Environmentally Friendly Polylactic Acid-Based Thermal Insulation

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General Research

Environmentally-friendly Polylactic Acid-based Thermal Insulation Foams Blown with Supercritical CO2 Pengjian Gong, Shuo Zhai, Richard E. Lee, Chongxiang Zhao, Piyapong Buahom, Guangxian Li, and Chul B Park Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05023 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Industrial & Engineering Chemistry Research

Environmentally-friendly Polylactic Acid-based Thermal Insulation Foams Blown with Supercritical CO2 Pengjian Gong1,2*, Shuo Zhai1,2, Richard Lee2, Chongxiang Zhao2, Piyapong Buahom2, Guangxian Li1 and Chul B. Park2* 1. College of Polymer Science and Engineering, Sichuan University, 24 Yihuan Road, Nanyiduan, Chengdu, Sichuan, People’s Republic of China, 610065 2. Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 * Corresponding authors: Chul B. Park. Email address: [email protected]. Tel: +1-416-978-3053; Pengjian Gong. Email address: [email protected]. Tel: +86-181-8075-6171

Table of Contents

ABSTRACT A supercritical CO2 (scCO2) foaming technology was used to develop a PLA foam with a thermal conductivity as low as 30 mW/m-K. The PLA foam’s larger optimal expansion ratio and strong infrared (IR) block ability greatly helped to achieve this outcome. Unlike the PS foams, in which non-biodegradable carbon particles are often added to block the IR thermal radiation, the PLA foams’ intrinsic IR-absorbing characteristic, which acted via the ester group in the PLA molecular chain, further enhanced its environmental impact. Overall, environmentally-friendly PLA foams, made by using the non-toxic scCO2 foaming method, offer a sound alternative to PS foams. 1 / 22 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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KEYWORDS: Polylactic acid, thermal insulation, supercritical CO2 foaming, intrinsic infrared thermal radiation blocking

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1.

INTRODUCTION The development and use of environmentally-friendly thermal insulation

materials represents an approach that addresses our urgent economic and environmental concerns. The use of thermal insulation materials protects our ecosystem and preserves our dwindling energy resources. Specifically, the European Commission started the “Energy Savings 2020” program in 2010 to promote energy waste reduction.1 They found that the heating and cooling systems in industrial and residential buildings consumed more than 60% of the total energy produced in the world.2 As a result, thermal insulation polystyrene (PS) foams are now being used in vast quantities worldwide. Their light weight, good insulation performance, and low production costs add to their energy efficiency. In contrast, the petroleum-based PS foams take a long time to decompose, cause an enormous amount of waste, and intensely threaten the Earth’s ecosystem.3 In addition, the residual styrene in the PS matrix is carcinogenic, and thus threatening human health.4 To solve the PS waste problem, which is the primary source of urban litter, numerous cities have restricted or even banned the use of PS foams for food packaging. Therefore, an environmentally-friendly replacement, which is biodegradable, and which is made from sustainable resources, is urgently needed to help solve the related health and environmental challenges. Furthermore, when PS foams have a large expansion ratio, the infrared (IR) thermal radiation that passes through them has to be blocked to achieve a low thermal conductivity.5 This is because the heat transfer that occurs in PS foams includes heat that is conducted from the gas phase and the polymer matrix, and also heat from the IR radiation.6 Due to the PS’s weak IR-absorbing capacity, carbonaceous particles with strong IR-absorbing capability, such as graphite and carbon black, are frequently added to reduce the radiative conductivity.7, 8 Furthermore, the black color resulted from carbonaceous particles greatly limited the application of PS foams, especially as commodities. Therefore, environmentally-friendly foams with intrinsically strong IR-absorbing characteristic are preferred because they do not need to use any 3 / 22 ACS Paragon Plus Environment


