Sintering of PLLA powders for rotational molding

Thermochimica Acta 582 (2014) 59–67

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Sintering of PLLA powders for rotational molding Antonio Greco ∗ , Alfonso Maffezzoli, Stefania Forleo Department of Innovation Engineering, University of Salento, Italy

a r t i c l e

i n f o

Article history: Received 10 January 2014 Received in revised form 26 February 2014 Accepted 4 March 2014 Available online 12 March 2014 Keywords: Rotational molding Biodegradable Sintering

a b s t r a c t This paper is aimed to study the suitability of poly-lactic acid (PLLA) for the production of components by rotational molding. To this purpose, the sintering behavior of PLLA powders was studied by thermomechanical analysis (TMA), in order to identify the onset and endset temperatures of sintering and the onset temperatures of degradation. The results indicate that sintering of PLLA is characterized by two different steps, namely powder coalescence and void removal. The first process is fast, occurring just above the melting temperature, whereas the second one occurs at much higher temperatures. Finally, at higher temperatures, degradation involves the formation of gas in the bulk of the polymer, leading to a decrease of the bulk density. The different phenomena occurring during heating of PLLA powders were interpreted by means of dimensionless numbers. The use of such approach allowed identifying the processing window for PLLA powders, defined as the difference between the endset of sintering and the onset of degradation. In agreement with experimental results, the dimensionless analysis confirmed that wider processing windows are obtained for slower heating rates of PLLA powders. On the other hand, it is well known that potential materials for rotational molding should be characterized by an adequate toughness, essentially related to de-molding of parts. Therefore, PLLA was mixed with two different plasticizers, a non-biodegradable one, i.e. di-ethyl-hexyl-phthalate (DEHP) and a biodegradable one, i.e. poly-ethylene glycol (PEG). The plasticizers were responsible for a reduction of viscosity and therefore a faster sintering process. On the other hand, the decrease of thermal stability due to the addition of plasticizer is expected to significantly decrease the width of the processing window. © 2014 Elsevier B.V. All rights reserved.

1. Introduction A growing environmental awareness is motivating researchers from industry and academia to apply and optimize the standard technologies used for polymer processing (closed molding processes, film forming, etc.) to several new bio-based and biodegradable polymers and composites [1]. Among bio-based and biodegradable polymers, poly(lactic acid) (PLLA) is already substituting petroleum-based plastics in many applications [2] because of its high stiffness and strength, which are comparable to polystyrene, at least at room temperature [3]. However, the inherent high modulus and low elongation at break have been the major drawbacks and limited its application only to the rigid thermoformed packing industry, film forming or fiber spinning. Several attempts have been proposed to improve the mechanical properties of PLLA [4], such as copolymerization with other monomers [5], polymer blending [6] and the use of plasticizer [7], in order to

∗ Corresponding author. Tel.: +39 0832297233. E-mail address: [email protected] (A. Greco). http://dx.doi.org/10.1016/j.tca.2014.03.005 0040-6031/© 2014 Elsevier B.V. All rights reserved.

extend its applications to different technologies, eventually keeping its full biodegradability. Rotational molding is a process for manufacturing hollow or double-walled plastic products, in the absence of any externally applied force. The development of components produced by rotational molding is severely limited by the specific requirements that a polymer must possess. Low viscosity is required in order to achieve an efficient sintering of powders and hence void-free products [8]. Further, an adequately high toughness is required in order to allow the extraction of the part [9]. Nowadays, only few classes of thermoplastic polymers are processed by rotomolding, and most of them are different grades of polyethylene, in particular linear low density polyethylene (LLDPE). Nevertheless, in recent years, great attention has been devoted to the development of new materials for rotational molding. Incorporation of inorganic particles at nano, micro-, or macro-scale [10–12], has been considered in order to increase the stiffness of rotational molded products, despite the decrease of toughness [13], and sinterability of the material [14]. Other approaches developed in order to increase the mechanical properties of the rotomolded products involve the use of different types of polymers, such as polyamides [12,15], polypropylene [16], high density polyethylene [17], or combination of different

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materials in multi-walled products [18,19]. Recently, it was shown that the rotational molding equipment can be readily adapted to the production of long fiber reinforced composites by the use of thermoplastic prepreg in a bladder molding process [19,20] or by selective reinforcement by pultruded glass fibers [21]. The use of biodegradable polymers in rotational molding industry can lead to the production of an innovative class of products, which can be disposed in a composter and are capable to be degraded in soil. Therefore, this paper is aimed to study the suitability of PLLA for the production of bio-degradable hollow components by rotational molding. As a preliminary step toward the development of such innovative products, a detailed analysis on the sintering behavior of PLLA has been performed. In view of a potential use in rotomolding industry, also the sinterability of PLLA, toughened by the addition of two types of plasticizers of different molecular weight, has been studied.

