Characterization of Different Chemical Blowing Agents and Their Applicability to Produce Poly(Lactic Acid) Foams by Extrusion

applied sciences

Article Characterization of Different Chemical Blowing Agents and Their Applicability to Produce Poly(Lactic Acid) Foams by Extrusion

Ákos Kmetty 1,2,* , Katalin Litauszki 1 and Dániel Réti 1 1 Department of Polymer Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics, M˝uegyetemrkp. 3., H-1111 Budapest, Hungary; [email protected] (K.L.); [email protected] (D.R.) 2 MTA–BME Research Group for Composite Science and Technology, M˝uegyetemrkp. 3., H-1111 Budapest, Hungary * Correspondence: [email protected]; Tel.: +36-1-463-2004

Received: 25 September 2018; Accepted: 11 October 2018; Published: 17 October 2018

Abstract: This study presents the applicability of different types (exothermic and endothermic) of chemical blowing agents (CBAs) in the case of poly(lactic acid) (PLA). The amount of foaming agent is a ﬁxed 2 wt%. We used a twin-screw extruder and added the individual components in the form of dry mixture through the hopper of the extruder. We characterized the PLA matrix and the chemical blowing agents with different testing methods. In case of the produced foams we carried out morphological and mechanical tests and used scanning electron microscopy to examine cell structure. We showed that PLA can be successfully foamed with the use of chemical blowing agents. The best results were achieved with an exothermic CBA and with PLA type 8052D. The cell population density of PLA foams produced this way was 4.82 × 105 cells/cm3, their expansion was 2.36, their density 0.53 g/cm3 and their void fraction was 57.61%.

Keywords: chemical blowing agent; poly(lactic acid) foam; foam extrusion

1. Introduction Nowadays the number and amount of polymers produced from renewable resources are increasing considerably. These polymers are also readily available [1]. Global market forecasts predict a fourfold increase until 2019 (7.8 million ton/year) [2]. Polymer foams nowadays are mostly produced from petroleum-based materials. Foamed products, made by physical [3], chemical [4] and bead foaming techniques [5], in addition there are innovative examples of metallic foams as well [6]. Although physical foaming technique produce a lower foam density (density reduction up to 80%), chemical foaming agents can be used without modifying the extruder [7]. Foaming can be divided into four sections. In the initial phase, the gas enters the polymer matrix and the two phases mix. In the next section, the matrix material is saturated with the foaming gas at an elevated temperature and pressure. In the third section, the homogeneous system expands as the pressure decreases. In the ﬁnal section, the structure stabilizes [8]. An important foaming method is chemical foaming, when the foaming agent requires certain conditions to decompose and in the process a gas phase is generated as a result of a chemical change. It is important to choose the decomposition temperature range of the foaming agent to match the processing temperature of the matrix polymer. Chemical blowing agents can be used in different temperature ranges. Their decomposition products are various gases. The most important is the gas that is produced in the largest quantity [9]. Foaming agents can be endothermic and exothermic. The most often used exothermic foaming agent is azodicarbonamide (ADCA), even though ADCA decomposes at a relatively high temperature, at 230 ◦C. When 5% 4,40-oxybis

Appl. Sci. 2018, 8, 1960; doi:10.3390/app8101960 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1960 2 of 17

(benzenesulfonylhydrazide) (OBSH) is added, it reduces the initial temperature of decomposition to 205 ◦C. However, this temperature is still higher than the processing temperature of some very common polymers (e.g., P-PVC, U-PVC, EVA and PLA). With the use of activators (zinc stearate, zinc oxide, naphthenate, urea or benozate), the decomposition temperature range can be reduced to as low as 40 ◦C. The decomposition process typically yields 220 cm3/g gas. Its decomposition products are nitrogen (N2) 65%, carbon monoxide (CO) 24%, carbon dioxide (CO2) 5% and ammonia (NH3) 5% [10,11]. It can produce larger cells, greatly reduce density from 1.24 g/cm3 to 0.599 g/cm3 (in case of neat, extruded PLA) but the yellowish colour of the foaming agent affects the colour of the product [12]. The most often used endothermic foaming agent is a mixture of sodium bicarbonate and citric acid. It is typically whitish, can produce finer cells and less reduction in density from 1.24 g/cm3 to 0.645 g/cm3 in case of neat, extruded PLA [12–16]. Sodium bicarbonate starts to decompose at around 160 ◦C, while citric acid only starts decomposing at around 210 ◦C. About 120 cm3/g of gas is produced. Foaming agents working in different temperature ranges can be produced by mixing the two components at different ratios [17,18]. Such CBAs typically have water among their decomposition products. If the polymer matrix is sensitive to moisture (e.g., polycarbonates or polyesters), a CBA should be chosen which does not have water among its decomposition products (or only a very small amount) because the water can hydrolyse the polymer [9]. A great disadvantage of petroleum-based polymeric materials and products made from them is that they are difficult and costly to recycle, cannot be decomposed biologically and are a considerable load on the environment after they lose their function. Renewable resource-based and biodegradable polymers offer an environmentally friendly alternative. Of all biopolymers, poly(lactic acid) (PLA) receives the most attention nowadays [19]. PLA can be foamed in many ways (extrusion foaming, foam injection moulding, bead foaming) but currently the application of supercritical fluid state carbon dioxide receives the most attention [20,21]. Foaming this biopolymer with chemical blowing agents is not fully researched yet. Commercially available CBAs are mostly designed for the foaming of petroleum-based polymers and their applicability with biodegradable polymers needs more research [22]. Our goal was to analyse a wide range of commercially widely available conventional exothermic and endothermic foaming agents so we selected two endothermic, one exothermic CBAs recommended for polyesters and a new endothermic CBA specially designed for the foaming of PLA. Such a CBA, especially recommended for PLA, has never been examined in the literature. We characterized these CBAs and investigated their applicability for PLA in detail, using continuous extrusion as manufacturing technology.

2. Materials and Methods

2.1. Material

We used poly(lactic acid) type 2003D (4.3 mol% D-lactide content) and 8052D (4.5 mol% D-lactide content), (Ingeo™ Biopolymer, NatureWorks© LLC, Minnetonka, MN, USA). The PLAs have a melting temperature of 150.9 ◦C and of 153.3 ◦C (determined by DSC from the ﬁrst melting curve, 5 ◦C/min), a Melt Flow Index of 2 g/10 min and 7 g/10 min (CEAST 7027.000, 2.16 kg, 190 ◦C), respectively, as measured by the authors. The number average molecular weight was 100,422 Da, the weight average molecular weight was 180,477 Da, the polydispersity index was 1.79, the number average molecular weight was 85,562 Da, the weight average molecular weight 153,235 Da and the polydispersity index was 1.79 (Figure1, determined by gel permeation chromatography (GPC) measurements). Appl. Sci. 2018, 8, 1960 3 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 17

1.4 2003D 1.2 8052D

1.0

0.8

0.6 dw/dlogM

0.4

0.2

0.0 1 100 10000 1000000 Molecular weight [Da]

FigureFigure 1. 1.The The molecular molecular weight weight distribution distributi ofon differentof different poly(lactic poly(lactic acid) ac (PLA)id) (PLA) granules granules based based on a gelon a permeationgel permeation chromatography chromatography (GPC) (GPC) test. test.

We used exothermic and endothermic CBAs in 2 wt% for the extrusion foaming of PLA. Table1 We used exothermic and endothermic CBAs in 2 wt% for the extrusion foaming of PLA. Table 1 contains the exact types and their speciﬁcations as found on their datasheets. contains the exact types and their specifications as found on their datasheets.