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non-biodegradable carbonaceous particles. Polylactic acid (PLA) is a biodegradable and compostable polymer derived from renewable resources such as cornstarch and sugarcane. As a potential replacement for petroleum-based and non-biodegradable polymers, PLA has attracted wide industrial and academic interest.9, 10 From the IR spectrum of PLA, it is noted that PLA would have strong IR absorption capability,11 and this would greatly aid in reducing the foams’ transmitted IR thermal energy. The fundamental characteristic of intrinsic strong IR absorption can enhance the foams’ thermal insulation performance, without compromising their environmentally-friendly nature. Consequently, when the PLA is foamed to form a porous structure with a high void fraction, it would be possible to achieve light weight and thermal insulation characteristics12, 13 without the use of any carbonaceous particles. Supercritical CO2 (scCO2) foaming14, 15 is a preferential way to ensure the green PLA foams not contaminated at the processing step.16 Although various blowing agents have been developed to produce polymeric foams, CO2 is an inexpensive environmentally-friendly physical blowing agent.17 This is because CO2 is nontoxic, nonflammable, environmentally benign, and abundant in the air.18 Furthermore, CO2 has a moderate supercritical point, which is 73.8 bar and 31.1oC.19 Beyond that point, CO2 is in a supercritical condition, indicating a large fluid density similar to a liquid, which gives it a fast diffusion rate that resembles a gas. These characteristics ensure the efficient and environmentally-friendly production of PLA foams with a satisfied cellular structure.20, 21 To successfully apply scCO2 in the produce of PLA foams, especially microcellular PLA foams with a large expansion ratio, a challenge caused by the PLA’s low-melt strength has to be overcome.22 Crystallization control is an effective way to significantly manage PLA’s foaming ability.23-25 For example, the strength of the PLA matrix’s semi-crystalline structure will be increased by the rigid crystalline phase, and this will help to restrict the PLA foam’s cell coalescence.26, 27 However, too high crystallinity will result in less gas dissolution in the PLA matrix. In the end, cell expansion will be decreased and cell nucleation may be suppressed. 4 / 22 ACS Paragon Plus Environment

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Therefore, an optimal crystallization degree during processing is needed to ensure the PLA foam has both a high expansion ratio and a large cell density.28, 29 Compared with the commodity PS foam products blown with flammable hydrocarbons, such as butane and pentane, a much greener process that uses scCO2 to prepare PLA foams is a great advantage. Hence, industry has been showing an increasing interest in scCO2 foaming of PLA foams. In our study, we prepared environmentally-friendly PLA foam product through the non-toxic scCO2 foaming route. Because of the intrinsic strong IR-absorption characteristic of PLA, the prepared PLA foam with a larger optimal expansion ratio had a low radiative conductivity. Accordingly, the thermal conductivity of the PLA foam reached as low as 30 mW/m-K, which showed an excellent thermal insulation performance similar to that of commercial PS foam products. Due to environmental and health concerns, PS foams have been restricted or partially banned worldwide in many cities. However, the thermally insulating PLA foam derived from natural resources and processed by scCO2 foaming is a purely green product. It is capable of biodegradation and does not contain any non-biodegradable carbonaceous additives. Therefore, PLA foam would be an excellent candidate to replace PS foam for use in green thermal insulation applications, where energy resources should be preserved with reduced CO2 emission.

2.

EXPERIMENTAL

2.1 Materials The commercially available polylactic acid, PLA IngeoTM biopolymer 8052D with a D-lactide molar content of 4.5 mol% (melt flow rate = 14 g/10 min at 210°C/2.16 kg, density = 1.24 g/cm3) was purchased from NatureWorks LLC. Joncryl ADR-4368C, an epoxy-based multi-functional oligomeric chain extender, was purchased from BASF Inc. The detailed information about the molecular structure and chain extension mechanism of Joncryl ADR-4368C can be found in the reference.30 The carbon dioxide (Linde Gas, purity over 99%) was applied as a physical blowing 5 / 22 ACS Paragon Plus Environment


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agent.