Hot stage microscopy was performed on PLLA powders in order to follow the evolution of the coalescence process during thermal treatment of powders. A Zeiss AxioCam MRC5 microscope, equipped with a Linkam AV4 heating stage, was used. PLLA powders were heated from room temperature to 463 K at 2 K/min. Thermomechanical analysis (TMA) was performed in order to measure the evolution of powders bulk density during thermal treatment, using a Perkin Elmer TMA 7 equipped with an expansion probe. Powders were placed in an aluminum pan (6 mm diameter) and heated in the TMA apparatus from room temperature to 573 K at different heating rates (5–10–15–20 K/min), holding a constant pressure (1 mN, corresponding to 35 KPa). During each test, holding a constant force on the sample and increasing the temperature resulted in a decrease of the sample thickness, due to polymer sintering [22,23]. Neglecting the density variation of the polymer due to thermal expansion, the thickness of the sample ı(t) was related to the bulk density B by B (t, T ) =

2. Experimental

(1)

where AT is the surface area of the sample. The rate of sintering was obtained by time differentiation of Eq. (1) as Mass dı dB = dt AT ı2 dt

(2)

Themogravimetric analysis (TGA) was performed in order to measure the thermal stability of PLLA and plasticized PLLA, which is a relevant issue in severe thermo-oxidative conditions as those commonly found in rotational molding. TGA tests were performed on pulverized samples using a Mettler Toledo TGA 1 instrument, heating 10 mg samples between 300 and 973 K at 20 K/min in air flow at 60 ml/min. Each result reported for DSC, TMA, TGA, and rheological analysis is obtained as the average of three tests. 3. Results and discussion DSC traces of neat PLLA, obtained at 2 and 10 K/min, are reported in Fig. 1. Two melting peaks can be observed, at both heating rates. The presence of two melting peaks is attributed to recrystallization phenomena occurring in the melting range, with a concomitant increase in the thickness of the crystal lamellae [24]. The different behavior of the material at different heating rates is related to the crystallization kinetics. At lower heating rates, cold crystallization occurs at lower temperatures. As a consequence, thinner crystals are formed and the onset of melting is also shifted at lower temperatures.

20

2 K/min 10 K/min

endo

dH/dT(J/(g*K))

The material used is a poly-lactic acid (PLLA) Synterra 1010, characterized by a density of 1.25 g/cm3 and melt flow rate of 1010 g/600 s at 190 ◦ C. According to the producer technical data sheet, the polymer is mainly composed of l-isomer, with a d-isomer content lower than 1%. The PLLA grade was chosen due to its low viscosity, which is expected to lead to an efficient sintering during powder processing. The two plasticizers used for toughening of PLLA are di-ethylhexyl-phthalate (DEHP) by Merckx, characterized by a molecular weight MW = 390 g/mole, and a poly-ethylene-glycol (PEG) with MW = 800 g/mole, purchased by Sigma–Aldrich. Although DEHP is not biodegradable, it was considered as a reference plasticizer, in view of its efficiency in increasing the toughness of PLLA. The plasticized PLLA were produced in a Haake R Rheomex PTW 16/25 D twin screw extruder with 20% by weight of plasticizer content. The extrusion process was run at a screw temperature profile of 453–463–483–483–473 K with a screw speed of 20 rpm. Materials were pulverized with a Retsch ZM 100 mill, using a 500 ␮m sieve. Powders with an average diameter of 400 ␮m were obtained. The plasticizer content was chosen based on a preliminary mechanical characterization of the materials, with particular attention paid to the specific requirements of rotational molding. In facts, a very high toughness of the materials is required in order to extract the part after processing. On the other hand, the very slow cooling rates usually attained in rotational molding (less than 10 K/min) are responsible of the formation of a crystalline phase, characterized by the presence of thick lamellae, which are known to adversely affect the toughness of the material. For this reason, a quite high amount of plasticizer was added to PLLA, despite the expected decrease of modulus and potential migration of the plasticizer. Differential scanning calorimetry (DSC) analysis was performed on a Mettler Toledo 822 instrument under a nitrogen flux of 30 mL min−1 applying a heating scan between 298 and 473 K at 2 and 10 K/min. DSC analysis was performed in order to measure the melting range of PLLA and plasticized PLLA, which is of primary importance in determining the processing range. Tests were performed on pulverized samples. Rheological analysis was performed by measuring the dynamic viscosity in a ARES instrument, using a parallel plate geometry. Tests were performed at 1% deformation, 1 Hz frequency, heating the materials between 437 and 453 K at 2 K/min. The deformation was chosen, after a preliminary evaluation, within the limit of the linear viscoelastic behavior of the material. Rheological properties were measured, since low viscosity is considered a strict requirement for an efficient sintering process.