Table 1. Chemical blowing agents (CBAs) used and their properties. Table 1. Chemical blowing agents (CBAs) used and their properties. Effective CBACBA Trade Trade Name Type Type of of CBA Blowing Agent * Effective Other InformationOther * Abbreviation Blowing Agent * Gases * Abbreviation Name CBA NGases, CO, * Information * Tracel IM 3170 MS exothermic azodicarbonamide 2 - Tracel3170 Tracel IM CON22,, NH CO,3 exothermic azodicarbonamide water is formed,- Tracel3170 citric acid and 3170Tracel MS IMC 4200 endothermic COCO,H2, NHO 3 it contains a Tracel4200 baking soda 2 2 waternucleating is formed, agent Tracel IMC citric acid and recommended for Hydrocerol CT 3168endothermic endothermic n.d.CO n.d.2, H2O it contains a Hyd_3168Tracel4200 4200 baking soda PLA Luvobatch PE BA 9537 endothermic n.d. n.d.nucleating carrier PE agent Luv_9537 Hydrocerol *—Information from Technical Datasheet, n.d.—no data from therecommended manufacturer. endothermic n.d. n.d. Hyd_3168 CT 3168 for PLA 2.2.Luvobatch Testing Methods endothermic n.d. n.d. carrier PE Luv_9537 PE BA 9537 2.2.1. Gel Permeation Chromatography (GPC) *—Information from Technical Datasheet, n.d.—no data from the manufacturer. The tests were performed on an Agilent PL-GPC 50 System (Santa Clara, CA, USA) GPC/SEC device.2.2. Testing The samples Methods were first dissolved in chloroform (ca. 30 mg/mL), then filtered through a PTFE filter. Four Agilent PL-Gel columns (3 × PL-Gel Mixed C (5 µm) and 1 × PL-Gel Mixed E (3 µm) columns)2.2.1. Gel were Permeation used in series, Chromatography with HPLC grade(GPC) chloroform (amylene-stabilised) as the eluent, at a flow −1 ◦ rate ofThe 1 mL tests min wereat performed 30 C, on a on Waters an Agilent Alliance PL-GPC system 50 equipped System with(Santa an Clara, Alliance CA, 2695 USA) Separation GPC/SEC Module.device. The samples polymer were number-average first dissolved molecular in chlorofo weightrm (ca. (M 30n) mg/mL), and polydispersity then filtered index through (Mw a/M PTFEn; PDI)filter. were Four calibrated Agilent againstPL-Gel lowcolumns dispersity (3 × PL-Gel polystyrene Mixed standards C (5 μm) withand a1 3rd× PL-Gel order polynomialMixed E (3 μ fit,m) linearcolumns) across were molar used mass in ranges.series, with HPLC grade chloroform (amylene-stabilised) as the eluent, at a flow rate of 1 mL min−1 at 30 °C, on a Waters Alliance system equipped with an Alliance 2695 2.2.2. Differential Scanning Calorimetry (DSC) for Chemical Blowing Agents Separation Module. The polymer number-average molecular weight (Mn) and polydispersity index (MwDifferential/Mn; PDI) were scanning calibrated calorimetry against (DSC) low dispersity measurements polystyrene of chemical standards blowing with agents a 3rd wereorder performedpolynomial with fit, a linear Setaram across DSC molar 92 (Caluire, mass ranges. France) device. The measurement temperature range was ◦ ◦ 25–250 C, the heating rate was 10 C/min, the mass of the samples was between 3 mg and 6 mg

Appl. Sci. 2018, 8, 1960 4 of 17 and the tests were performed in nitrogen protective gas (20 mL/min) and with a nitrogen measuring atmosphere (20 mL/min).

2.2.3. Differential Scanning Calorimetry (DSC) for Biopolymer Foams Differential scanning calorimetry (DSC) measurements of biopolymer foams were carried out with a TA Instruments Q2000 (New Castle, DE, USA) automatic sampling device. The measurement temperature range was 0–200 ◦C, the heating rate was 5 ◦C/min, the mass of the samples was between 3 mg and 6 mg and the tests were performed in nitrogen protective gas (20 mL/min) and with a nitrogen measuring atmosphere (20 mL/min). The degree of crystallinity (χc) was calculated according to Equation 1, where ∆Hm is the crystallization enthalpies. The degree of crystallinity (χcf) created via foam processing was calculated according to Equation (2), where ∆Hm is the melt and ∆Hc is the crystallization enthalpy. PLA100% is the theoretical melting enthalpy of 100% crystalline PLA, which is 93 J/g [23]. ∆Hm χc = x 100 [%], (1) PLA100%

∆Hm − |∆Hc| χcf = x 100 [%], (2) PLA100%

2.2.4. Thermogravimetric Analysis-Fourier-Transform Infrared Spectrometry (TGA-FTIR) In the TGA-FTIR test, the two devices were connected so that the gas phases released in the TGA test travel through the sensors of the FTIR device. The gases went through a silicon tube; ﬂow was maintained with nitrogen background gas. The FTIR test can determine the chemical composition of the gas from the TGA test. With this method, the composition of the gas can be determined across the whole temperature range. The TGA-FTIR measurements were performed with a TA Instruments Q5000 (New Castle, DE, USA) automatic sampling device. The measurement temperature range was 40–800 ◦C, the heating rate was 10 ◦C/min, the mass of the samples was between 1 mg and 4 mg and the tests were performed in nitrogen protective gas (25 mL/min) and with nitrogen measuring atmosphere (50 mL/min). The TGA device was connected to a Bruker Tensor 37 FTIR (Billerica, MA, USA) device. Gas was released at different temperatures in the case of the different CBAs and the spectrum was evaluated at the maximum of effective gas generation in each case (in the case of Tracel3170 it was 190 ◦C, in the case of Tracel4200 it was 175 ◦C, in the case of Luv_9537 it was 157 ◦C, while in the case of Hyd_3168 it was 232 ◦C). We took into account the effective gases which are generated below manufacturing temperature.

2.2.5. Thermogravimetric Analysis Performed in Isothermal Conditions TGA measurements were performed with a TA Instruments Q500 (New Castle, DE, USA) automatic sampling device. The measurement temperature was 190 ◦C, the heating rate was 100 ◦C/min, the mass of the samples was between 1 mg and 4 mg and the tests were performed in nitrogen protective gas (40 mL/min) and with industrial air measuring atmosphere (60 mL/min). These conditions are closer to those in an extruder. The isothermal temperature was chosen to be 190 ◦C because this is a usual processing temperature during the extrusion of PLA. The background gas was chosen to be air, because the CBA may get into contact with air during the manufacturing process. The samples reached the isothermal 190 ◦C in 3 min and they were kept at this temperature for another 330 s.

2.2.6. Scanning Electron Microscopy (SEM) Scanning electron micrographs were taken from fracture surfaces with a Jeol JSM-6380LA (Tokyo, Japan) SEM with an acceleration voltage of 10 kV. Prior to the test, the samples were sputter-coated with a gold/palladium alloy. Appl. Sci. 2018, 8, 1960 5 of 17

2.2.7. Foam Characterisation The volume of foam structures was measured with a 10 mL glass cylinder (accuracy 0.1 cm3); the measuring medium was distilled water. Mass was measured with a Sartorius BP121S type balance (Göttingen, Germany). Its range is 120 g, its measuring accuracy is 0.1 mg and its resolution is 0.1 mg. Density was calculated according to Equation (3).

mfoam ρfoam = , (3) Vfoam

Void fraction was calculated according to Equation (4) [24].

ρfoam Vf = 1 − , (4) ρpolymer where Vf [-] is void fraction, ρfoam is the density of the foamed polymer and ρpolymer is the density of unfoamed polymer. Cell population density was calculated based on the SEM images, according to Equation (5), where n is the number of cells counted in the recorded image, A [cm2] is the cross section area of the sample, M [-] is the magniﬁcation factor and ER [-] is the expansion ratio [24].

! 3 n ∗ M2 2 1 Nc = x , (5) A 1 − Vf

The expansion ratio was calculated with Equation (6), where ER is the rate of expansion, ρfoam is the density of the foamed polymer, ρpolymer is the density of unfoamed polymer and Vf is the void fraction [16]. ρpolymer 1 ER = = , (6) ρfoam 1 − Vf

2.2.8. Measuring Foam Strength Foam compressive strength tests were performed with a Zwick Z005 (Ulm, Germany) universal testing machine (in compression mode). A Mess & Regeltechnik KAP-TC (Dresden, Germany) type load cell was used (measuring range 0–5000 N, preload 1 N). The measurement speed applied was 2 mm/min. The specimens were cylindrical, with a diameter of 3 mm and a height of 10 mm (Figure2). The test was continued until a deformation of 10% was reached. Five samples were tested and the average of the results was taken. Foam compressive strength was calculated in accordance with Equation (7). F10% σ10% = , (7) Afoam where compressive strength [MPa] is the compression strength at 10% deformation, F10% [N] is the 2 force at 10% deformation and Afoam [mm ] is the cross-sectional area of the foam specimen. Extrusion foaming was carried out with a Teach-line ZK25T (Collin GmbH, Ebersberg, Germany) twin-screw extruder (screw diameter: 25 mm, L/D = 24). The temperature proﬁles of the ﬁve zones starting from the hopper were 155 ◦C, 160 ◦C, 175 ◦C, 190 ◦C and 190 ◦C. Screw speed was 10 revolutions per minute, the residence time of the polymer was 330 s, pressure drop [MPa] was calculated according to Equation (8), where pmelt [MPa] is the pressure that we measured during extrusion in the die, patm [MPa] is the standard atmospheric pressure, 0,1 MPa. The extrusion die was a rod type with a circular cross section and with a nominal diameter of 3 mm. All foaming agents were added to the PLA granules during extrusion in an amount of 2 wt%, by dry mixing. Prior to manufacturing, the PLA granules were dried at 80 ◦C for 6 h in a WGL-45B type drying oven.