2.2 Preparation of the PLA matrix Reactive melt blending is an efficient method to prepare long-chain branched PLA product. The masterbatch of PLA/chain extender blends (30 wt%) was supplied by Polyvel Inc. The PLA matrix with a 0.7 wt% chain extender was then produced by mixing the masterbatch with the pristine PLA material in a DSM twin-screw compounder (Berstorff ZE25, screw length 48D, chamber volume 15 ml) at 210°C and 100 rpm for 5 min. Both the pristine PLA material and the PLA/chain extender masterbatch had been pre-dried at 60oC for at least 10 h to reduce the moisture in the pellets, which was a major cause of viscosity degradation in the subsequent reactive blending. The prepared PLA matrix was to be treated by the following scCO2 foaming. The thermal conductivity of the prepared PLA matrix was measured to be 230 mW/m-K, based on the transient plane source (TPS) measurement.

2.3 Supercritical CO2 foaming PLA rod sample with a diameter of ~ 5 mm and a length of 30 mm was first placed in an autoclave to dissolve the CO2. Before the temperature was increased, the autoclave was purged by CO2 to remove any moisture. Then, the autoclave was heated up to 200oC and held for 10 min with high-pressure CO2 present to melt all crystals and to remove their thermal history. Then, the temperatures were reduced to 110 oC, 120 oC, or 130oC within 30 min, and held for 0 or 30 min to induce isothermal crystallization. The saturation pressures were 69 or 138 bar. After the saturation, foaming was induced through the rapid depressurization at the isothermal temperature in 2 s. Before characterization, all of the foam samples were first vacuumed for one week, and then left for over one month to ensure that the complete removal of CO2.

2.4 Characterization Scanning Electron Microscopy (SEM) was applied to observe the foam 6 / 22 ACS Paragon Plus Environment

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morphology. Cryogenic fractural surfaces were obtained by freezing the samples in liquid nitrogen. The fractural surfaces were then coated with gold for 180 s to enhance their electrical conductivity, and observed under the SEM (JEOL JSM-606) with the acceleration voltage of 15 kV and the wide distance of 30 mm. Image-J was the image processing software applied to calculate the average cell diameter, Φc, and the cell density, with respect to the solid polymer, Nf. The strut fraction, fs, for polymeric foams produced by scCO2 foaming is normally 0.2.5 The bulk densities of the samples before and after foaming were measured using the water-displacement method (the ASTM D792-00). The cell density, with respect to the unfoamed bulk polymer, was then calculated using Equation 1 as follows: 1.5

ρ n  N f = s  cell  ρf  A 

(1)

where ncell is the number of bubbles in a total area, A. ρf and ρs are the densities of the foam and the unfoamed bulk, respectively. The measured solid density of the prepared PLA matrix is 1.24 g/cm3. Here,

ρs

ρ f is also known as the foam expansion ratio

and the void fraction (ԑVF) is then expressed as (1 −

ρf

ρs ).

To simulate the isothermal melt crystallization behavior of the prepared PLA matrix with high-pressure CO2, thermal analysis using high-pressure differential scanning calorimetry (HP-DSC, NETZSCH DSC 204 HP, Germany) was done at atmospheric pressure and at two CO2 pressures of 30 bar and 45 bar. The calibration was carried out by measuring the heat of fusion and the melting points for In, Bi, Sn, Pb, and Zn in an ambient atmosphere and under high-pressure CO2. In the isothermal analysis, the samples were first heated from room temperature to 200oC at the rate of 20oC/min, and were then equilibrated for 10 min to remove their thermal history. After that, the samples were cooled to a certain isothermal temperature at the rate of 20oC/min, and were then equilibrated until a full crystallization reached. To analyze the isothermal crystallization kinetics, the Avrami equation was 7 / 22 ACS Paragon Plus Environment


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applied as follows:31

ln  − ln (1 − Χ ( t ) )  = n ln t + ln k

(2)

where X(t) is the relative crystallinity at crystallization time t, n is the Avrami exponent related to the corresponding mechanisms of crystal nucleation and growth, and k is the crystallization kinetic constant for the crystal nucleation and growth rates. By linear regressing the experimental data, the obtained slope is n and the intercept is lnk. The crystallization half time (t1/2) is defined as the duration from the onset of the crystallization up to 50% of the full crystallization. The reciprocal of t1/2 is then considered as the crystallization rate (G). These can be calculated as follows:32 1n

 ln 2  t1 2 =    k 

G=

(3)

