A Review of Engineering Biodegradable Polymer Blends - American

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A Review of Engineering Biodegradable Polymer Blends: Morphology, Mechanical Property, and Flame Retardancy Seongchan Pack,*,1 Menahem Lewin,# and Miriam H. Rafailovich1 1Department

of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275 #Deceased *E-mail: [email protected]

We first review recent flame retardant formulations on biodegradable polymer blends, such as either starch/ poly(butylene adipate-co-terephthalate) (PBAT) or poly(lactic acid)/poly(butylene adipate-co-terephthalate) (PLA/PBAT), where interfacial tensions of the biodegradable polymer blends can play an important role on mechanical and flame retardant properties. The improved material properties are also mainly influenced by morphology of the blends. In addition, a novel approach to increase not only compatibility but also flame retardancy of the biodegradable polymer blends is reviewed. In this method, an aryl phosphate flame retardant, resorcinol di(phenyl phosphate) (RDP), is adsorbed onto the added particles. This is shown to have the following advantages; (a) it can act as a surfactant on either starch or clays, leading to better dispersion in the polymer matrices, (b) enhance compatibilization of PLA and ECOFLEX by localizing the particles at the blend interfaces, and (c) segregated to the blend surface when heated, and reacted with both polymer and starch. These factors allowed for the formulation of self-extinguishing PLA/ECOFLEX blends, with unusual properties that distinguished them from other blends using standard flame retardant halogen formulations. The reactive properties allowed the formation of a shell like layer whose

© 2012 American Chemical Society In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

modulus was much higher than the interior polymer, and which dissipated the heat of the approaching front. Analysis of the chars formed after combustion in a cone calorimeter, indicated that addition of the RDP soaked clays did not affect the ductility of the chars or their ability to encase the combustion products which sustained internal pressures from the decomposition gases allowing the release at a steady rate.


Introduction The rapid accumulation of polymer wastes in landfills has raised significant concern regarding their use, and slowed their rapid growth as replacements for other structural materials, such as wood, metals, and glass (1). Furthermore, studies on environmental impact of the traditional halogenated flame retardant formulations have shown that they made their way into regional water supply causing them to be banned in some locations (2–4). These events have led to a rise in interest for developing biodegradable polymers, as well as environmentally benign flame retardant formulations, as replacements for the traditional petroleum polymers. This has poses several challenges for the industry, which have to overcome before a sustainable substitute is produced on an industrial scale. Unlike the conventional polymers, biodegradable polymers tend to be more polar, absorb water more easily, and have higher reactivity in ambient conditions. This makes them more difficult to thermally process, control their mechanical properties, form blends, and find appropriate flame retardant agents which combine with their decomposition products. In this chapter we review the basic properties of some of the common biodegradable polymers currently in use, the flame retardant formulations that have been developed, and the synergy which the materials properties, such as surface tension and compatibility, can play in enhancing their flame retardant response.

Review of Common Biodegradable Polymers: Morphology and Mechanical Properties Poly(lactic acid) (PLA) is a transparent linear aliphatic polyester, which derived from renewable sources such as sugarcane and starch. (5). PLA also belongs to the class of Polyhydroxyalikanoates (PHAs) (6). PLA biodegrades to carbon oxide, water and humus since the synthesis of lactic acid is a process of bioconversion to ring-opening polymerization (7–9). The tensile properties of PLA, are listed below; the polymer has a very high modulus and strength, which can be comparable to oil-based thermoplastic polymers (10). However, since PLA has a low glass transition temperature and is semicystalline, it does not have good processability and toughness. Furthermore, it is not a good char former and is not flame retardant according to most standard testing protocols, which restrict its applications to electronics, construction, and automobile parts. (11, 12). The thermal properties of biodegradable polymers are shown in Table 1. 428 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Table 1. The material properties and chemical structures of common biodegradable polymers

Poly (butylene adipate-co-terephthalate) (PBAT), commercialized as Ecoflex, is another popular biodegradable polymer which is digested by enzymes, to produce (13). PBAT is amorphous and can be characterized as an elastomeric polymer, with low modulus, but high elongation at break similar to the properties of butylene rubber. Hence PBAT is frequently blended with the PHA group of polymers to engineer biodegradable blends with improved mechanical strength and impact toughness (14, 15). For example, Pack et al. (15) investigated a biodegradable blend of PLA/Ecoflex with different nanofillers, and showed that PLA was well dispersed within the Ecoflex matrix (Figure 1a) and that the Ecoflex forms a fibrillar structure which further imparts strength and impact resistance to the material (Figure 1b). Other mechanical properties of the two materials are listed in Table 2. 429 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 1. TEM/SEM of a blend of PLA/Ecoflex (80/20 wt%) (the scale bar is 2 µm).