Mass Mass = Volume AT ı(t, T )

10

0

-10

320

340

360

380

400

420

temperature (K) Fig. 1. DSC traces of neat PLLA at heating rates of 2 and 10 K/min.

A. Greco et al. / Thermochimica Acta 582 (2014) 59–67

61

Table 1 Transition temperatures of PLLA and plasticized PLLA obtained by DSC tests at 10 K/min. Tg (K)

Ton,melt (K)

Tpeak,melt (K)

Tend,melt (K)

332 ± 0.4 310 ± 0.6 311 ± 0.5

412 ± 0.3 408 ± 0.5 408 ± 0.4

425 ± 0.4 422 ± 0.2 424 ± 0.5

428 ± 0.5 425 ± 0.3 425 ± 0.6

Nevertheless, experimental results show that the endset of melting is not influenced by the heating rate. For every heating rate in the range between 2 and 10 K/min, the endset of melting is observed at 428 K. The thermal properties of PLLA and plasticized PLLA, determined by DSC, reported in Table 1, show that addition of plasticizers reduces both the glass transition and the melting temperature range. The reduction of glass transition is a direct consequence of the increase of the molecular mobility, due to the addition of the plasticizer [25]. On the other hand, the reduction of the melting range can be attributed to two different effects. The first effect, which accounts for the thermodynamics of melting, is due to the reduction of the equilibrium temperature of melting as a consequence of the addition of the plasticizer. [26]. In addition, the increased mobility is responsible for the reduction of the cold crystallization temperature. Therefore, plasticized PLLA is characterized by thinner crystals, which in turn involves a further decrease of the melting temperature range. The thermal properties of the PLLA plasticized by DEHP are close to the thermal properties of PLLA plasticized by PEG. In order to gain a better understanding about the significance of the DSC analysis, one-way ANOVA analysis was performed, considering the type of material as the source of variation. The results for the F value obtained for the four different characteristic temperatures are reported in Table 2. Such values must be compared with the critical F value, which for two groups (k = 2) and three tests (repetitions) for each group, is FCV (1,4) = 7.7 at 0.05 significance level. When F is higher than FCV , the average values must be considered significantly different; if not, the means are not considered to be significantly different. The results of Table 2 show that the thermal properties of PLLA are significantly different from the thermal properties of plasticized PLLA. On the other hand, the thermal properties of DEHP plasticized PLLA are not significantly different from those of PEG plasticized PLLA. The only exception is the melting peak temperature. The evolution of viscosity of PLLA compounds as a function of temperature is reported in Fig. 2. Plasticized PLLA is characterized by a viscosity much lower than the viscosity of neat PLLA. In particular, the addition of DEHP, which is characterized by a lower molecular weight, involves a more significant reduction of viscosity compared to the addition of higher molecular weight PEG. Above melting, the PLLA compounds show a behavior which can be well represented by a VFT (Vogel–Fulcher–Tammann) model:  = ∞ exp

A T − To

(3)

where ∞ is the viscosity extrapolated at infinite temperature, A is an exponential factor, and T0 is the temperature at which viscosity diverges. According to DSC analysis, it was assumed that the viscosity is infinite below the endset of melting, and therefore,

1000 viscosity (Pa*s)

PLLA PLLA DEHP PLLA PEG

PLLA PLLA_DEHP PLLA_PEG VFT fitting

100

435

438

441 444 447 temperature (K)

450

453

Fig. 2. Viscosity curves for PLLA compounds as a function of temperature and VFT model fitting.

Table 3 VFT fitting parameters for PLLA compounds.

PLLA PLLA DEHP PLLA PEG

∞ (Pa s)

A (K)

T0 (K)

455 ± 43 23 ± 4 119 ± 1

17 ± 1 32 ± 2 22 ± 2

428 ± 0.5 425 ± 0.3 425 ± 0.6

fitting of experimental data by Eq. (3) was performed by assuming that T0 = Tend,melt . The fitting parameters for PLLA compounds are reported in Table 3. The pictures obtained by optical microscopy during heating of PLLA powders in the hot stage are reported in Fig. 3. Even at temperatures higher than the onset of melting, which is 412 K, no evidence of shape change can be detected, as reported in Fig. 3a. This indicates that, though being, at least partially, in the molten state, the polymer is still characterized by a too high viscosity, which prevents powder coalescence. Only starting from 434 K, (Fig. 3b), which is a temperature higher than the endset of melting reported in Table 1, a change in the shape of the powders is observed. At this stage, the viscosity decreases enough to allow powders coalescence. At about 436 K, Fig. 3c, the coalescence process is evidenced by the increase of the size of the neck between the two PLLA granules. The neck continues to grow, until the particles form a single granule, at about 441 K. The sintering process occurs in a narrow temperature range, between 434 and 441 K. The images of Fig. 3 were used for measuring the coalescence neck, y. The normalized coalescence neck y/a0 was obtained dividing y by the particle radius a0 = 200 ␮m. The increase of y/a0 as a function of temperature, reported in Fig. 4, evidences that coalescence occurs between 434 and 441 K. Also, for comparison purposes, the evolution of powder bulk density obtained by TMA is reported in Fig. 4. The coalescence neck and density do not increase until melting temperature is exceeded, since powder sintering of semicrystalline polymers can only occur in the molten phase [8,27]. Above melting, the increase of density observed by TMA results from the increase of the coalescence neck. At higher temperatures, above 443 K, the density continues to increase, even if the coalescence step can be considered completed. This further increase of the