1

(4)

t1 2

Fourier transform infrared spectroscopy (FTIR, PerkinElmer spectrum one) was used to measure the spectral properties of the PLA foams and films. The airborne H2O and CO2 background noise was registered before the samples’ IR transmittance was recorded. The foams were first cut into plate shapes with a diameter of 15 mm and a thickness ranging from 0.2 to 2 mm. The spectral data of eight scans for each foam sample in a spectral wavelength range from 2.5 to 25 µm (4,000 to 400 cm-1) was then collected. At least six plate samples with different thicknesses were measured to calculate the foam samples’ radiative conductivity. As for PLA’s complex refractive index, both the reflectance (Ref) and transmittance (τ) of the film with a 45 µm thickness (lfilm) were recorded. The imaginary part of the complex refractive index (Img), corresponding to the IR absorption index of PLA, is calculated as follows:33

Img = − ln τ film λ 4π l film

(5)

The film internal transmittance (τfilm) can be expressed as follows:33 

τ film = 

2 Ref ( 2 − Ref

)

 C − C 2 − 4 Ref ( 2 − Ref 

)

1 − 1 τ 

(6) 8 / 22

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2 2 where C is calculated as C = 1 + 2Ref + τ − Ref to simplify the equations.

The transient plane source (TPS) hot disk thermal constants analyzer (Therm Test Inc., TPS 2500, Sweden) was applied to measure the thermal conductivity of the PLA’s solids and foams. To mimic the situation inside the sample, the sensor was placed between two pieces of the sample. In our study, the measurement of thermal conductivity was performed using two sample pieces at room temperature. Each sample was measured three times, and the experimental deviation was less than 5%. Details of the TPS method can be found in the references 34 and 35.

3.

RESULTS AND DISCUSSION

3.1 Thermal and radiative properties of the PLA matrix Crystallization greatly affected the polymer matrix’s foaming behavior. This was especially so in a polymer matrix with a low melt strength. Because of PLA’s low melt strength, cell coalescence would reduce the PLA foam’s expansion ratio, increase the foam’s solid thermal conductivity, and deteriorate the foam’s thermal insulation performance. Meanwhile, due to the strong interaction between the CO2 and the PLA, the PLA’s crystallization behavior was also significantly affected by the existence of the high-pressure CO2.22,

25, 36, 37

Therefore, it was necessary to study the PLA’s

crystallization kinetics under high-pressure CO2. In our study, we used HP-DSC to investigate the effect of different isothermal temperatures (Tiso) on the PLA’s melt crystallization at atmospheric pressures (Patm) and at a high CO2 pressure (PCO2). Figure 1 shows the isothermal crystallization curves.

Figure 1 Melt crystallization of the PLA matrix at various isothermal temperatures 9 / 22 ACS Paragon Plus Environment


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and atmospheric pressures (a) or high pressure CO2 (b, c)

Figure 2 Avrami double-log plots at various isothermal temperatures and atmospheric pressures (a) or with high pressure CO2 (b, c)

We applied the Avrami equation to study the kinetics of crystallization based on the relative crystallinity obtained from the DSC curves. Figure 2 shows the corresponding plots according to the Avrami equation. Table 1 lists the Avrami parameters derived from the double-log plots, n and lnk, for each isothermal condition, together with the crystallization half-time, t1/2, and the crystallization rate, G. The mechanism of crystal nucleation and growth can be deduced from the Avrami exponent parameter, n. At atmospheric pressure, the value of n was close to 3, indicating the crystal nucleation and growth was three-dimensional and heterogeneous. At a high CO2 pressure, the n value decreased to near 2, suggesting the crystal growth was two-dimensional, homogeneous and spherulitic.32,

38

The existence of CO2

plasticizes the PLA molecular chains, promotes crystal nucleation, and enhances chain mobility in the beginning.25,

36, 37

However, as the crystals grow, the mobility of

molecular chains would be restricted in the later stage of crystallization.25,

36, 37

Therefore, the minimum crystallization half time decreased at a higher CO2 pressure due to the larger number of crystals.

