional melt compounding method was also used to prepare solid pellets for comparison. In this paper, pellets prepared by SGAP were labeled with the prefix "F-" because of foaming, which took place during the SGAP process resulting in visible gas cells being dispersed in the extruded matrix. Solid ASTM 638 Type V tensile bars were then injection molded with both SGAP foamed pellets and solid pellets from the conventional melt compounding method. As will be discussed below, thermal investigations, mechanical properties, and SEM morphology images were performed to study the effects of melt compounding of neat materials and their blends.
Materials. Ingeo 300ID--an injection grade PLA in pellet form--was purchased from NatureWorks (Minnetonka, MN). It had an MI of 22 g/10 min (ASTM D1238), a density of 1.26 g/ [cm.sup.3], 1.4% D-LA content, and a weight-average molecular weight of 63,145 and 123,482 g/mol, respectively. Dowlex[TM] 2045--a LLDPE in pellet form--was purchased from Dow Company (Texas) and is a heterogeneously branched ethylene/1octene copolymer. It had an MI of 1 g/10 min (ASTM D1238), a density of 0.92 g/[cm.sup.3], contained 19.9 branches per 1,000 main-chain carbons, a number and weight-average molecular weight of 25,700 and 109,900 g/mol , respectively, and a lower amount of the highly branched fraction but a greater portion of the medium branched fraction , Purified PW beads were obtained from LorAnn Products (Michigan) with a density of 0.88 g/[cm.sup.3]. Nitrogen (N2, purchased from Airgas) was used as the physical blowing agent for the subcritical gas-assisted processing (SGAP) extrusion process.
Equipment and Experiments. PLA and LLDPE pellets were dried at 80[degrees]C for 6 h prior to compounding. The blends were blended via a Leistritz ZSE-18 co-rotating twin screw extruder with a screw diameter of 18 mm and an UD ratio of 42. For the SGAP process, a standard N2 cylinder and a pressure regulator purchased from Airgas were connected to the barrel of the extruder by a metal hose (1/8" diameter) with Swagelok fittings. When SGAP was employed, the gas was set to be ~3.4 bars (50 psi) greater than the measured melt pressure at the die exit . To ensure the same thermal-mechanical history, neat PLA was also processed using the same conventional melt compounding and SGAP processes. Before the injection molding process, the extruded pellets were again dried in a vacuum oven at 40[degrees]C for 24 h. Although foamed pellets from the SGAP process were used for injection molding, the premolding drying and elapsed time from melt compounding to injection molding allowed the gas to diffuse out from the SGAP-foamed pellets prior to injection molding. In addition, the plastication within the injection molding barrel and the relatively high pack/hold pressure eliminated any trapped air or pre-existing voids, thereby producing solid tensile test bars. In this study, ASTM 638 Type V tensile bars were injection molded using a Boy XS injection molding machine. Some key processing conditions are listed in Table 2.
Differential Scanning Calorimetry (DSC). Thermal property measurements of the neat materials and blends were performed with a TA Instruments Q20. All samples were taken from the extruded pellets and placed in hermetically sealed aluminum pans. Samples were then heated to 200[degrees]C at a rate of 5[degrees]C/min and held for 5 min to erase any previous thermal history. The samples were cooled to -60[degrees]C and then reheated to 200[degrees]C at a rate of 5[degrees]C/min. All tests were carried out under a nitrogen protective atmosphere with a constant nitrogen purge flow of 50 mL/min.
The degrees of crystallinity ([X.sub.c]) of neat PLA, LLPDE, PW, and their blends were evaluated from their corresponding melting enthalpies using Eq. 1 . The 100% crystalline melting enthalpies for PLA and PE were taken as 93.0 J/g  and 293.0 J/g , respectively,
[X.sub.c] = [DELTA][H.sub.m] - [DELTA][H.sub.cold]/[DELTA][H.sub.m100] x [W.sub.f] x 100% (1)
where [DELTA][H.sub.m] was the measured melting enthalpy, [DELTA][H.sub.cold] was the cold crystallization enthalpy, [DELTA][H.sub.ml00] was the melting enthalpy of the 100% crystalline polymer matrix, and [W.sub.f] was the weight fraction of polymer in the blends.
Thermogravimetric Analysis (TGA). TGA analyses were performed on a TA Instruments Q50 ramping from 100[degrees]C to 600[degrees]C at a rate of 10[degrees]C/min to measure weight loss. All of the extruded neat PLA, LLDPE, and their blends were placed on a platinum pan at a constant nitrogen flow rate of 50 mL/min. The residual weight of the specimen as a function of the temperature was recorded.
Scanning Electron Microscopy (SEM). SEM images of the fracture surface and phase morphology were obtained on a Zeiss LEO 1530 with a 3 kV accelerating voltage using an in-lens detector. Prior to SEM, the surfaces of the samples were gold coated for 60 s at 50 mA in a Denton Vacuum Desk V sputter coater.