Pressure drop = pmelt − patm (8) Appl. Sci. 2018, 8, 1960 6 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 17

Figure 2. Measurement layout of measuring foam strength. Figure 2. Measurement layout of measuring foam strength. 3. Results of the Investigation of the Foaming Agents Extrusion foaming was carried out with a Teach-line ZK25T (Collin GmbH, Ebersberg, 3.1.Germany) The Thermal twin-screw and Morphological extruder (screw Properties diameter: of CBAs, 25 TGA, mm, TGA-FTIR, L/D = 24). DSCThe temperature profiles of the fiveFirst, zones we starting examined from the the decomposition hopper were 155 temperature °C, 160 °C, ranges 175 °C, of 190 chemical °C and foaming 190 °C. Screw agents speed by TGA. was The10 testrevolutions showed theper massminute, fraction the residence of the CBAs, time the of fractionthe polymer of the was gas producer330 s, pressure useful fromdrop the[MPa] aspect was ofcalculated foaming andaccording what is to the Equation proportion (8), ofwhere carrier p polymer [MPa] and is the other pressure residual that materials. we measured The samples during wereextrusion measured in the with die, two p background [MPa] is the gases, standard so it wasatmospheric possible topressure, compare 0,1 the MPa. processes The extrusion occurring die was a rod type with a circular cross section and with a nominal diameter of 3 mm. All foaming agents in the different media. We used a nitrogen (N2) atmosphere to get measurement results from an inertwere atmosphere, added to the while PLA airgranules was used during because extrusion that isin whatan amount is present of 2 duringwt%, by extrusion; dry mixing. therefore, Prior to themanufacturing, measurement the approximates PLA granules real-life were extrusiondried at 80 better. °C for Figure6 h in 3a showsWGL-45B that type the decompositiondrying oven. of CBAs occurred in several steps. In the case of Luv_9537, mass reduction starts at 136 ◦C, this CBA is Pressure drop = p − p (8) followed by Tracel4200, then Tracel3170. While in the case of the exothermic Tracel3170, mass reduction occurs in one step up to 200 ◦C, the endothermic Tracel4200 and Luv_9537 exhibit mass reduction 3. Results of the Investigation of the Foaming Agents in two steps. Between 150–250 ◦C during the thermal decomposition of CBAs probably effective gases, taking an active part in foaming are released, above this temperature it is likely that the carrier 3.1. The Thermal and Morphological Properties of CBAs, TGA, TGA-FTIR, DSC materials decompose. In the case of Hyd_3168 and Luv_9537, a great reduction of mass can be observed at 300First,◦C and we 444examined◦C, while the indecomposition the case of Tracel3170 temperature and ranges 4200, of an chemical extended foaming gradual agents decrease by TGA. can beThe observed test showed at low the intensity. mass fraction Comparing of the CBAs, the same the fr measurementsaction of the gas in producer N2 and air,useful we from can conclude the aspect thatof foaming gas formation and what occurs is the in proportion the same temperature of carrier polymer range. Inand air, other the curveresidual is continuouslymaterials. The sloping, samples whilewere in measured nitrogen with the decomposition two background ranges gases, separate; so it was the possible degradation to compare of the the carrier processes polymer occurring and the in accompanyingthe different media. mass reduction We used only a nitrogen starts at (N around2) atmosphere 400 ◦C. Tableto get2 containsmeasurement the measurement results from results: an inert theatmosphere, decomposition while temperature air was used ranges because based that on dTGis what and is the present percentage during ratio extrusion; of mass therefore, reduction atthe 190measurement◦C. The data approximates indicate that real-life in the case extrusion of the better. investigated Figure CBAs,3 shows in that nitrogen the decomposition and air atmosphere, of CBAs atoccurred 190 ◦C, massin several reduction steps. was In the nearly case the of same.Luv_9537 There, mass is a reduction greater difference starts at only136 °C, in thethis case CBA of is Tracel3170;followed by in nitrogenTracel4200, mass then reduction Tracel3170. was 7.5%,Whilewhile in the in case air it of was the only exothermic 2.8%. The Tracel3170, greatest mass mass reductionreduction in occurs nitrogen in wasone observedstep up to in 200 the °C, case th ofe Tracel3170endothermic (7.5%), Tracel4200 it is followed and Luv_9537 by Tracel4200 exhibit (6.9%), mass Luv_9537reduction (4.8%) in two and steps. Hyd_3168 Between (0.1%). 150–250 The °C data duri indicateng the that thermal Hyd_3168 decomposition probably cannotof CBAs produce probably a foameffective at 190 gases,◦C, the taking characteristic an active processingpart in foaming temperature are released, of PLA. above All fourthis temperature foaming agents it is produced likely that residualthe carrier mass materials at 800 ◦C, decompose. which is probably In the case a nucleating of Hyd_3168 agent. and Luv_9537, a great reduction of mass can be observed at 300 °C and 444 °C, while in the case of Tracel3170 and 4200, an extended gradual decrease can be observed at low intensity. Comparing the same measurements in N2 and air, we can conclude that gas formation occurs in the same temperature range. In air, the curve is continuously sloping, while in nitrogen the decomposition ranges separate; the degradation of the carrier polymer and the accompanying mass reduction only starts at around 400 °C. Table 2 contains the measurement results: the decomposition temperature ranges based on dTG and the percentage ratio of mass reduction at 190 °C. The data indicate that in the case of the investigated CBAs, in nitrogen and air atmosphere, at 190 °C, mass reduction was nearly the same. There is a greater difference only in the case of Tracel3170; in nitrogen mass reduction was 7.5%, while in air it was only 2.8%. The greatest mass reduction in nitrogen was observed in the case of Tracel3170 (7.5%), it is followed by Tracel4200 (6.9%), Luv_9537 (4.8%) and Hyd_3168 (0.1%). The data indicate that Hyd_3168 probably

Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 17 cannot produce a foam at 190 °C, the characteristic processing temperature of PLA. All four foaming Appl. Sci. 2018, 8, 1960 7 of 17 agents produced residual mass at 800 °C, which is probably a nucleating agent.

(a) (b)

FigureFigure 3. ThermogravimetricThermogravimetric analysis analysis (TGA) (TGA) results results of of foaming foaming agents agents in in nitrogen nitrogen and and air air ( (aa)) mass mass reductionreduction as as a a function function of of temperature temperature (50–800 (50–800 °C),◦C), ( (bb)) the the rate rate of of mass mass reduction reduction as as a a function function of of temperaturetemperature (50–800(50–800 ◦°C).C). TheThebroken broken line line indicates indicates the the nitrogen nitrogen atmosphere, atmosphere, while while the the continuous continuous line lineindicates indicates air. air.