Table 1 The crystallization half-time (t1/2), the crystallization rate (G) and the parameters based on the Avrami equation (n and lnk) at different combination of isothermal temperatures and pressure conditions. 10 / 22 ACS Paragon Plus Environment

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The radiative conductivity of polymeric foams with a high expansion ratio is strongly related to the material’s intrinsic IR absorption efficiency.5 Based on the spectral data obtained from the FTIR, the PLA’s IR absorption efficiency can be calculated by Equation 5. Figure 3 summarizes the result for the PLA, together with the PS’s absorption efficiency and spectral radiant emittance of thermal radiation.39, 40 According to Planck’s law, the radiative energy is a function of wavelength, and over 90% of the emitted radiative energy is located in the mid-infrared wavelength region of 2.5-25 µm at room temperature.41 It is noted from Figure 3 that the PLA’s ester group contributed greatly to the IR absorption in the wavelength range of 5 to 10 µm. For example, the carbonyl -C=O stretch and bend deformations were attributed to the IR absorption peaks at 5.7 µm and 8.2 µm.42 The -C-O- stretch was then attributed to a strong absorption area from 8 µm to 10 µm, covering the maximum point of the spectral radiant emittance curve at a wavelength of 9.5 µm.40, 43 In the PS, its aromatic =C-H out-of-plane deformation vibrations were attributed to the IR absorption peaks 11 / 22 ACS Paragon Plus Environment


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at 13.2 µm and 14.3 µm, and the out-of-plane ring deformation vibrations were attributed to the IR absorption peak at 18.5 µm.44 Although the PS’s functional group had a certain degree of IR absorption, the PS’s absorption efficiency was very weak in the wavelength ranges of 5 ~ 12 µm (over 50% of the emitted radiant energy was in this area). Therefore, the PLA showed a greater capacity for IR absorption than the PS.

Figure 3 Spectral radiant emittance of thermal radiation and IR absorption index of PLA and PS

3.2 Cellular structure of PLA foams The PLA foams in this study were prepared by scCO2 foaming. Figure 4 summarizes the cell size, the cell density, and the expansion ratio for these foam samples. Applying different foaming conditions enabled us to prepare PLA foams with various expansion ratios ranging from 18- to 39-fold and various cell densities from 1×107 to 2×1010 #/cm3. Figure 4b also shows the SEM micrographs of the four typical PLA foams. The remaining four PLA foam samples were not shown in Figure 4b because they had a similar cell size. Thus, they had similar SEM micrographs. When the expansion ratio is over 15-fold for microcellular polymeric foams, the cell wall thickness becomes optically thin,45 and conductive conduction is significantly 12 / 22 ACS Paragon Plus Environment

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decreased. Hence, radiation starts to affect the total thermal conductivity. All of the PLA foams in this study expanded over 17-fold and the corresponding cell wall thickness was around 1 µm. The thin film (cell wall) interference induced a large amount of transmitted IR radiation.45 Thus, the radiative heat-transfer behavior through these foams should be investigated.

Figure 4 (a) Cell size and cell density as a function of expansion ratio and (b) SEM micrographs of PLA foams

3.3 Conduction through PLA foams Based on Equations S2 and S3 in the supporting information, both the gas and solid conductivities were related to the void fraction, or to the expansion ratio. Figure 5 summarizes the PLA foams’ calculated conduction conductivities of the solid phase, the gas phase, and the solid and gas phases. As is shown in Figure 5a, the solid conductivity decreased with an increased expansion ratio. In contrast, the gas conductivity showed an opposite trend with the expansion ratio (Figure 5b). But eventually, the “solid+gas” conductivity decreased with the expansion ratio (Figure 5c). Because the unfoamed PLA had a much larger thermal conductivity of 230 mW/m-K (k0solid) than that of the air at 26 mW/m-K (k0gas), the increasing expansion ratio from 18- to 39-fold greatly reduced the foam’s solid conductivity by 4.2 mW/m-K. But this slightly increased the foam’s gas conductivity by 0.7 mW/m-K. Therefore, increasing the expansion ratio effectively reduced the heat transfer via 13 / 22 ACS Paragon Plus Environment


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conduction.