Tensile Properties. Tensile properties of injection molded ASTM 638 Type V samples were tested on an Instron 5967 with a 30 kN load cell. All samples were tested at a crosshead speed of 1 mm/min and then the stress-strain curves were acquired to examine the consistency of the results. After tensile testing, the fracture surfaces of the tensile bars were frozen in liquid nitrogen and the phase morphology and fractured surface characteristics were examined via SEM.
X-ray Diffraction (XRD). X-ray diffraction information was obtained on a Bruker D8 Discovery with a Cu-Ka (A = 1.54A) emitter and a VANTEC 500 two-dimensional detector covering a two-theta (20) angle of 10[degrees] to 40[degrees] and an X-ray beam with a 2 mm diameter. The scanned samples were injection molded tensile bars at room temperature.
Rheology. The viscosity of the neat material and blends were tested using a rheometer, TA Instruments AR 2000ex. A 25 mm parallel plate fixture at 180[degrees]C was used to test the extruded pellets from a high of 100 Hz to a low of 0.1 Hz at a gap of 500 [micro]m. The complex viscosity of each sample was collected and analyzed.
RESULTS AND DISCUSSION
The DSC results of extruded neat PLA, LLDPE, PW, and their blends are shown in Fig. 1. There were four additional thermal characteristic temperatures associated with PLA namely, the glass transition ([T.sub.g]), cold crystallization peak ([T.sub.cc]), recrystallization peak ([T.sub.c]), and melting peak ([T.sub.m]) temperatures  as shown in Fig. 1a and b. The melting temperatures of PW, LLDPE, and PLA were indicated as [T.sub.m1], [T.sub.m2], and [T.sub.m3], respectively. These DSC thermograms suggested that PLA and LLDPE were immiscible and LLDPE and PW were miscible. Numerical values of these transition temperatures, as well as the corresponding melt enthalpies and degrees of crystallinity, are tabulated in Table 3.
From Table 3, it can be seen that the crystallinity of PLA increased after adding a secondary phase material, either LLDPE or PW, in the PLA matrix. With PW, the increased degree of crystallinity in PLA was due to the nucleating and lubricating effects of the PW phase in the matrix . Since PW is a low molecular weight material, only weak van der Waals forces hold the chains together in the wax . Once the temperature reaches the melting temperature of PW, the chains will start to relax as a result of the phase change from solid to liquid. During the phase change process, the intramolecular interactions of PW collapsed, contributing to the change in the crystallinity and crystallization temperature of the blends . Additionally, the short chains of PW will also increase the free volume in the amorphous phase of the PLA matrix, which will give rise to higher chain mobility . Similarly, the LLDPE also served as nucleating agent in the PLA matrix and facilitating the rearrangement of the chains so that the crystallinity increased.
Interestingly, mixing all three materials together--PLA, LLDPE, and PW--decreased the crystallinity of PLA slightly but increased that of LLDPE. The crystallinity of LLDPE in PLA/LLDPE/PW increased from 9.7% to 12.0%, whereas the crystallinity of PLA decreased from 24.4% to 21.5%. This was due to LLDPE and PW both having similar chemical structures so that they tend to interact first before they interact with PLA during extrusion. The SGAP-processed samples also exhibited the same trend although the crystallinity variations of PLA and LLDPE were smaller than their conventional counterparts.
The thermal stability and thermal degradation behavior of conventionally compounded neat polymer and blend pellets, as well as SGAP-processed neat polymer and blend pellets, were measured by TGA (cf. Fig. 2 and Table 4). Thermal degradation was assessed by examining the onset temperature of degradation, [T.sub.onset], at 5% weight loss, and the maximum degradation temperature of the matrix, [T.sub.max], at the point of maximum slope. Typical immiscible, two-component blends would normally show a two-step degradation process with different degradation temperatures corresponding to the two constituent polymers. The second maximum degradation temperature corresponding to the minor phase, [T*.sub.max], for those immiscible blends, if applicable, is also listed in Table 4.
The ratio of LLDPE in the matrix plays an important role with regard to thermal stability. Because LLDPE has higher thermal stability than PLA, blends with LLDPE showed increased [T.sub.onset] and [T.sub.max]. Since PW exhibited a lower onset degradation temperature (at 246.9[degrees]C), adding PW into the matrix induced an earlier onset of the decomposition. Interestingly, the maximum degradation temperature of LLDPE/PW and PLA/ LLDPE/PW blends from both compounded methods increased by 5[degrees]C compared to neat LLDPE. This was because the PW had turned into char residue as it degraded completely at the high temperature. A previous study concluded that the presence of char residue affected the thermal decomposition and the rate of thermal decomposition of the remaining LLDPE .