Table 2. The decomposition range of CBAs (dTG ranges relevant from the aspect of PLA extrusion Table 2. The decomposition range of CBAs (dTG ranges relevant from the aspect of PLA extrusion processing) and mass reduction at 190 ◦C. processing) and mass reduction at 190 °C. dTG Range Mass Reduction dTG Range Mass Reduction at SampledTG Range Mass Reduction◦ at 190 °C dTG Range Mass Reduction◦ at 190 °C Sample N2 at 190 CN2 Air 190 C Air N2 N2 Air Air ◦ ◦ [-] [-][°C] [ C][%] [%][°C] [ C] [%][%] Tracel3170Tracel3170 145–239 145–239 7.5 7.5147–212 147–212 2.82.8 145–188,145–188, 147–187,147–187, Tracel4200Tracel4200 6.9 6.9 6.56.5 204–245204–245 196–232196–232 Hyd_3168Hyd_3168 186–237 186–237 0.1 0.1193–237 193–237 0.10.1 136–168,136–168, 137–165,137–165, Luv_9537Luv_9537 4.8 4.8 3.93.9 168–225168–225 174–214174–214

We evaluated the gases released at the maximum intensity of decomposition with a TGA-FTIR We evaluated the gases released at the maximum intensity of decomposition with a TGA-FTIR measurement. The results can be seen in Figure 4. We determined the type of gases generated under measurement. The results can be seen in Figure4. We determined the type of gases generated under the the processing temperature (Table 3). The maximum intensity of gas generation in the case of the processing temperature (Table3). The maximum intensity of gas generation in the case of the different different CBAs: Tracel3170—183 °C, Tracel4200—168 °C, Luv_9537—147 °C, while in the case of CBAs: Tracel3170—183 ◦C, Tracel4200—168 ◦C, Luv_9537—147 ◦C, while in the case of Hyd_3168 there Hyd_3168 there was no observable gas generation intensity maximum under 200 °C, the maximum was no observable gas generation intensity maximum under 200 ◦C, the maximum occurred at 225 ◦C. occurred at 225 °C. Based on the spectrum measured by FTIR, in the case of Tracel3170, peaks Based on the spectrum measured by FTIR, in the case of Tracel3170, peaks indicating carbon dioxide indicating carbon dioxide have the highest intensity but carbon monoxide is also present with lower have the highest intensity but carbon monoxide is also present with lower absorption. Also, a C=N absorption. Also, a C=N bond appears as well, which indicates isocyanate (~2200–2350 cm−1), which bond appears as well, which indicates isocyanate (~2200–2350 cm−1), which is a typical decomposition is a typical decomposition product of azodicarbonamide. In the case of Tracel4200, a strong product of azodicarbonamide. In the case of Tracel4200, a strong absorption carbon dioxide peak can absorption carbon dioxide peak can be observed but no carbon monoxide. In the case of Luv_9537, be observed but no carbon monoxide. In the case of Luv_9537, the absorption wavelength of carbon the absorption wavelength of carbon dioxide can be observed. In the case of Hyd_3168, carbon dioxide can be observed. In the case of Hyd_3168, carbon dioxide appears as an effective gas but with dioxide appears as an effective gas but with a small intensity peak. It is important to note that the a small intensity peak. It is important to note that the type of effective gas also has an effect on the type of effective gas also has an effect on the final foam structure. CO2 has higher solubility (20 wt%) ﬁnal foam structure. CO2 has higher solubility (20 wt%) in the PLA matrix compared to N2 (2 wt%). in the PLA matrix compared to N2 (2 wt%). As a result, CO2 effectively lowers melt pressure during As a result, CO2 effectively lowers melt pressure during extrusion and increases the percentage of extrusion and increases the percentage of crystalline fraction [25]. The reaction range of CBAs was crystalline fraction [25]. The reaction range of CBAs was also analysed with a DSC test. Figure5 below also analysed with a DSC test. Figure 5 below contains the measurement results. contains the measurement results.

Appl. Sci. 2018, 8, 1960 8 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 17

Tracel4200 Luv_ 9537 Tracel3170 Hyd_3168 Absorbance

3800 3400 3000 2600 2200 1800 1400 1000 600 Wavenumber [cm-1]

−1 FigureFigure 4. The 4. The Fourier Fourier transform transform infrared infrared (FTIR) (FTIR) spectrum spectrum of foaming of foaming agents agents (in the (in 600–4000 the 600–4000 cm range).cm−1 range).Table 3. The results of the TGA-FTIR measurement (10 ◦C/min, nitrogen measuring atmosphere).

We compared the absorbanceAbsorbance spectrum of Maximums generated [-] effective gases with the intensity peaks of Foaming Agent Type of Gas Generated the spectra of materials in Wavethe IR Number database [cm of− the1] NationalTemperature Institute of [◦C] Standards and Technology [26].

600–750, 2250–2350, 3600–3750 183 CO2 TableTracel3170 3. The results of the TGA-FTIR2050–2250 measurement (10 °C/min, 183 nitrogen measuring atmosphere). CO 750-1250, 1450–1800, 3200–3500 183 NH Absorbance Maximums [-] 3 Foaming Agent 600–750, 2250–2350, 3600–3750 168Type of COGas Generated Tracel4200 Wave Number [cm−1] Temperature [°C] 2 1600–1700, 3500–3700 168 H2O 600–750, 2250–2350, 3600–3750 183 CO2 2050–2250 147 CO Tracel3170Luv_9537 2050–2250 183 CO 600–750, 2250–2350, 3600–3750 147 CO2 750-1250, 1450–1800, 3200–3500 183 NH3 2050–2250 225 CO Hyd_3168 600–750, 2250–2350, 3600–3750 168 CO2 Appl. Sci.Tracel4200 2018, 8, x FOR PEER600–750, REVIEW 2250–2350, 3600–3750 225 CO2 9 of 17 1600–1700, 3500–3700 168 H2O 2050–2250 147 CO Luv_9537 600–750, 2250–2350,Tracel3170 3600–3750 147 CO2 2050–2250Tracel4200 225 CO Hyd_3168 Hyd_3168 600–750, 2250–2350, 3600–3750 225 CO2 Luv_9537

Figure 5 shows Endo that Tracel3170 has an exothermic conversion peak, while in the case of Tracel4200, Luv_9537 and Hyd_3168, more endothermic reaction occurred. The conversion temperatures measured by DSC can be compared to the temperature ranges measured by TGA and this provides more accurate information concerning the temperature range of the decomposition process. Table 4 contains these data. The initial chemical change temperature of DSC is nearly the same as the beginning of the decomposition range measured by TGA but the end of the decomposition range (accordingHeat flow[W/g] to dTG) falls a little after the end of the changing temperature range registered by DSC. This is because the DSC data describe the changing process of the foaming agent and only the beginning part of this process is the same as TGA. TGA, on the other hand, does not 2 W/g show the energy change before and after the start of decomposition; it only registers the mass reduction effect of the decomposition50 75 100 product 125 released 150 175in the 200gas phase, 225 which 250 continues after the reaction, as well. Temperature [°C] Figure 5. Differential scanning calorimetry (DSC) resultsresults of foaming agents (50–250(50–250°C,◦C, first ﬁrst heating, ◦ 10 °C/min).C/min).

Table 4. Comparison of decomposition processes registered with DSC and TGA tests.

Decomposition Range Decomposition Range State Change ** Sample According to dTG * According to dTG * N2 N2 Air [-] [°C] [°C] [°C] Tracel3170 145–239 147–212 165–200 145–188, 147–187, 150–180, Tracel4200 204–245 196–232 210–225 153–169, Hyd_3168 186–237 193–237 208–229 136–168, 137–165, 136–160, Luv_9537 168–225 174–214 175–200 * TGA measurement, ** DSC measurement.

3.2. Isothermal Thermogravimetric Analysis Foaming agents have to produce effective gas at the processing temperature of PLA. In the previous tests, a constant heating rate was used (10 °C/min) but thermal conditions are different during actual manufacturing. Therefore, as a next step, we investigated decomposition by TGA at a constant temperature, 190 °C. Figure 6a,b show the results of the isothermal TGA test. Figure 5 shows that as a result of constant temperature, decomposition also occurs. For evaluation, however, the results must be compared to the previous results so that any possible differences could be seen. Table 5 contains the numerical data. Comparing the TGA and isothermal TGA measurement data, we can draw conclusions about the behaviour of foaming agents. There is no big difference between the results of the two types of measurement but mass reduction was greater in the case of the isothermal process. The biggest difference was observed in the case of Tracel3170, where isothermal mass reduction was 4.9%, which is 2.1% greater than in the case of a constant heating rate. There is a difference in the case of the other three foaming agents too but it is smaller.

Appl. Sci. 2018, 8, 1960 9 of 17

We compared the absorbance spectrum of generated effective gases with the intensity peaks of the spectra of materials in the IR database of the National Institute of Standards and Technology [26]. Figure5 shows that Tracel3170 has an exothermic conversion peak, while in the case of Tracel4200, Luv_9537 and Hyd_3168, more endothermic reaction occurred. The conversion temperatures measured by DSC can be compared to the temperature ranges measured by TGA and this provides more accurate information concerning the temperature range of the decomposition process. Table4 contains these data. The initial chemical change temperature of DSC is nearly the same as the beginning of the decomposition range measured by TGA but the end of the decomposition range (according to dTG) falls a little after the end of the changing temperature range registered by DSC. This is because the DSC data describe the changing process of the foaming agent and only the beginning part of this process is the same as TGA. TGA, on the other hand, does not show the energy change before and after the start of decomposition; it only registers the mass reduction effect of the decomposition product released in the gas phase, which continues after the reaction, as well.