Table 2. Mechanical properties of neat ecoflex, PLA, and a blend of 80% PLA to 20% Ecoflex

*

Polymer

Elongation at break (%)

Toughness (MPa)

Tensile Strength (MPa)

Young’s Modulus (MPa)

Storage Modulus (Pa)

PLA

6.5

1.9

53.4

1205.5

1.51E+09

Ecoflex

*

*

12.5

59.5

-

PLA/Ecoflex (80/20%)

270.8

82.3

36.6

916.7

1.17E+09

Pure Ecoflex did not rupture.

Review of Flame Retardant of Biodegradable Polymers PLA is not known for its flame retardant properties. In fact the polymer has a poor LOI compared to aromatic polymers, such as polycarbonate (16, 17). Reti et al. demonstrated that addition of both ammonium polyphosphate (APP) and either starch or lignin to PLA could achieve the vertical flammable designation (18). Zhan et al. also showed that the addition of bisphosphorate diphosphoryl melamine had an impact on reducing flammability of PLA (19). Metal oxides were also being used to decrease flammability of the biodegradable polymer (16, 20, 21). Moreover, they developed a synthesis process of being attached a phosphours oligomers to the back bone of PLA (16). As a result of that, good flame retardant properties were obtained. However, these processes produced significant amounts of hazardous chemical waste and involved complicated multiple step synthesis procedures. 430 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Starch is well known to have good flame retardant properties, due to the carbonization process of the starch occurs at exposed surface from heat fronts (15, 22). However, crystallization of starch is a major drawback for melt-blending and extruding it together with other polymers (23). Owing to the fact that PLA is derived from starch, it is one of the few polymers which can be melt-blended with PLA, especially when the starch is treated with glycerol (24, 25). Wang et al reported adding 10% starch to PLA slightly increased the LOI of neat PLA from 20 to 23 (26). They also investigated that a combination of melamine with ammonium polyphosphate (APP) could render the starch/PLA be self-extinguished, which was rated as UL-94-V0. The improvement though occurred at a relatively large loading of the flame retardants. So it might reduce in the impact toughness from the pure PLA polymer.

Table 3. Properties of thermal stability of Resorcinol bis (diphenyl phosphate) (RDP)

Recently, Park et al. have succeeded that when starch can be soaked with the RDP, a common flame retardant compound (see Table 3), it is also possible to blend it with PBAT to produce a flame retardant compound. In this case though, they showed that the addition of a third component, namely Halloysite tubes, This compound was achieved with acceptable impact toughness, and was also able to pass the UL-94-V0, which is shown in Figure 2. In order to understand the process, the samples were cross sectioned and analyzed with STXM, a technique which allows one to identity the different chemical components and map their distribution within the blend. The STXM spectra are shown in Figure 3a, where the distinct abortion peaks corresponding to the components are shown. From Figure 3b-d we can see that the distribution of the starch within the PBAT improved when the starch is soaked in RDP, but a major improvement in the distribution is achieved when the Halloysite tubes are added. The ability of large aspect ratio nanoparticles to compabiltize polymer blends had been shown previously (27, 28). 431 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 2. (a) Results of Impact Strength and (b) UL94-V0 flammable test of RDP starch/Ecoflex with HNTs. Reproduced with permission from reference (1). Copyright 2012 Elsevier.