In Fig. 2, it can be seen that the gap between the PLA/ LLDPE blend and the PLA/LLDPE/PW blend was noticeably different between the two compounding methods. The differences were caused by the distribution of LLDPE in the blends. In the SGAP process, the equibiaxial extensional flow [22, 34] resulted in a better phase dispersion of LLDPE in the blends, which yielded a higher thermal resistance and thus a lower gap difference and slower weight loss.
Phase Morphology of the Neat Material and its Blends
Conventionally melt compounded and SGAP-processed pellets were injection molded into tensile bars and their morphology was examined. The molded tensile bars were first fractured in the middle in liquid nitrogen (cryogenic-fractured), and then the as-fractured cross-sections were examined via SEM; the results of which are shown in Fig. 3 (conventional) and Fig. 4 (SGAP). Due to the low softening temperature of the PLA/PW blends as mentioned above, the PLA/PW blends tended to melt prematurely during processing and stick to the surface of the processing equipment, such as the injection screw. As a result, the PLA/PW blends could not be easily processed or injection molded. However, as reported below, after adding LLDPE into either PW or PLA/PW, the LLDPE/PW and PLA/LLDPE/PW blends could be easily and continuously injection molded, demonstrating that LLDPE improved the processability of the blends.
The morphology of cryogenic-fractured neat LLDPE tensile bars displayed a uniform and layered structure, as shown in Figs. 3a and 4a. As the lubricating PW was added to the LLDPE matrix, the morphology became smoother than neat LLDPE as shown in Figs. 3b and 4b. Additionally, a number of short wedges can be seen. In Figs. 3c and 4c, due to the brittleness of neat PLA and F-PLA, the fracture surface showed a typical smooth, brittle surface topography. Figures 3d and 4d displayed that the surface roughness increased with LLDPE in the PLA matrix. LLDPE particles, voids, and phase separation were found on the fracture surface of the PLA/LLDPE blends.
Furthermore, distinct particle interfaces and large dispersed phase particles could be seen, thus indicating that the interfacial adhesion of PLA and LLDPE was poor  and the materials were immiscible. For the PLA/LLDPE/PW blends, while LLDPE and PW were miscible they were immiscible to PLA. Therefore, particles of the minor phases can be clearly seen on PLA matrix as shown in Figs. 3e and 4e. Note that all of the samples extruded via the SGAP process (cf. Fig. 4) exhibited a smoother surface and finer morphology with less voids and less particle pullout.
After tensile tests, the morphology of the fractured tensile bar surface of the various neat resins and blends was also examined using SEM. Unlike the cryogenic-fractured samples shown in Figs. 3 and 4, these tensile bars were tested and fractured at room temperature. All the tensile fractured cross-section samples except neat PLA were highly fibrillated along the tensile direction in the necking region. The fractured surface of a neat LLDPE tensile bar is shown in Fig. 5a, which features a deformed region (b) and a fibrillated region (c) at the tip of the fractured bar. In Fig. 5b, numerous small and parallel wrinkles, referred to as ridges in the matrix, can be seen . In Fig. 5c, wedges grew in size and pulled away from the surrounding LLDPE matrix. Wedge deformation usually occurs at low strain rates in relatively high-strength thermoplastics. Upon loading, wedges start to nucleate and grow inwards into the material. In materials like polyethylene, wedging is the dominant mode of surface and bulk deformation, and fracture is characterized by severely deformed fibrils. This type of deformation is defined as fibrillated deformation .
As shown in Figs. 6a and 7a, wedge, crazing, ridges, and crazing/tearing phenomena could be found at the surface of the LLDPE/PW blends, similar to the neat LLDPE matrix. Fibrillated deformation of LLDPE/PW blends can also be seen in Figs. 6b and 7b. In Figs. 6c and 7c, due to the brittleness of neat PLA, the fracture surface showed a typical brittle, smooth surface topography. In the PLA/LLDPE blends, although the matrix was PLA, numerous fibrils can be seen after tensile testing, as shown in Figs. 6d and 7d. Moreover, an abundance of short wedges came up when comparing LLDPE/PW blends to the neat LLDPE matrix. This was because the brittle PLA domain would fracture first upon loading. This caused the formation of short wedges and curling fibrils. Meanwhile, the rest of the LLDPE kept developing fibrillated fibers in the tensile loading direction. Figures 6e and 7e also exhibited the same morphology as Figs. 6d and 7d.
Mechanical Properties of the Neat Material and its Blends
The mechanical properties in terms of tensile stress-strain curves of neat PLA, LLDPE, and their blends are shown in Fig. 8, and the Young's moduli and stress-strain values are listed in Table 5. The addition of either PW or LLDPE in the PLA matrix increased the elongation-at-break. This can be attributed to two well-known reasons: (1) PW acts like a ductile material especially at low strain rates , and (2) LLDPE is highly flexible and elongates under stress.