Table 4. Comparison of decomposition processes registered with DSC and TGA tests.

Decomposition Range Decomposition Range State Change ** Sample According to dTG * According to dTG * N2 N2 Air [-] [◦C] [◦C] [◦C] Tracel3170 145–239 147–212 165–200 145–188, 147–187, 150–180, Tracel4200 204–245 196–232 210–225 153–169, Hyd_3168 186–237 193–237 208–229 136–168, 137–165, 136–160, Luv_9537 168–225 174–214 175–200 * TGA measurement, ** DSC measurement.

3.2. Isothermal Thermogravimetric Analysis Foaming agents have to produce effective gas at the processing temperature of PLA. In the previous tests, a constant heating rate was used (10 ◦C/min) but thermal conditions are different during actual manufacturing. Therefore, as a next step, we investigated decomposition by TGA at a constant temperature, 190 ◦C. Figure6a,b show the results of the isothermal TGA test. Figure5 shows that as a result of constant temperature, decomposition also occurs. For evaluation, however, the results must be compared to the previous results so that any possible differences could be seen. Table5 contains the numerical data. Comparing the TGA and isothermal TGA measurement data, we can draw conclusions about the behaviour of foaming agents. There is no big difference between the results of the two types of measurement but mass reduction was greater in the case of the isothermal process. The biggest difference was observed in the case of Tracel3170, where isothermal mass reduction was 4.9%, which is 2.1% greater than in the case of a constant heating rate. There is a difference in the case of the other three foaming agents too but it is smaller. Appl. Sci. 2018, 8, 1960 10 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 17

(a) (b)

FigureFigure 6.6. TGATGA decompositiondecomposition of foamingfoaming agents atat 190190 ◦°CC inin isothermalisothermal conditionsconditions inin aa measuringmeasuring atmosphereatmosphere ofof airair ((aa)) massmass reductionreduction asas aa functionfunction ofof time,time, ((bb)) dTGdTG asas aa functionfunction ofof timetime (the(the blueblue lineline showsshows thethe momentmoment ofof reachingreaching 190190 ◦°C).C).

Table 5. The decomposition range of CBAs at an isothermal temperature. Table 5. The decomposition range of CBAs at an isothermal temperature. Mass Reduction Mass Reduction Sample Mass Reduction Mass Reduction (10 ◦C/min, 190 ◦C) Air (Isothermal, 190 ◦C, 330 s) Air Sample (10 °C/min, 190 °C) (Isothermal, 190 °C, 330 s) [-] [%]Air [%] Air Tracel3170[-] 2.8[%] 4.9 [%] Tracel4200 6.5 6.8 Tracel3170 2.8 4.9 Luv_9537 3.9 5.4 Tracel4200Hyd_3168 0.16.5 0.9 6.8 Luv_9537 3.9 5.4 4. Chemical FoamingHyd_3168 of Poly(Lactic Acid) 0.1 0.9

Foaming,4. Chemical Processing Foaming of Poly(Lactic Acid)

Foaming,The temperatureProcessing parameters of extrusion foaming were selected based on the typical processing temperature of PLA; technical datasheets recommend processing at 190–200 ◦C. Table6 contains the designationsThe temperature of the manufactured parameters of specimens extrusion foaming and Table were7 contains selected the based manufacturing on the typical parameters. processing Wetemperature experienced of PLA; during technical manufacturing datasheets that recommend in the case processing of 8052_4200 at 190–200 and 2003_4200, °C. Table melt6 contains pressure the decreaseddesignations drastically of the manufactured from the initial specimens 74 to 43 and bar Table and the 7 contains viscosity the of manufacturing the melt changed parameters. considerably We whenexperienced it exited during the die manufacturing and in the case ofthat 8052_Hyd_3168 in the case of and 8052_4200 2003_Hyd_3168, and 2003_4200, we did notmelt see pressure signs of successfuldecreased celldrastically formation from in the materialinitial 74 ﬂowto 43 exiting bar and the the die viscosity – the sample of the continued melt changed to be considerably transparent, onlywhen its it colourexited becamethe die and whiter. in the The case extruded of 8052_Hyd_3168 reference rod and is basically 2003_Hyd_3168, smooth and we transparent. did not see Figuresigns of7 showssuccessful that cell in the formation case of 8052_4200, in the material 2003_4200 flow exitin and 8052_3170,g the die – 2003_3170,the sample foaming continued produced to be transparent, a different surfaceonly its oncolour the became specimens. whiter. Tracel4200 The extruded produced refere ance surface rod is similar basically to PLA smooth but and with transparent. a little whitening Figure and7 shows the cells that are in visible.the case Tracel3170 of 8052_4200, produced 2003_4200 a coarser and surface 8052_3170, and the2003_3170, cells can befoaming felt on produced the surface. a Althoughdifferent surface Tracel3170 on the is an specimens. AZO type Tracel4200 foaming agent, produced 8052_3170, a surface 2003_3170 similar to show PLA only but awith very a small little yellowishwhitening tint.and the Foaming cells are agents visible. that Tracel3170 can produce produced effective a coarser gases surface at 190 ◦andC reduced the cells melt can be viscosity felt on duringthe surface. extrusion. Although Tracel3170 is an AZO type foaming agent, 8052_3170, 2003_3170 show only a very small yellowish tint. Foaming agents that can produce effective gases at 190 °C reduced melt viscosity during extrusion.

Appl. Sci. 2018, 8, 1960 11 of 17

Table 6. The designation of the samples and their recipes.

Sample Code PLA Used CBA Used CBA Dosing Note [-] [Type] [Type] [wt%] [-] 2003_ref 2003D - 0 extruded once 8052_ref 8052D - 0 extruded once 2003_3170 2003D Tracel3170 2 - 8052_3170 8052D Tracel3170 2 - 2003_4200 2003D Tracel4200 2 - 8052_4200 8052D Tracel4200 2 - 2003_Luv_9537 2003D Luv_9537 2 - 8052_Luv_9537 8052D Luv_9537 2 - 2003_Hyd_3168 2003D Hyd_3168 2 - Appl. Sci. 2018, 88052_Hyd_3168, x FOR PEER REVIEW 8052D Hyd_3168 2 - 11 of 17

Table 6. The designation of the samples and their recipes. Table 7. Manufacturing parameters. Sample Code PLA Used CBA Used CBA Dosing Note Sample Code T p Pressure Drop [-] [Type] [Type]melt melt [wt%] [-] ◦ 2003_ref 2003D- C- bar 0 MPa extruded once 8052_ref 8052D2003_ref 196- 74 0 7.3 extruded once 2003_3170 2003D8052_ref Tracel3170 196 43 2 4.2 - 8052_3170 2003_31708052D Tracel3170 196 32 2 3.2 - 8052_3170 195 21 2.0 2003_4200 2003D Tracel4200 2 - 2003_4200 196 14 1.3 8052_4200 8052_42008052D Tracel4200 196 13 2 1.2 - 2003_Luv_9537 2003_Luv_95372003D Luv_9537 196 18 2 1.7 - 8052_Luv_9537 8052_Luv_95378052D Luv_9537 196 34 2 3.3 - 2003_Hyd_3168 2003_Hyd_31682003D Hyd_3168 197 56 2 5.5 - 8052_Hyd_3168 8052_Hyd_31688052D Hyd_3168 196 46 2 4.5 -

FigureFigure 7.7.The The surface surface of2003_3170, of 2003_3170, 2003_4200 2003_4200 and 8052_3170, and 8052_3170, 8052_4200 8052_4200 foam structures foam manufacturedstructures atmanufactured 190 ◦C. at 190 °C.