Recent Work on PLA/PBAT Blends: Common “Environmental” FR Formulations and Flammability In order to achieve better mechanical properties, as well as flame retardance, it is important to greatly reduce the amount of additives. This is usually achieved by blending polymers to achieve a new material with desirable properties. Here we discuss blends of PLA and PBAT, where we vary the mechanical and impact toughness. This variation is essential if biodegradable materials are produced as effective replacements for the petroleum based polymers. In this section we show that either the RDP coated starch or RDP coated nanoparticles can be very effective as flame retardant agents and compatibilizers in biodegradable polymers. The synthesis of the RDP-coated starch, montmorillontie clay, and Halloysite nanotubes (HNTs) is straightforward and no harmful chemical wastes are generated. The process consists of pouring 20 wt % of RDP into a beaker containing clays or starch and then the beaker is mixed in a high shear mixer 432 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


at 60°C. A typical SEM image of the RDP-coated particles is shown at Figure 4. From the figure we can see that Halloysites tubes are completely coated with RDP, which is confirmed by the EDAX spectra obtained from the corresponding the SEM image.

Figure 3. (a) The NEXAFS spectra of Starch, Ecoflex, and RDP and STXM images: (b) Starch/Ecoflex (50/50 wt%) (c) RDP Starch/Ecoflex (50/50 wt%), (d) RDP Starch/Ecoflex/Halloysites (60/40/5 wt%) (the scale bar is 3 µm and the image was taken at 284.7 eV). Reproduced with permission from reference (15). Copyright 2012 Elsevier.

In previous section, Pack et al. showed that the addition of HNTs could make the blends of RDP starch/Ecoflex(60/40 wt%) self-extinguishing, whereas the same ratio of neat starch/Ecoflex blend was not. They also showed that the addition of RDP starch could have same impact on flame retardant properties of the blend of PLA/Ecoflex, where the addition of the RDP starch can increase the degree of dispersion of the Ecoflex domains, which is shown in Figures 5 and 6. 433 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 4. SEM images: (a) RDP-coated HNTs (b) EDAX of the square box (the scale bar is 0.3 µm).

From the figures we can see that dark phases are Ecoflex, which are aligned in the direction of extrusion and fairly dispersed in PLA matrix. These morphology dramatically changes when 5 % RDP starch is introduced to the blend. As a result, the fiber-like domains become spherical ones. It can lead to changing tensile properties. Pack et al. reported that the absorption of RDP to starch surfaces could increase compatibility of the PLA/Ecoflex blend, which led to an increase of strength and toughness. They also showed that surface energy of RDP starch could play a significant role on flame retardant properties, where the surface tension of RDP (~48 mN/m) is relatively higher than that of starch (35~45 mN/m) thereby the strong RDP absorption occurs at the starch surfaces. The surface tensions of the polymers are shown in Table 4. Moreover, from the table, we can see that the surface energy of PBAT is much higher than that of PLA, which may cause RDP starch be segregated at the interfaces between Ecoflex and PLA. This may be confirmed in the TEM image of the PLA/Ecoflex blends in Figure 5b.

Table 4. The surface tensions of polymers are obtained from contact angle measurement at a room temperature (at 25°C) Polymer

PLA

PBAT

Starch

RDP

Surface Tension (mN/m)

37

53

35-42

48

434 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 5. TEM images at PLA/Ecoflex blends with different RDP coated particles: dark phases are Ecoflex domains; (a) PLA/Ecoflex (80/20 wt%), (b) PLA/Ecoflex/RDP Starch (80/20/5 wt%), and (c) PLA/Ecoflex/RDP clays (80/20/5 wt%) (d) A high magnification of PLA/Ecoflex/RDP clays(80/20/5 wt%). (Reproduced with permission from reference (15). Copyright 2012 Elsevier).

On the other hand, the addition of RDP clay can change the Ecoflex domain in morphology. In Figure 6 we show the STXM data obtained from PLA/Ecoflex blends (wt percent 79/19) comparing the morphology when 5% starch or 5% RDP impregnated starch is added. In each case we can clearly see that the domains of the minority phase( Ecoflex), are significantly smaller in the samples where the starch was first impregnated with RDP. From Figure 5c-d we can also see that as the long fibers is aligned in the shear direction, the sharp interfaces between the blends become opaque. It can be indicated that the blend is more compatible since the RDP clays are localized at the interfaces. The localization of RDP clays may be predicted by a balance of surface energy between PLA/RDP and PBAT/RDP clays, where interfacial energy can be reduced by the inclusion of RDP clays in the blend. 435 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