Tensile testing is a destruction of the original LLDPE crystallites, turning them into reoriented crystallites to form a needle-like structure. Such a needle-like structure eventually led to fibrillar morphology after tensile testing. LLDPE has poorly defined yield regions at break. Generally speaking, LLDPE resins do not form a fibrillar structure until they fracture. Upon loading, the crystallites and the intercrystalline bridges are highly aligned along the tensile direction. Meanwhile, the nanocrystalline plates become thinner and denser, while the number and thickness of the intercrystalline bridges increase, eventually causing fibrillated deformation . Therefore, with the additional amount of LLDPE in the PLA matrix, the elongation-atbreak, as well as the area under curve, increased significantly at the expense of the Young's modulus. That is, for the conventional compounded blends, the elongation-at-break increased remarkably from 3.37% to 14.76%, the area under the curve increased from 1.41 to 2.01 N/mm2, and the Young's modulus decreased from 2,954.5 to 1,524.8 MPa. In the case of SGAPprocessed blends, the elongation-at-break increased from 3.26% to 16.03%, the area under curve increased from 1.33 to 2.13 N/ mm2, and the Young's modulus decreased from 2,910.9 to 1,413.8 MPa. This demonstrates that LLDPE acted as a toughness modifier in the matrix.
The presence of PW in the LLDPE matrix caused a decrease in both the Young's modulus and elongation-at-break. Interestingly, after adding PW to the PLA/LLDPE blends, the tendency of the Young's modulus of the PLA/LLDPE/PW and F-PLA/ LLDPE/PW blends differed, yet the area under curve of both blends increased. The previous research  found that with a 20% PW content in the PLA matrix, the Young's modulus was slightly higher than that of neat PLA. Additionally, from the SEM images, the SGAP-processed blends exhibited better phase dispersion. The better the dispersion of the secondary phase materials, the greater and more uniform the mechanical properties that could be achieved. Thus, for the conventional compound blends, the area under the curve increased from 2.01 to 2.66 N/[mm.sup.2], and the Young's modulus and elongation-at-break both decreased from 1,524.8 to 1,449.6 MPa, and 14.76% to 11.68%, respectively. Conversely, for the SGAP samples, the elongation-at-break decreased from 16.03% to 12.12%, and both the Young's modulus and the area under the curve increased from 1,413.8 to 1,487.7 MPa, and 2.13 to 2.70 N/[mm.sup.2], respectively. This suggests that the toughness of PLA/LLDPE blends increased with the additional of PW.
Figure 9 shows the diffraction patterns of neat materials and their blends using different compounding methods. LLDPE and PW have similar 2[theta] values. Their characteristic diffraction peaks were 21.4[degrees] and 23.7[degrees], corresponding to the (110) and (200) crystal planes, respectively. The 29 value of PLA was 16.8[degrees]. In Fig. 9a, the broad band of LLDPE suggests the presence of an amorphous structure. On the contrary, PW had a highly crystalline structure [29, 48].
In the injection molding process, PLA was rapidly cooled from 190[degrees]C to 50[degrees]C in 45 s. Owing to the very slow crystallization rate of PLA during cooling, the PLA tensile bars were anticipated to be amorphous. This is demonstrated in Fig. 9b and c by the broad band of the PLA curve. The additional LLDPE in the PLA matrix increased the crystallization rate of PLA, and a higher intensity 2[theta] = 16.8[degrees] peak could clearly be seen in the PLA/LLDPE blends. The added PW in the PLA/ LLDPE blends not only enhanced the intensity of the 21.4[degrees] and 23.7[degrees] diffraction peaks but lowered the intensity of the 16.8[degrees] diffraction peak, demonstrating the same results as DSC. That is, the degrees of crystallinity of LLDPE and PW increased while the degree of crystallinity of PLA decreased in the PLA/LLDPE/ PW blend. In addition, peaks from the SGAP-processed samples displayed a higher intensity than their conventional counterparts, especially for PLA. This increase in the degree of crystallinity was attributed to the plasticizing effect of the physical blowing agent and better phase dispersion in the SGAP-processed blends. Based on both of the DSC and XRD results, PW was found to have a higher crystallinity than LLDPE and the short chains of PW were able to facilitate higher mobility in the polymer matrix. Therefore, the additional amount of PW in the polymer matrix played an important role in inducing higher crystallinity.