5. Results of the Foamed Poly(LacticTable Acid) 7. Manufacturing parameters.

5.1. Scanning Electron MicroscopySample Code Tmelt p Pressure Drop - °C bar MPa We evaluated the cell structure2003_ref of the manufactured196 74 foam samples7.3 based on scanning electron microscope (SEM) images.8052_ref Figure 8 shows196 characteristic 43 SEM4.2 images of the foam structure. They indicate that foaming2003_3170 took place in the196 case of both32 polymer3.2 matrices and all three foaming ◦ agents (Tracel3170, Tracel42008052_3170 and Luv_9537).195 Hyd_316821 applied2.0 at 190 C could not form a cell structure in the case of either2003_4200 PLA matrix. This196 corresponds14 to the1.3 TGA results, which showed that ◦ Hyd_3168 could not form effective8052_4200 gas at 190 196C in 33013 s (the modelled1.2 extrusion time) and therefore a cell structure could not form.2003_Luv_9537 196 18 1.7 8052_Luv_9537 196 34 3.3 2003_Hyd_3168 197 56 5.5 8052_Hyd_3168 196 46 4.5

5. Results of the Foamed Poly(Lactic Acid)

5.1. Scanning Electron Microscopy We evaluated the cell structure of the manufactured foam samples based on scanning electron microscope (SEM) images. Figure 8 shows characteristic SEM images of the foam structure. They indicate that foaming took place in the case of both polymer matrices and all three foaming agents (Tracel3170, Tracel4200 and Luv_9537). Hyd_3168 applied at 190 °C could not form a cell structure in the case of either PLA matrix. This corresponds to the TGA results, which showed that Hyd_3168 could not form effective gas at 190 °C in 330 s (the modelled extrusion time) and therefore a cell structure could not form.

Appl. Sci. 2018, 8, 1960 12 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 12 of 17

8052_3170 2003_3170

Background

(a) (b) 8052_4200 2003_4200

(e) (f) 8052_Hyd_3168 2003_Hyd_3168

(g) (h)

FigureFigure 8. ScanningScanning electron electron microscopy microscopy (SEM) (SEM) images images of of samples samples foamed foamed with with different foaming agentsagents (manufactured (manufactured at at 190 190 °C)◦C) (a ()a 8052_3170,) 8052_3170, (b) ( ba) 2003_3170, a 2003_3170, (c) (8052_4200,c) 8052_4200, (d) (2003_4200,d) 2003_4200, (e) 8052_Luv_9537,(e) 8052_Luv_9537, (f) a (f 2003_Luv_9537,) a 2003_Luv_9537, (g) (8052_Hyd_3168,g) 8052_Hyd_3168, (h) (2003_Hyd_3168h) 2003_Hyd_3168 20× 20 magnification.× magniﬁcation.

5.2.5.2. The The Morphology Morphology of of Foam Foam Structures Structures WeWe graded graded foam foam structures structures by by cell cell population population dens density,ity, expansion, expansion, void void fraction fraction and density. and density. The resultsThe results can canbe found be found in Figure in Figures 9 and9 and 10. 10 The. The exothermic exothermic Tracel3170 Tracel3170 produced produced more more cells cells in unitunit 5 3 volume in the case ofof thethe 8052_31708052_3170 reciperecipe recommendedrecommended forfor foaming foaming (4.82 (4.82× × 10105 cells/cmcells/cm3)) than than in in the case of 2003_3170 (4.17 × 105 cells/cm3). The 8052D polymer not only produces a higher number of cells but the expansion of the cells is also higher, 2.36, as opposed 2.18 in the case the 2003_3170. As

Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 17

a result, density with 8052_3170 was lower, 0.53 g/cm3, while in the case of 2003_3170, it was 0.57 g/cm3. This means that a void fraction of 57.61% was achieved in the case of the 8052_3170 recipe (considering that the density of PLA is 1.24 g/cm3. The good results can be attributed to the nucleating agent Tracel3170 contained based on the manufacturer’s data and the TGA results (residual material content of 40.96% at 800 °C). The endothermic Tracel4200 produced fewer cells per unit volume in the case of the 8052_4200 recipe recommended for foaming (5.58 × 104 cells/cm3) than in the case of the 2003_4200 recipe (1.91 × 105 cells/cm3). In the case of the 2003D the expansion of the cells is also better, 1.53, as opposed to 1.24 in the case of the 8052_4200. This is because the decomposition products of Tracel4200 include water, which probably hydrolysed the PLA, causing its melt pressure, viscosity and melt strength to decrease during the foaming process. Since the Mw of 8052D is lower (153,235 Da) than that of 2003D (180,477 Da), it resisted degradation by hydrolysis and produced better cell population density and expansion than the 8052D type PLA. The density of foam structures manufactured with Tracel4200 was 1.00 g/cm3 in the case of the 8052_4200, while 0.81 g/cm3 in the case of the 2003_4200, which is less than the density reduction achieved with Tracel3170. The endothermic Luv_9537 foaming agent produced more cells per unit volume in the case of the 8052_Luv_9537 recipe recommended for foaming (1.67 × 105 cells/cm3) than in the case of the 2003_Luv_9537 recipe (5.51 × 104 cells/cm3). Although the cells expanded, expansion in the case of the 2003D polymer was 1.22 and in the case of the 8052_Luv_9537 recipe, it was 1.23, which is quite little in both cases. The SEM images show that the size and shape of the cells in the case of the 8052_Luv_9537 are not homogeneous. The densities of PLA foams made with the Luv_9537 foaming agent are about 1 g/cm3 in both cases. In the case of 2003_Luv_9537, density is 1.02 g/cm3, while in the caseAppl. Sci.of 8052_Luv_9537,2018, 8, 1960 it is 1.01 g/cm3. 13 of 17 We did not characterize the 2003_Hyd_3168 and 8052_Hyd_3168 recipes with the methods used for the characterization of foam structures, since no foam structure was formed during the case of 2003_3170 (4.17 × 105 cells/cm3). The 8052D polymer not only produces a higher number manufacturing. Only the PLA matrix formed a blend with the carrier of the CBA without the effective of cells but the expansion of the cells is also higher, 2.36, as opposed 2.18 in the case the 2003_3170. gases being able to form a gas phase and nucleate cells. As a result, density with 8052_3170 was lower, 0.53 g/cm3, while in the case of 2003_3170, it was In the case of exothermic foaming, PLA foamed to a much greater degree than in the case of 0.57 g/cm3. This means that a void fraction of 57.61% was achieved in the case of the 8052_3170 recipe endothermic foaming and so densities were lower. This is due to the type of the endothermic reaction (considering that the density of PLA is 1.24 g/cm3. The good results can be attributed to the nucleating products on the one hand and to the effective gas formation temperature ranges unfavourable for agent Tracel3170 contained based on the manufacturer’s data and the TGA results (residual material PLA and the foaming agent fraction of the CBA granules. content of 40.96% at 800 ◦C).

2.6 1.4 2.4 2003 1.3 2003 1.2 2.2 8052 8052 2.0 1.1 1.8 1.0 ] 1.6 3 0.9 0.8 1.4 0.7 1.2 0.6 1.0 0.5 0.8 Density [g/cm 0.4 Expansionratio [-] 0.6 0.3 0.4 0.2 0.2 0.1 0.0 0.0 Tracel3170 Tracel4200 Luv_ 9537 PLA Tracel3170 Tracel4200 Luv_9537 PLA Type of CBA Type of CBA

(a) (b)

Appl. Sci.Figure 2018, 9.8, Foamedx FOR PEER sample REVIEW ( a)) expansion expansion values values ( (b)) density density values values (manufacturing (manufacturing temperature temperature 190 190 °C).◦14C). of 17

100 2003D 90 80 8052D 70 60 50 40 30 Voidfraction [%] 20 10 0 Tracel3170 Tracel4200 Luv_ 9537 PLA Type of CBA (a) (b)

FigureFigure 10. 10. (a(a) )Cell Cell population population density density and and (b (b) )porosity porosity in in the the case case of of th thee different different foaming foaming agent agent types types (manufacturing(manufacturing temperature temperature 190 190 °C).◦C).