Figure 5 (a) The solid conductivities, (b) the gas conductivities, and (c) the “solid+gas” (conduction) conductivities of PLA foams as a function of the expansion ratio

3.4 Radiation through PLA foams The FTIR spectrometry effectively measures radiation by recording the IR-transmitted energy through the foams. The higher the transmittance, the more radiative conductivity there will be. Figure 6a shows the spectral transmittance of PLA foam for sample thicknesses of 0.17, 0.278, 0.585, 1.222, and 1.368 mm, respectively. And with an increase in the sample thickness, the IR transmittance decreased. This is because a thicker sample has a larger amount of the PLA matrix in which to absorb more radiative energy before it reaches the FTIR’s detector. The FTIR data obtained from the foam samples with different thicknesses were then used to calculate the spectral extinction coefficient, Ke,λ, based on Equation S4 in the supporting formation. Figure 6b shows the Ke,λ value of the PLA foam, together with the spectral radiant emittance’s curve. For comparison, the Ke,λ value of the PS foam is also included in Figure 6b. The higher the Ke,λ value (that is, the spectral extinction coefficient) was, the higher the attenuating IR radiation was. In this case, the PLA foam had a much higher Ke,λ in the wavelength range, where the spectral radiant emittance peaked. The Rosseland extinction coefficient (Ke,R) then became an averaged value of Ke,λ considering the energy distribution of the spectral radiant emittance. Consequently, the radiative conductivity (krad) of the foams can be quantitatively calculated from Ke,R, based on Equation S8 in the supporting formation. 14 / 22 ACS Paragon Plus Environment

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Figure 6 (a) FTIR spectral transmittance of PLA foam with various sample thicknesses, and (b) the PLA foam’s spectral extinction coefficient value in this study and PS foam from Reference 5

Figure 7 summarizes the PLA foams’ radiative conductivities as a function of the expansion ratio. It can be seen that the PLA foams’ radiative conductivity increased with an increased expansion ratio. The PLA foam with a larger expansion ratio had thinner cell walls which were less capable to block the radiative energy despite the larger number of cell walls.45 For comparison, Figure 7 also shows the radiative conductivities, conduction conductivities, and total thermal conductivities of the PS and PS/carbon nanotube (CNT) foams reported from reference 39. At a large expansion ratio, the total thermal conductivity of PS foams was greatly affected by the radiative conductivities. Therefore, adding the CNTs with strong IR absorption effectively decreased the radiative conductivity and correspondingly decreased the total thermal conductivity, in spite of the PS matrix’s weak IR absorption efficiency. In the PLA foams without adding the CNTs, the intrinsic IR-absorbing characteristic on the other hand effectively blocked the IR radiation and, correspondingly, decreased the radiative conductivity. It is noteworthy that the PLA foams without any non-biodegradable IR-absorbing additives had relatively low radiative conductivities, and were comparable to the PS/CNT 0.5~1.0 wt% foams.



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Figure 7 Radiative conductivity (Rad) of PLA foam from this study and conduction conductivity (Con), radiative conductivity (Rad), and total thermal conductivity (Total) of PS foam from Reference 39

3.5 Conduction + Radiation through PLA foams Figure 8a summarizes the “gas+solid” conduction conductivity, the radiative conductivity, and the total thermal conductivity (from both experimental and calculated perspectives). The total thermal conductivity calculated is the sum of Equations S2, S3, and S8 in the supporting information. Although the radiative conductivity increased with the increasing expansion ratio, the greatly reduced conduction conductivity decreased the total thermal conductivity. Thus, a minimum value of 30.1 mW/m-K was achieved at a higher expansion ratio of 38.9-fold. Figure 8b shows each heat transfer term’s contribution in the PLA foams. With an increased expansion ratio, the radiative contribution increased to 10.3%. This happened because a smaller amount of the PLA’s matrix absorbed the IR thermal radiation in the foam. And the smaller amount of the PLA matrix, which was due to the foam’s larger expansion ratio, also decreased the solid conductivity contribution to 11%. Subsequently, the gas conductivity contribution increased to 78.7%, with most of the 16 / 22 ACS Paragon Plus Environment

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heat transfer occurring through the PLA foams.