As an expectation by the interfacial energy between RDP particles, a combination RDP-coated starch with RDP-coated clay could be given to more improvement on toughness and flame retardant properties of the biodegradable blends, which was reported in ref. (15). Moreover, In their previous report, a combination of two different particles might lead to a synergistic effect at interfacial area (29). Since the RDP starch particles were more likely to be segregated at interfaces between PLA and Ecoflex, RDP clays could be segregated into Ecoflex domains. As a result of that, more exfoliated RDP clays are observed, which is shown in Figure 5d. Thus, the ability of segregation of RDP-coated particles to either polymers or interfaces is mainly contributed to surface energy of each particle.

Figure 6. STXM images: (a) PLA/Ecoflex/starch (b) PLA/Ecoflex/RDP starch, taken at 284.7 eV; (c) PLA/Ecoflex/starch (d) PLA/Ecoflex/RDP starch, taken at 288.5 eV (the scale bar is 5 µm). In Figure 7, we show that a series of SEM images of chars surfaces is examined in terms of microstructure. It is indicated that the different chars morphology can be confirmed by EDAX from each of chars surface. From the figure we can see that, in the case of neat starch/Ecoflex blend (60/40 wt%), the char surfaces seem 436 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


to consist of strand-like bundles with many small holes. about a few micro sizes in diameter. However, when the HNTs are added, the small holes looks covered with the nanotubes, where the peak intensity of Si/Al is relatively higher than that of C/ O in the EDAX. Moreover, when RDP clays are added in the blend, a network-like structures is observed at on the surface of the chars, as shown in Figure 8i. The structures become segregated when 2% HNTs/3% RDP clays is replaced with the 5% RDP clays. Later, the segregated network-like structures appears little at the char surface when 3% C20A is introduced in the blend, instead of the 3% RDPcoated clays. These morphology evolutions may be contributed to a competition of the surface tensions of either RDP-coated clays/HNTs or C20A/HNTs. Since we have reported that the surface tension of RDP is relatively higher than that of the di-tallow on the C20A (28), in the case of RDP clays/HNTs, RDP clays are more segregated to HNT clays, which can interrupt the network-like structures. The relatively lower surface energy of HNTs becomes a relatively high surface energy when C20A is added in the blends. As a result of that, there are no network-like structures at the exposed surface. Therefore, the two nanoparticles may compete with each other at interfaces. This competition of surface energy can determine the morphology of chars surface. The morphology is further confirmed by the EDAX spectra of the chars which are shown adjacent to the SEM images. In Figure 7b we see that for the unfilled polymer, we see a large carbon peak and no Si or Al peaks are present. In Figures 7d we see that when neat HNT are added the carbon peak is reduced, while two peaks of similar amplitude are present corresponding to Si and Al. The spectra in Figure 7f corresponding to the addition of Closite 20A clay is similar, except that the Aluminum content relative to the silicate content is reduce in the clays. The reduced carbon peak in both spectra indicates that the HNT and the Cloisite 20A particles are enriched in the char at the expense of the polymer. In Figure 7h and 7j we show the spectra corresponding to the char formed when RDP impregnated clay is added. Here we find similar Aluminum to Si ratio, but the carbon peak is much larger and comparable to that found in the polymer sample, indicating that the near surface layer was mostly composed of the polymer matrix and the RDP clays were further into the interior of the char. Hence, even though biodegradable polymers have higher surface energies than most non-biodegradable polymers, the surface energy of RDP is still higher, addition of RDP clay causes enrichment of the polymer to the air surface when heated, and the larger carbon content in the char. Conversely, the high surface energy of the polymers allows enrichment of the surface with the relatively low surface energy neat HNT and Cloiste 20A particles, reducing the carbon component in the char. It is well established that the char formation occurs at a nearly exposed surface from heat, which could prevent melt polymers underneath the chars (30–32). Both the rate of heat release and the rate of mass loss from cone calorimety have been typically used to interpret thermal behavior of polymers. In particular, charring polymers are well investigated since the mass loss rate dramatically reduces when chars are formed at an exposed surface against heat fronts (28, 33). Recently, Pack et al. reported that the combination carbon nanotubes with clays could reduce the flammability since the carbon nanotubes were entangled with clay platelets, which may increase heat capacity of the system (29). 437 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