The rheological properties of neat PLA, LLDPE, PW, and their blends were investigated. Figure 10 shows the complex viscosity ([[eta].sup.*]) of these materials either compounded conventionally or with the SGAP extrusion process. Only neat PLA displayed the Newtonian behavior at frequencies lower than 1 Hz. Beyond that, it exhibited shear thinning behavior. The rest of the blends, as well as the neat LLDPE matrix, did not exhibit a Newtonian plateau. Polymers with broad molecular weight distributions tend to display a broader Newtonian to shear-thinning transition [47, 49, 50] as shown in Fig. 10.
The increased molecular weight and low free volume of LLDPE in the blends caused the complex viscosity to shift to higher values . Adding 4 to 10 wt% PW to the matrix caused the complex viscosity to decrease remarkably compared to neat LLDPE, PLA/LLDPE blends, and PLA/PW blends. The decreased complex viscosity was caused by the short chain and lubrication behavior of PW. The free volume of the matrix increased with the addition of PW, thus the friction and resistance of the molten polymer decreased, resulting in a lower complex viscosity. The foamed structure also led to a lower complex viscosity, due to the physical foaming agent plasticizing the melt, as well as increased free volume. Furthermore, the nitrogen that dissolved in the extruded strands slowly dissipated into the atmosphere for about 50 h after extrusion and created voids in the blends. The voids left by the nitrogen, if present in the sample, would also lower the complex viscosity. The additional PW and foamed structure both lowered the complex viscosity, thereby, improving the fluidity and processibility of the blends.
LLDPE has been found to greatly improve the thermal stability of PLA/PW blends. It is also beneficial to apply the SGAP extrusion process during melt compounding. LLDPE and PW both acted as nucleating agents in the PLA blends. PW enhanced the mobility in polymer matrix and thus induced a higher crystallinity in the blends. DSC results revealed that in PLA/LLDPE/PW blends, PW interacted more with LLDPE than with PLA, hence the crystallinity of LLDPE increased while the crystallinity of PLA decreased. XRD results also confirmed this finding as the diffraction peaks corresponding to LLDPE and PW in the PLA/LLDPE/PW blends became higher while the crystal peak associated with PLA decreased. The SGAP extrusion method yielded more consistent material properties and better dispersion morphology. Owing to the more uniformly dispersed minor phase material in the matrix, the SGAP-processed samples exhibited better mechanical properties. In the F-PLA/LLDPE/PW blends, the addition of PW increased the Young's modulus and toughness. Samples with LLDPE exhibited numerous fibrils on their fractured surfaces. Furthermore, the complex viscosity of the blends was lowered due to the presence of PW and foam structure.
The authors acknowledge the support of the Wisconsin Institute for Discovery (WID) and the College of Engineering at the University of Wisconsin-Madison, as well as the Ministry of Education of Taiwan.
[1.] T. Mekonnen, P. Mussone, H. Khalil, and D. Bressler, J. Mater. Chem. A, 1, 13379 (2013).
[2.] R.E. Drumright, P.R. Gruber, and D.E. Henton, Adv. Mater., 12, 1841 (2000).
[3.] D. Garlotta, J. Polym. Environ., 9, 63 (2001).
[4.] R. Auras, B. Harte, and S. Selke, Macromol. Biosci., 4, 835 (2004).
[5.] N. Kawamoto, A. Sakai, T. Horikoshi, T. Urushihara, and E. Tobita, J. Appl. Polym. Sci., 103, 198 (2007).
[6.] R.M. Rasal, A.V. Janorkar, and D.E. Hirt, Prog. Polym. Sci., 35, 338 (2010).
[7.] J. Wang, S.J. Severtson, and A. Stein, Adv. Mater., 18, 1585 (2006).
[8.] R. Ibarra-Gomez, R. Muller, M. Bouquey, J. Rondin, C.A. Serra, F. Hassouna, Y. El Mouedden, V. Toniazzo, and D. Ruch, Polym. Eng. Sci., 55, 214 (2015).
[9.] R. Tokoro, D.M. Vu, K. Okubo, T. Tanaka, T. Fujii, and T. Fujiura, J. Mater. Sci., 43, 775 (2008).
[10.] A. Shakoor and N.L. Thomas, Polym. Eng. Sci., 54, 64 (2014).
[11.] D. Pholharn, Y. Srithep, and J. Morris, Polym. Eng. Sci., (2017). https://doi.org/10.1002/pen.24603
[12.] K. Bernland and P. Smith, J. Appl. Polym. Sci., 114, 281 (2009).
[13.] G. Biresaw and C. Carriere, J. Polym. Sci. Part B: Polym. Phys., 40, 19 (2002).
[14.] N. Reddy, D. Nama, and Y. Yang, Polym. Degrad. Stab., 93, 233 (2008).
[15.] T. Takayama and M. Todo, J. Mater. Sci., 41, 4989 (2006).
[16.] B.Y. Chen, X. Jing, H.Y. Mi, H. Zhao, W.H. Zhang, X.F. Peng, and L.S. Turng, Polym. Eng. Sci., 55, 1339 (2015).