Figure 8 (a) The “gas+solid” conduction conductivity, the radiative conductivity and the total thermal conductivity (both by experiment and calculation); (b) percentage contribution of each heat transfer term

The thermal insulation performance of the obtained PLA foams was also compared with that of the PS foams reported in the references.7, 39,

46-49

Figure 9

summarizes their total thermal conductivities. The PLA foams prepared by Ameli et al. using foam injection molding had total thermal conductivities of over 60 mW/m-K.49 This was because the heat transfer in polymeric foams with an expansion ratio of less than 5-fold is dominated by solid conduction. Therefore, in such a high-density PLA foam system, an increase in the expansion ratio will significantly decrease the total thermal conductivity. To decrease the foam’s thermal conductivity to below 40 mW/m-K, its expansion ratio has to be at least 10-fold. However, in addition to increasing the expansion ratio further, the thermal conductivity attributed to the IR radiation hinders the decrease in total thermal conductivity. This is especially so for foam made from weak IR-absorbing polymers, such as PS.39 Therefore, adding carbonaceous materials, like carbon nanotubes (CNTs) and graphites, to the PS matrix to effectively absorb the IR thermal radiation is important to reduce the PS foam’s total thermal conductivity to 30 mW/m-K. With the PLA foams, the intrinsic ester group in the PLA molecular chain had strong IR absorption efficiencies. Thus, it 17 / 22 ACS Paragon Plus Environment


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effectively blocked IR radiation through the PLA foams, even with large expansion ratios. Therefore, PLA foam without any non-degradable additives successfully achieved a total thermal conductivity of 30 mW/m-K at an expansion ratio of 38.9-fold.

The

IR-absorbing

function

is

of

great

importance

in

environmentally-friendly foams, where the application of thermal insulation preserves energy resources. The results indicate that the total thermal conductivity of PLA foam may decrease further with a higher expansion ratio, but it is practically very difficult to make a low-density foam with an expansion ratio over 40.50, 51 Figure 9 also shows the total thermal conductivity of commercially available EPS foam products.48 We especially note that the thermal insulation performance of our PLA foam was comparable with the EPS foams.

Figure 9 Total thermal conductivity of PLA and PS foams

4.

CONCLUSION In this work, we prepared the PLA foams using environmentally-friendly CO2 as

the physical blowing agent. All of the obtained PLA foams had an expansion ratio of 18 / 22 ACS Paragon Plus Environment

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over 18-fold and, therefore, their corresponding solid conductivities were all less than 8 mW/m-K. Among them, one PLA foam had a large expansion ratio of 38.9-fold and hence a low thermal conductivity of 30 mW/m-K. These values were comparable to commercial PS foam products which were lightweight and thermally insulated. This indicates that PLA foam is a good candidate to replace PS foam as a thermal insulation material. Furthermore, the PLA matrix had an intrinsic IR-absorbing ester group. Without adding any non-biodegradable carbon particles, the PLA foam itself had strong IR absorption. Thus, it had a low radiative conductivity, even at a large expansion ratio. Consequently, from the perspective of environmentally-friendly technologies, biodegradable PLA foams prepared via a greener scCO2 foaming method are excellent replacements for PS foams in the green application of thermal insulation.

Supporting Information IR transmittance and reflectance of PLA film, IR transmittance of PLA foams, SEM micrographs of PLA foams, and the equations for thermal conductivities

Acknowledgements The authors are grateful to the National Natural Science Foundation for the Youth of China (No. 51703146) for their financial support of this project. We are also thankful to the Programme of Introducing Talents of Discipline to Universities (B13040) and the Fundamental Research Funds for the central Universities.

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