438 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Figure 7. SEM images of the Residues: (a-b) Starch/Ecoflex (60/40 wt%), (c-d) Starch/Ecoflex/HNTs (60/40/5 wt%), (e-f) Starch/Ecoflex/C20A/HNTs (60/40/3/2 wt%) , (g-h) Starch/Ecoflex/RDP-coated clays/HNTs (60/40/3/2 wt%), and (i-j) Starch/Ecoflex/ RDP-coated clays (60/40/5 wt%), the scale bar is 20 micrometers.

Figure 8. (a) The MLR of starch/Ecoflex with different nanoparticles in gasification at 50kW/m2 under N2. (Reproduced with permission from reference (15). Copyright 2012 Elsevier). (b) The MLR of PLA/Ecoflex with RDP starch in gasification at 50kW/m2 under N2. A combination of two different nanoparticles can be also applied to biodegradable polymers in order to obtain the synergy in flammability. The MLR of the 60% starch/40% Ecoflex blends with different nanoparticles is shown in Figure 8a, where the addition of Cloisite 20A (C20A) leads to a greater reduction of the MLR trace of the blend with HNTs, which is about 12g/m2s. Since the surface energy of C20A is relatively lower than the other components in the blend the C20A could segregate to the exposed surfaces, forming a char layers with the HNTs as a heat barrier, which may cause to lowering mass loss rate at the beginning in the burning process. However, the addition of RDP-coated MMT (RDP Na+) does not reduce the initial mass loss rate of the starch/Ecoflex blend since the surface energy of the RDP nanoparticles is higher than that of the blend, and hence the RDP clay would not segregate to the surface. Rather, the MLR is similar to that of the starch homopolymer. Furthermore, in the case of the blend with RDP Na+ and HNTs, no steady plateau is obtained at the MLR, which may be attributed to formation of a softer char. This kind of thermal response also occurs in blends of PLA/Ecoflex with nanoparticles. Since the surface energy of PLA is much lower than that of Ecoflex, it is reasonable to assume that the surface of the blend is enriched in the PLA component. Hence the MLR of all the compounds is determined by the component at the surface, or the one having the lower surface energy, which is shown in Figure 8b. from the figure we can see that the peak of MLR of the neat Ecoflex is 439 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


dramatically reduced to about 30g/m2s when PLA is added, which is similar to that of the neat PLA. However, when 5% Cloistie Na+ is added in PLA/Ecoflex blend, the MLR trace of the blend is slightly increased, and the whole burning process is finished at an earlier time, about 275 sec. This may be attributed to a fact that Cloisite Na+ clays could be regarded as an agglomerate in the blend, where local temperature/pressure gradient along the interfaces between the clays and polymers may be high enough to vaporize the polymer matrix. Since RDP is well known as a phosphours-based flame retardant and mainly retards combustion process of polymers in condense phases as a charring agent, the addition of RDP Na+ clays somewhat could reduce the MLR trace of the blend. It may be contributed to an excess of RDP on the clays. On the other hand, the addtition of RDP-coated starch can affect the rate of heat release(RHR) of the PLA/Ecoflex blend in cone calarimetry. In particular, the peak RHR of the blend is greately reduced when the RDP starch is added, which is shown in Figure 9. Thus, the addition of RDP coated particles could not only increase the compatibility but also improve the flammability of the blend.

Figure 9. The RHR of starch/Ecoflex with RDP starch at 35kW/m2 under air.

In previous paragraph , we discuss that surface energy of biodegradable polymers can be altered by the addition of nanoparticles, which also leads to different chars formation in microstructure. Pack et al. (28, 29) reported that the flexibility of the chars layers might be important in reducing the MLR since the chars are most effective when they keep the hot gases from diffusing throughout the matrix, accelerating the decomposition process. The char formed by the unfilled polymer blend is usually the most ductile and free of holes. From Figure 7 above we showed that the morphology of the chars can be very different depending on the type of nanoparticle filler used. We showed that it was possible 440 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


to quantify the hardness and modulus of the chars using nanoidentation (15). The results are tabulated in Table 5, where we see that soft char also produced the best MLR spectra. On the other hand, when the chars became too soft, and could not contain the gases, i.e. the results for the formulation with 5%HNT, the MLR we not reduced. Hence the best results were obtained with chars of intermediate rigidity, where had both, good mechanical integrity as well as ductility. The malfunctioned expandability of chars layers could decrease flame retardant properties, which was reported in ref (15). Therefore, measuring elastic modulus and hardness of chars surface is to understand a mechanism of stretching chars.