[17.] X. Jing, H.Y. Mi, X.F. Peng, and L.S. Turng, Polym. Eng. Sci., 55, 70 (2015).
[18.] S.H. Im, C.W. Lee, G. Bibi, Y. Jung, and S.H. Kim, Polym. Eng. Sci. (2017). https://doi.org/10.1002/pen.24681
[19.] J. Lobos, S. Iasella, M.A. Rodriguez-Perez, and S.S. Velankar, Polym. Eng. Sci., 56, 9 (2016).
[20.] V. Volpe, M.D. Filitto, V. Klofacova, F.D. Santis, and R. Pantani, Polym. Eng. Sci. (2017). https://doi.org/10.1002/pen. 24730
[21.] S. Sankarpandi, C.B. Park, and A.K. Ghosh, Polym. Eng. Sci., 57, 365 (2017).
[22.] Y.-J. Chen, A. Huang, T. Ellingham, C. Chung, and L.-S. Tumg, Eur. Polym. J., 98, 262 (2018).
[23.] A. San, Energy Corners. Manage, 45, 13 (2004).
[24.] H. Inaba and P. Tu, Heat Mass Transfer, 32, 307 (1997).
[25.] F. Chen and M.P. Wolcott, Eur. Polym. J., 52, 79 (2014).
[26.] F. Chen and M. Wolcott, Sol. Energy Mater. Sol. Cells, 137, 44 (2015).
[27.] T.P. Gumede, A.S. Luyt, R.A. Perez-Camargo, and A.J. Muller, J. Appl. Polym. Sci., 134, 2 (2017).
[28.] I. Krupa, G. Mikova, and A. Luyt, Eur. Polym. J., 43, 4695 (2007).
[29.] D. Kim, I. Park, J. Seo, H. Han, and W. Jang, J. Polym. Res., 22, 2 (2015).
[30.] M.A. AlMaadeed, S. Labidi, I. Krupa, and M. Ouederni, Arab. J. Chem., 8, 388 (2015).
[31.] A.B. Strong, Plastics: Materials and Processing, Prentice Hall, Upper Saddle River, NJ (2006).
[32.] J.A. Molefi, A.S. Luyt, and I. Krupa, Thermochim. Acta, 500, 88 (2010).
[33.] T. Ellingham, L. Duddleston, and L.-S. Turng, Society of Plastics Engineering TPO Conference, Detroit, Oct. 2-5 (2016).
[34.] T. Ellingham, L. Duddleston, and L.-S. Turng, Polym. J., 117, 132 (2017).
[35.] A.K. Whittaker, Radiat. Phys. Chem., 48, 601 (1996).
[36.] P.-W.S. Chum, R.P. Markovich, G.W. Knight, and S.-Y. Lai, U.S. Patent 6,316,549 (2001).
[37.] C.L. Simoes, J.C. Viana, and A.M. Cunha, J. Appl. Polym. Sci., 112, 345 (2009).
[38.] J.F. Turner, A. Riga, A. O'Connor, J. Zhang, and J. Collis, J. Therm. Anal. Calorim., 75, 257 (2004).
[39.] D.L. Wilfong and G. Knight, J. Polym. Sci. Part B: Polym. Phys., 28, 6 (1990).
[40.] A.M. Harris and E.C. Lee, J. Appl. Polym. Sci., 107, 2246 (2008).
[41.] L.H. Sperling, Introduction to Physical Polymer Science, John Wiley & Sons, Hoboken, NJ (2005).
[42.] S. Bandi and D.A. Schiraldi, Macromolecules, 39, 6537 (2006).
[43.] M. Molaba, D. Dudic, and A. Luyt, Polym. Lett. 9, 901 (2015).
[44.] C.L. Beyler and M.M. Hirschler, SFPE Handbook of Fire Protection Engineering, Vol. 2, NFPA (2002).
[45.] K.S. Anderson, S.H. Lim, and M.A. Hillmyer, J. Appl. Polym. Sci., 89, 3757 (2003).
[46.] A. Dasari and R. Misra, Acta Mater., 52, 1683 (2004).
[47.] A. Peacock, Handbook of Polyethylene: Structures, Properties, and Applications, CRC Press, Boca Raton, FL (2000).
[48.] K.A. Moly, H.J. Radusch, R. Androsh, S.S. Bhagawan, and S. Thomas, Eur. Polym. J., 41, 1410 (2005).
[49.] P.C. Dartora, R.M.C. Santana, and A.C.F. Moreira, Polimeros, 25, 531 (2015).
[50.] J.P. Hogan, C.T. Levett, and R.T. Werkman, SPEJ., 23, 11 (1967).
[51.] M. Chanda, Introduction to Polymer Science and Chemistry: A Problem-Solving Approach, CRC Press, Boca Raton, FL (2013).