5.3. DifferentialThe endothermic Scanning Tracel4200 Calorimetry produced fewer cells per unit volume in the case of the 8052_4200 recipe recommended for foaming (5.58 × 104 cells/cm3) than in the case of the 2003_4200 recipe We examined the morphology that formed after foaming process, using differential scanning (1.91 × 105 cells/cm3). In the case of the 2003D the expansion of the cells is also better, 1.53, as opposed calorimetry, the results of which can be seen in Figure 11 and Table 8. Figure 11 shows the DSC curves to 1.24 in the case of the 8052_4200. This is because the decomposition products of Tracel4200 include of the single-processed reference poly(lactic acid) (extruded granules, processed similarly to foamed water, which probably hydrolysed the PLA, causing its melt pressure, viscosity and melt strength to samples 2003_ref and 8052_ref) and the foamed samples in the case of the first heating curve. In the decrease during the foaming process. Since the Mw of 8052D is lower (153,235 Da) than that of 2003D case of foams manufactured with the same processing conditions, the glass transition temperature of (180,477 Da), it resisted degradation by hydrolysis and produced better cell population density and the polymer (Tg), which is the first registered endothermic change in the DSC curve, was not different expansion than the 8052D type PLA. The density of foam structures manufactured with Tracel4200 from the reference value in the case of the Tracel4200 foaming agent and the 8052_3170 recipe. On the other hand, in the case of the other foaming agents Tg decreased by 2–4 °C (Table 8). A change in the second registered heat flow is the exothermic phenomenon of cold crystallization, which, in the case of foam structures, shifts considerably to lower temperatures. The foaming agent causes cold crystallization to happen at a much lower temperature than in the case of neat PLA. The second, bigger endothermic peak is the crystalline fraction melting range. It can be observed that the extruded foam samples are capable of cold crystallization during the DSC test, which is an exothermic reaction. This is because after processing, due to fast cooling, the crystalline fraction cannot fully form, since the molecular chains do not have time to get ordered. During the DSC test, due to the relatively slow temperature rise (5 °C/min), the molecular chains can become more ordered, which is accompanied by the release of heat. These crystalline fractions melt later, at a higher temperature and the polymer melts. In the case of 8052D, two crystalline melting peaks can be observed. Frackowiak et al. [27] provided the explanation that crystals with lower melting temperatures are smaller and formed during cold crystallization, while, crystal types with a higher melting temperature formed during the primary crystallization process, therefore, in our case, they are created by the technology. Therefore, foaming agents influence the crystalline structure that forms in the polymer. The crystalline melting range has two peaks and is composed of two different crystalline types: the α and α’ types. The α’ type has a lower melting temperature, while the α type has a higher melting temperature and is also thermodynamically more stable. The range between the melting of the two phases is called α′–α phase transition [27]. The crystalline fraction forming during manufacturing is typically low, due to the cooling of the polymer in open air (<5%). The foam structure with the highest population density and highest expansion has the highest crystalline fraction from manufacturing (4.3%). Crystalline fraction was typically higher than that of reference PLA; the nucleating agents in the CBAs probably had a beneficial effect on the forming of crystalline nuclei. Thus, the foaming agent affected the crystalline state and structure of the samples and also their thermal properties. In the presence of the foaming agent, transition temperatures typically shifted to lower temperatures compared to the reference poly(lactic acid).

Appl. Sci. 2018, 8, 1960 14 of 17 was 1.00 g/cm3 in the case of the 8052_4200, while 0.81 g/cm3 in the case of the 2003_4200, which is less than the density reduction achieved with Tracel3170. The endothermic Luv_9537 foaming agent produced more cells per unit volume in the case of the 8052_Luv_9537 recipe recommended for foaming (1.67 × 105 cells/cm3) than in the case of the 2003_Luv_9537 recipe (5.51 × 104 cells/cm3). Although the cells expanded, expansion in the case of the 2003D polymer was 1.22 and in the case of the 8052_Luv_9537 recipe, it was 1.23, which is quite little in both cases. The SEM images show that the size and shape of the cells in the case of the 8052_Luv_9537 are not homogeneous. The densities of PLA foams made with the Luv_9537 foaming agent are about 1 g/cm3 in both cases. In the case of 2003_Luv_9537, density is 1.02 g/cm3, while in the case of 8052_Luv_9537, it is 1.01 g/cm3. We did not characterize the 2003_Hyd_3168 and 8052_Hyd_3168 recipes with the methods used for the characterization of foam structures, since no foam structure was formed during manufacturing. Only the PLA matrix formed a blend with the carrier of the CBA without the effective gases being able to form a gas phase and nucleate cells. In the case of exothermic foaming, PLA foamed to a much greater degree than in the case of endothermic foaming and so densities were lower. This is due to the type of the endothermic reaction products on the one hand and to the effective gas formation temperature ranges unfavourable for PLA and the foaming agent fraction of the CBA granules.

5.3. Differential Scanning Calorimetry We examined the morphology that formed after foaming process, using differential scanning calorimetry, the results of which can be seen in Figure 11 and Table8. Figure 11 shows the DSC curves of the single-processed reference poly(lactic acid) (extruded granules, processed similarly to foamed samples 2003_ref and 8052_ref) and the foamed samples in the case of the first heating curve. In the case of foams manufactured with the same processing conditions, the glass transition temperature of the polymer (Tg), which is the first registered endothermic change in the DSC curve, was not different from the reference value in the case of the Tracel4200 foaming agent and the 8052_3170 recipe. On the ◦ other hand, in the case of the other foaming agents Tg decreased by 2–4 C (Table8). A change in the second registered heat flow is the exothermic phenomenon of cold crystallization, which, in the case of foam structures, shifts considerably to lower temperatures. The foaming agent causes cold crystallization to happen at a much lower temperature than in the case of neat PLA. The second, bigger endothermic peak is the crystalline fraction melting range. It can be observed that the extruded foam samples are capable of cold crystallization during the DSC test, which is an exothermic reaction. This is because after processing, due to fast cooling, the crystalline fraction cannot fully form, since the molecular chains do not have time to get ordered. During the DSC test, due to the relatively slow temperature rise (5 ◦C/min), the molecular chains can become more ordered, which is accompanied by the release of heat. These crystalline fractions melt later, at a higher temperature and the polymer melts. In the case of 8052D, two crystalline melting peaks can be observed. Frackowiak et al. [27] provided the explanation that crystals with lower melting temperatures are smaller and formed during cold crystallization, while, crystal types with a higher melting temperature formed during the primary crystallization process, therefore, in our case, they are created by the technology. Therefore, foaming agents influence the crystalline structure that forms in the polymer. The crystalline melting range has two peaks and is composed of two different crystalline types: the α and α’ types. The α’ type has a lower melting temperature, while the α type has a higher melting temperature and is also thermodynamically more stable. The range between the melting of the two phases is called α0–α phase transition [27]. The crystalline fraction forming during manufacturing is typically low, due to the cooling of the polymer in open air (<5%). The foam structure with the highest population density and highest expansion has the highest crystalline fraction from manufacturing (4.3%). Crystalline fraction was typically higher than that of reference PLA; the nucleating agents in the CBAs probably had a beneficial effect on the forming of crystalline nuclei. Thus, the foaming agent affected the crystalline Appl. Sci. 2018, 8, 1960 15 of 17 state and structure of the samples and also their thermal properties. In the presence of the foaming agent, transition temperatures typically shifted to lower temperatures compared to the reference poly(lacticAppl. Sci. 2018 acid)., 8, x FOR PEER REVIEW 15 of 17