Table 5. The Elastic Modulus and Hardness of Starch/Ecoflex blends with different nanoparticles Sample (wt%)

Reduced Elastic Modulus (GPa)

Hardness (GPa)

Starch/Ecoflex (60/40 wt%)

0.2

0.5

Starch/Ecoflex/HNTs (60/40/5 wt%)

5.3

0.78

Starch/Ecoflex/RDP clay (60/40/5 wt%)

2.5

0.43

Starch/Ecoflex/C20A/HNTs (60/40/3/2 wt%)

3.2

0.37

Starch/Ecoflex/RDP Na+/HNTs (60/40/3/2 wt%)

0.2

0.03

From the table we can see that when HNTs are added, the elastic modulus and hardness are 5.3 GPa and 2.5 GPa, respectively. However, when RDP clays are added to the blend, both E’ and H are reduced to 2.5 GPa and 0.43GPa, respectively, which may be explained with the morphology of chars formation in the previous paragraph, where the HNTs are uniformly segregated onto the char surface, whereas the RDP clays are well dispersed in the matrix. Therefore, the transformation of a ductile chars layer when the RDP clays are introduced can explain the mechanism of expanding chars formation, which ultimately lead to an improvement of flame retardant properties, such as MLR and RHR. Moreover, when 3% C20A is added in the blend, the char layers become more plastic deformed, which may indicate that a maximum capacity of expansible chars increases against heat fronts. This can be contributed to a balance of surface energy of between polymers/particles, where the HNTs may have highest surface tension against the polymer blend, Therefore, the mechanical properties of chars layers may be taken into consideration in order to reduce the flammability of polymer blends. 441 In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.


Conclusion We first review recent flame retardant formulations on biodegradable polymer blends, such as either starch/ poly(butylene adipate-co-terephthalate) (PBAT) or poly(lactic acid)/poly(butylene adipate-co-terephthalate) (PLA/PBAT), where interfacial tensions of the biodegradable polymer blends can play an important role on mechanical and flame retardant properties. The improved material properties are also mainly influenced by morphology of the blends. In addition, a novel approach to increase not only compatibility but also flame retardancy of the biodegradable polymer blends is reviewed. In this method, an aryl phosphate flame retardant, resorcinol di(phenyl phosphate) (RDP), can act as a surfactant on either starch or clays. The strong absorption of RDP to the particles leads to segregation of the Ecoflex domains in the PLA matrix. Therefore, the addition of RDP-coated particles could render the biodegradable polymer blends self-extinguishing since rigid shell-like chars are formed on the exposed surface which act as a barrier against the heat front. In addition, the shell also changes the effective viscosity and prevents dripping of polymer melts. We also found that the surface energy f\of the components in the polymer blend can determine the flame retardant characteristics. We found that the surface energy determines which component will segregate to the blend surface when the material is heated. The nature of this component then determines the time to ignition, as well as the mechanical properties of the char which forms during combustion Hence the char properties, as well as the time to ignition can be controlled by judicious choice of the lower surface energy component. Nanidentation measurements of the chars showed that the best results were obtained when the char was ductile and could sustain the large internal pressures from the decomposing gases. Containment of the gases resulted in steady state release, and lower MLR values.

Acknowledgments Support from the NSF MRSEC and CBET divisions is gratefully acknowledged. We would like to thank Prof. Chad Korach for performing the nanodientation measurements and discussion regarding their interpretation. The help of Neil Muir and Ezra Bobo was invaluable in sample preparation in the early part of this work.

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In Fire and Polymers VI: New Advances in Flame Retardant Chemistry and Science; Morgan, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.

A Review of Engineering Biodegradable Polymer Blends - American

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