Yann-Jiun Chen, (1, 2, 3) Emily Yu, (1, 2) Thomas Ellingham, (1, 2) Chunhui Chung, (3) Lih-Sheng Turng (iD) (1, 2)
(1) Department of Mechanical Engineering, Polymer Engineering Center, University of Wisconsin-Madison, Madison, Wisconsin 53706
(2) Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, Wisconsin 53715
(3) Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
Correspondence to: L.-S. Turng; e-mail: email@example.com
Contract grant sponsor: Wisconsin Institute for Discovery; contract grant sponsor: University of Wisconsin-Madison.
Published online in Wiley Online Library (wileyonlinelibrary.com).
Caption: FIG. 1. DSC thermograms of the second heating curve of (a) conventionally melt compounded material and (b) SGAP-foamed compounded material. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 2. TGA curves of (a) conventional PLA/PW blends and (b) SGAP-foamed F-PLA/PW blends. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 3. Cross-sections of SEM images: (a) neat LLDPE, (b) L90/PW10 blends, (c) neat PLA (d) P62 5/L37 5 blends, and (e) P62.5/L36/PW4 blends.
Caption: FIG. 4. Cross-sections of SEM images: (a) neat LLDPE, (b) F-L90/PW10 blends, (c) neat F-PLA, (d) F-P62.5/ L37.5 blends, and (e) F-P62.5/L36/PW4 blends.
Caption: FIG. 5. Images of LLDPE cross-sections after tensile testing: (a) fractured SEM sample after gold coating, SEM images showing (b) the deformation mechanism and (c) fibrillated fibers. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 6. Cross-sections of SEM images after tensile testing: (a) L90/PW10 blends, (b) fibrillated deformation of L90/PW10 blends, (c) neat PLA, (d) P62.5/L37.5 blends, and (e) P62.5/L36/PW4 blends.
Caption: FIG. 7. Cross-sections of SEM images after tensile testing: (a) F-L90/PW10 blends, (b) fibrillated deformation of F-L90/PW10 blends, (c) neat F-PLA, (d) F-P62.5/L37.5 blends, and (e) F-P62.5/L36/PW4 blends.
Caption: FIG. 8. Stress-strain curves of (a) conventional PLA/PW blends and (b) SGAP-processed F-PLA/PW blends. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 9. X-ray diffraction patterns of (a) neat material, (b) conventional blends, and (c) SGAP-foamed blends. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 10. Complex viscosity ([[eta].sup.*]) verses frequency of (a) conventionally compounded blends and (b) SGAP processed blends. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Designation of materials and their compositions. Composition Weight Description Materials (wt/wt/wt) ratio PLA PLA 100 -- L90/PW10 LLDPE/PW 90/10 9:1 P62.5/L37.5 PLA/LLDPE 62.5/37.5 5:3 P60/L36/PW4 PLA/LLDPE/PW 60/36/4 15:9:1 P93.75/PW6.25 PLA/PW 93.75/6.25 15:1 TABLE 2. Key process conditions. Extrusion Temperature ([degrees]C) 155/155/160/165/170/175/180 (sequence from hopper to die) Screw speed (rpm) 100 Feed rate (g/min) 30 SGAP injection pressure (bar) 31 Injection molding Nozzle temperature ([degrees]C) 190 Injection speed (cm3/s) 45 Cooling time (s) 45 Pack/hold pressure (bar) 620 Mold temperature ([degrees]C) 50 Back pressure (bar) 50 Pack/hold time (s) 7 TABLE 3. Different DSC results of neat PLA, PW, conventional PLA/PW blends, and SGAP/foamed F/PLA and F-PLA/PW blends. Sample [T.sub.g] [T.sub.cc] [T.sub.m] ([degrees]C) ([degrees]C) (J/g) LLDPE -32.6 NA 124.2 