Figure 11. The DSC curves of reference PLA and foamed samples (1st heat-up, 5 ◦C/min). Figure 11. The DSC curves of reference PLA and foamed samples (1st heat-up, 5 °C/min). Table 8. DSC results of foamed samples and unfoamed, extruded PLA (1st heat-up, 5 ◦C/min). Table 8. DSC results of foamed samples and unfoamed, extruded PLA (1st heat-up, 5 °C/min). Type of Sample Tg Tc1 ∆Hc Tm1 Tm2 ∆Hm χc χcf Type of Sample Tg Tc1 𝐇 Tm1 Tm2 𝐇   [-] [◦C] [◦C] [J/g]𝐜 [◦C] [◦C] [J/g]𝐦 [%]𝐜 𝐜𝐟 [%] [-] [°C] [°C] [J/g] [°C] [°C] [J/g] [%] [%] 2003_ref 59.6 116.7 23.8 151.1 - 26.4 28.4 6.9 8052_ref2003_ref 59.2 59.6 115.9 116.7 25.223.8 150.3151.1 - - 28.8 26.4 28.4 31 6.9 3.1 2003_31708052_ref 57.5 59.2 108.4 115.9 26.525.2 147.3150.3 153.8 - 28.1 28.8 30.2 31 3.1 1.7 8052_31702003_3170 59.6 57.5 107.5 108.4 27.426.5 147.6147.3 154.8153.8 31.428.1 30.2 33.8 1.7 4.3 2003_42008052_3170 59.3 59.6 114.5 107.5 29.527.4 147.2147.6 154.2154.8 32.231.4 33.8 34.6 4.3 2.9 8052_4200 59.6 107.9 29.3 143.7 154.9 30.5 32.8 1.3 2003_4200 59.3 114.5 29.5 147.2 154.2 32.2 34.6 2.9 2003_Luv_9537 57.2 105.6 28.8 146.4 154.4 30.1 32.4 1.4 8052_Luv_95378052_4200 55.4 59.6 105.4 107.9 33.729.3 147.1143.7 154.8154.9 34.530.5 32.8 37.1 1.3 0.9 2003_Hyd_31682003_Luv_9537 56.2 57.2 112.3 105.6 29.628.8 148.5146.4 155.2154.4 30.1 31 32.4 33.3 1.4 1.5 8052_Hyd_31688052_Luv_9537 57.5 55.4 108.3 105.4 27.433.7 148.5147.1 155.5154.8 28.334.5 37.1 30.4 0.9 1.0 2003_Hyd_3168 56.2 112.3 29.6 148.5 155.2 31 33.3 1.5 5.4. Foam Strength8052_Hyd_3168 57.5 108.3 27.4 148.5 155.5 28.3 30.4 1.0 Foam samples were finally tested mechanically. A widely used test of foamed products is the 5.4. Foam Strength foam strength test. We only performed the test on samples deemed acceptable (void fraction >40%), that isFoam on 2003_3170 samples were and 8052_3170finally tested samples. mechanically. Figure 12 A showswidely theused measurement test of foamed curves products selected is the as typical.foam strength The shape test. ofWe the only curves performed is characteristically the test on samples the same deemed as that acceptable of other known (void fraction foam strength >40%), curvesthat is coincides.on 2003_3170 At initialand 8052_3170 loading, insamples. the linear Figure section, 12 shows deformation the measurement is elastic, then curves a peak selected follows, as whichtypical. corresponds The shape of to the “yield curves strength.” is characteristical In elastic deformation,ly the same as the that cells of other take up known the load, foam then strength they arecurves restored coincides. to their At originalinitial loading, state when in the loading linear section, stops. At deformation the inflexion is elastic, part, however, then a peak the cellsfollows, are deformedwhich corresponds and damaged. to “yield The strength.” straight section In elastic (the de sectionformation, after the cells inflexion take point)up theappeared load, then in they the curveare restored of every to specimen their original within state the when 10% deformation loading stops. limit, At thatthe is,inflexion each specimen part, however, suffered the permanent cells are deformation.deformed and Indamaged. the case The of 2003_3170, straight section it is 22.8(the ±section0.6 MPa, after while the inflexion in the case point) of appeared 8052_3170, in it the is 22.4curve± of0.7 every MPa specimen. There was within no significant the 10% deformation difference between limit, that the is, compression each specimen strength suffered values permanent of foams produceddeformation. with In Tracel3170 the case of and 2003_3170, the two different it is 22.8 types ± 0.6 ofMPa, PLA; while the compressionin the case of strengths 8052_3170, and it standard is 22.4 ± deviations0.7 MPa. There are nearly was no the significan same. Thet difference aim was tobetween create biodegradablethe compression hard strength foams values with the of usefoams of commerciallyproduced with available Tracel3170 CBAs. and Compressive the two different strength types is higher, of PLA; compared the compression physically strengths foamed PLAand standard deviations are nearly the same. The aim was to create biodegradable hard foams with the use of commercially available CBAs. Compressive strength is higher, compared physically foamed PLA in the literature [21]. The results should be considered a reference value when later modified foam structures are developed later.

Appl. Sci. 2018, 8, 1960 16 of 17 in the literature [21]. The results should be considered a reference value when later modiﬁed foam structuresAppl. Sci. 2018 are, 8, developedx FOR PEER REVIEW later. Appl. Sci. 2018, 8, x FOR PEER REVIEW 16 of 17 16 of 17

30 30

25 25

20 20

15 15

10 10 Compressive strenght [MPa] 5 2003_3170Compressive strenght [MPa] 5 2003_3170 8052_3170 8052_3170 0 0 02468100246810 Deformation [%] Deformation [%] FigureFigure 12. Compression strength deformationFigure diagram 12. Compression in the case strength of 2003_3170 deformation and 8052_3170 diagram recipes. in the case of 2003_3170 and 8052_3170 recipes. 6. Conclusions 6. Conclusions 6. Conclusions We comprehensively examined chemical blowing agents and determined their properties. We comprehensively examined chemicalWe comprehensively blowing agents andexamined determined chemical their blowing properties. agents Then and determined their properties. Then Then we foamed poly(lactic acid) with endothermic and exothermic foaming agents recommended we foamed poly(lactic acid) with endothermicwe foamed poly(lacticand exothermic acid) withfoaming endothermic agents recommended and exothermic for foaming agents recommended for for polyester and an endothermic foaming agent specially recommended for poly(lactic acid). polyester and an endothermic foamingpolyester agent and specially an endothermic recommended foaming for agentpoly(lactic specially acid). recommended We for poly(lactic acid). We We examined the cell population density, cell expansion, void fraction, density, morphology examined the cell population density,examined cell exthepansion, cell population void fraction, density, density, cell exmorphologypansion, void and fraction, density, morphology and and mechanical properties of the manufactured foams. Our results indicate that the exothermic mechanical properties of the manufacturedmechanical foampropertiess. Our of results the manufactured indicate that foamthe s.exothermic Our results indicate that the exothermic azodicarbonamide foaming agent (Tracel IM 3170 MS) added to 8052D PLA in 2 wt% is a very azodicarbonamide foaming agent (Tracelazodicarbonamide IM 3170 MS) foaming added agentto 8052D (Tracel PLA IM in 31702 wt% MS) is addeda very to 8052D PLA in 2 wt% is a very promising foaming agent at the processing temperature of 190 ◦C. The cell population density of such promising foaming agent at the processingpromising temperatur foaminge agentof 190 at °C. the The processing cell population temperatur densitye of of 190 such °C. The cell population density of such foams is 4.82 × 105 cells/cm3, their expansion is 2.36, their density is 0.53 g/cm3, their void fraction is foams is 4.82 × 105 cells/cm3, their expansionfoams is 4.82is 2.36, × 10 their5 cells/cm density3, their is 0.53 expansion g/cm3, their is 2.36, void their fraction density is is 0.53 g/cm3, their void fraction is 57.61% and their compression strength is 22.4 ± 0.7 MPa. 57.61% and their compression strength57.61% is 22.4 and ± their0.7 MPa. compression strength is 22.4 ± 0.7 MPa.

Author Contributions: Conceptualization, Á.K.; methodology, Á.K. and K.L.; formal analysis, Á.K. and K.L.; Author Contributions: Conceptualization,Author Á.K.; Contributions: methodology, Conceptualization,Á.K. and K.L.; form Á.K.;al analysis, methodology, Á.K. and Á.K. K.L.; and K.L.; formal analysis, Á.K. and K.L.; investigation, D.R. and K.L.; writing—original draft preparation, Á.K. and K.L.; writing—review and editing, Á.K. investigation, D.R. and K.L.; writing—originalinvestigation, draft preparation,D.R. and K.L.; Á.K. writing— and K.L.;original writing—review draft preparation, and editing, Á.K. and K.L.; writing—review and editing, Funding:Á.K. Á.K.

Funding: Supported by theFunding: ÚNKP-17-4-III New SupportedNational Excellence by the ÚNKP-17-4-III Program of theNew Ministry National of Excellence Program of the Ministry of Human Capacities. This research was alsoHuman supported Capacities. by The This National research Research, was also De supportedvelopment by and The Innovation National Research, Development and Innovation Supported by the ÚNKP-17-4-III New National Excellence Program of the Ministry of Human Capacities. Office (NVKP_16-1-2016-0012). This research was also supported byOffice The (NVKP_16-1-2016-0012). National Research, Development and Innovation Office (NVKP_16-1-2016-0012).Acknowledgments: The author would likeAcknowledgments: to thank Tim Stoesser, The author Department would oflike Chemistry, to thank Tim Oxford Stoesser, University, Department of Chemistry, Oxford University, Acknowledgments:Oxford, UK for his helpThe with author the would GPC measurements. likeOxford, to thank UK for Tim his Stoesser, help with Department the GPC measurements. of Chemistry, Oxford University, Oxford, UK for his help with the GPC measurements. Conflicts of Interest: The authors declareConflicts no conflict of Interest: of interest. The Thauthorse funders declare had no no conflict role in ofthe interest. design ofTh thee funders had no role in the design of the Conflictsstudy; in the of Interest:collection,The analyses, authors or declare interpretationstudy; no conflictin the of collection,data of interest.; in the analyses, writing The funders ofor theinterpretation hadmanuscript, no role of inor data thein the; designin decisionthe writing of the to of the manuscript, or in the decision to study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. publish the results. publish the results.

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