PW 35.8 NA 56.5 L90/PW10 -27.5 NA 123.0 PLA 60.8 99.4 168.7 P62.5/L37.5 61.5 96.3 168.3 P60/L36/PW4 61.4 98.0 168.3 P93.75/PW6.25 NA 99.5 168.2 F-L90/PW10 -26.2 NA 121.9 F-PLA 61.0 97.5 168.5 F-P62.5/L37.5 61.5 96.1 168.7 F-P60/L36/PW4 61.2 97.8 168.1 F-93.75/PW6.25 NA 99.8 165.5 Sample [H.sub.c] [H.sub.m] [X.sub.c] (%) (J/g) (J/g) of PLA LLDPE NA 117.7 NA PW NA 193.6 NA L90/PW10 NA 126.9 NA PLA 24.9 35.6 12.1 [+ or -] 0.7 P62.5/L37.5 9.7 23.4 24.4 [+ or -] 0.8 P60/L36/PW4 7.6 19.2 21.5 [+ or -] 0.9 P93.75/PW6.25 25.2 37.7 14.4 [+ or -] 0.7 F-L90/PW10 NA 137.6 NA F-PLA 24.3 36.6 13.0 [+ or -] 0.3 F-P62.5/L37.5 9.2 23.6 23.1 [+ or -] 0.4 F-P60/L36/PW4 9.9 23.4 23.0 [+ or -] 0.6 F-93.75/PW6.25 25.8 38.9 15.1 [+ or -] 0.4 Sample [X.sub.c] (%) of LLDPE LLDPE 41.7 [+ or -] 0.6 PW NA L90/PW10 48.1 [+ or -] 0.4 PLA NA P62.5/L37.5 9.7 [+ or -] 0.9 P60/L36/PW4 12.0 [+ or -] 0.8 P93.75/PW6.25 NA F-L90/PW10 52.2 [+ or -] 0.8 F-PLA NA F-P62.5/L37.5 9.7 [+ or -] 0.5 F-P60/L36/PW4 10.3 [+ or -] 0.42 F-93.75/PW6.25 NA TABLE 4. TGA results of neat PLA, PW, conventional PLA/PW blends, and SGAP-processed F-PLA and F-PLA/PW blends. [T.sub.onset] [T.sub.max] [T*.sub.max] ([degrees]C) ([degrees]C) ([degrees]C) LLDPE 441.4 485 N/A PW 246.9 322 N/A L90/PW10 316.4 490.7 N/A PLA 330.8 376 N/A P62.5/L37.5 340.4 380.1 484.8 P60/L36/PW4 327.8 377.7 486.3 P95.75/PW6.25 323.2 367.1 N/A F-L90/PW10 328.6 488.6 N/A F-PLA 332.8 375.1 N/A F-P62.5/L37.5 338.6 377.5 486.4 F-P60/L36/PW4 325.3 375.4 488.4 F-P93.75/PW6.25 336.7 375.2 N/A [T.sub.onset]: onset temperature of degradation, [T.sub.max]: the maximum degradation temperature of the matrix, and [T*.sub.max]: second maximum degradation temperature corresponding to the minor phase. TABLE 5. Mechanical properties of PLA and its blends. Young's modulus (MPa) Tensile strength (MPa) LLDPE 121.87 [+ or -] 4.03 16.22 [+ or -] 1.51 L90/PW10 116.65 [+ or -] 8.96 15.14 [+ or -] 0.37 PLA 2,954.47 [+ or -] 26.3 65.75 [+ or -] 1.06 P62.5/L37.5 1,524.83 [+ or -] 56.82 35.31 [+ or -] 1.99 P60/L36/PW4 1,449.63 [+ or -] 66.27 32.78 [+ or -] 0.97 F-L90/PW10 102.00 [+ or -] 12.03 14.93 [+ or -] 0.46 F-PLA 2,910.95 [+ or -] 23.85 65.67 [+ or -] 1.10 F-P62.5/L37.5 1,413.82 [+ or -] 46.52 35.11 [+ or -] 1.26 F-P60/L36/PW4 1,487.65 [+ or -] 35.15 32.71 [+ or -] 3.42 Elongation-at-break (%) Area under curve (N/[mm.sup.2]) LLDPE 262.74 [+ or -] 3.69 36.95 [+ or -] 1.45 L90/PW10 121.77 [+ or -] 4.27 16.52 [+ or -] 1.52 PLA 3.37 [+ or -] 0.18 1.41 [+ or -] 0.11 P62.5/L37.5 14.76 [+ or -] 2.15 2.01 [+ or -] 0.48 P60/L36/PW4 11.68 [+ or -] 1.67 2.66 [+ or -] 0.51 F-L90/PW10 131.42 [+ or -] 4.09 17.95 [+ or -] 1.65 F-PLA 3.26 [+ or -] 0.25 1.33 [+ or -] 0.10 F-P62.5/L37.5 16.03 [+ or -] 3.45 2.13 [+ or -] 0.43 F-P60/L36/PW4 12.12 [+ or -] 0.91 2.70 [+ or -] 0.45
|Printer friendly Cite/link Email Feedback|
|Author:||Chen, Yann-Jiun; Yu, Emily; Ellingham, Thomas; Chung, Chunhui; Turng, Lih-Sheng|
|Publication:||Polymer Engineering and Science|
|Date:||Dec 1, 2018|
|Previous Article:||Residual Strength and Damage Mechanisms of Laminated Carbon Fiber Reinforced Polymer under Thermal Environments and Laser Irradiations.|
|Next Article:||Microcellular Silicone Rubber Foams: The Influence of Reinforcing Agent on Cellular Morphology and Nucleation.|