Table 2 F values from ANOVA analysis for the DSC characteristic temperatures.

PLLA vs PLLA DEHP PLLA vs PLLA PEG PLLA DEHP vs PLLA PEG

F value for Tg

F value for Ton,melt

F value for Tpeak,melt (K)

F value for Tend,melt (K)

2624 2899 4

157 214 0.46

117 6 39

90 50 0.1

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Fig. 3. Images from hot stage microscopy of PLLA powders during coalescence.

1.0 y/a0 0.8

1200

ρB

y/a0

3

ρB (g/cm )

1050 0.6 900 0.4 750 0.2 600 0.0 425

430

435 440 temperature (K)

445

450 450

Fig. 4. Comparison of neck radius obtained from hot stage and bulk density obtained from TMA.

density results from the second mechanisms involved in powder sintering, i.e. void removal [28]. TMA curves obtained for neat PLLA powders at different heating rates are reported in Fig. 5. All the curves show a similar qualitative behavior, with a first density increase, occurring above melting, which is due to powder coalescence, and a second densification step, occurring at higher temperatures, due to void removal. Finally, at higher temperatures, above 550 K, a significant density reduction is observed, as a consequence of gas formation deriving from polymer degradation. The curves are shifted at higher temperatures with increasing heating rate, due to the kinetic nature of the sintering and degradation processes. Furthermore, the maximum density attained by powders at the lower heating rates is almost equal to the density of fully compacted PLLA, which indicates an efficient sintering. In contrast, the sintering efficiency decreases when the sample is heated at 20 K/min. The rate of sintering curves, reported in Fig. 6, show a distinct peak in correspondence of the powder coalescence step. Its position

A. Greco et al. / Thermochimica Acta 582 (2014) 59–67

2 K/min 5 K/min 10 K/min 20 K/min

1400 10 9 8

1000

600 400 400

7

2 K/min 5 K/min 10 K/min 20 K/min

800

6 5

G1

3

ρB(Kg/m )

1200

63

4 3

450 500 550 temperature (K)

600

2 1 0 430

Fig. 5. TMA sintering curves of PLLA at 2, 5, 10 and 20 K/min.

435

440

445

450

455

temperature (K) is influenced by the heating rate, being the maximum difference between the curve at 2 K/min and the curve at 20 K/min of about 5 K. On the other hand, the peak associated to void removal is much smaller and visible in the inset of Fig. 6 between 440 and 480 K. The void removal step is shifted at significantly higher temperatures at increasing heating rates, being the maximum difference observed between the peak temperature measured at 2 K/min and 10 K/min, equal to 40 K. The differential curve obtained for the scan at 20 K/min was not reported in Fig. 6, since in this case the peak due to the void removal step was not observed. The absence of the void removal step justifies the lower density attained during the test at 20 K/min. The characteristic values of the onset (Ton,coalescence ) and endset (Tend,coalescence ) temperatures of coalescence, the endset temperature of sintering, including void removal (Tend,sintering ) the onset temperature of degradation (Ton,degradation ), and the maximum value of density are reported in Table 4 for the tested heating rates. The value of the endset of sintering is not reported for the curve at 20 K/min, since in this case, as discussed, the void removal step is not completed. The relevance of heating rate on the coalescence process was studied by referring to the Frenkel model [8]. In isothermal conditions, the dimensionless number G1 , which represents the ratio between the characteristic time of the experiment and the characteristic time of coalescence, is obtained as a function of the surface tension  , the time of the experiment tc , the viscosity  and the average powder radius a0 [23]: G1 =

Fig. 7. Evolution of dimensionless number G1 at different heating rates.

In non-isothermal conditions, the rate of neck growth can be expressed as [8] d  = f () a0 dt

Being the contact angle between sintering particles,  = tan−1 (y/a0 ) and f() can be obtained by the Frenkel or equivalent models. On the other hand, the bulk density is a function of : B = g()

where g is a function which depends on the geometry of coalescing particles [29]. Inversion of Eq. (5) yields  = g −1 (B ) = s(B )

(6)

And therefore, combining Eqs. (4) and (6): dB  u(B ) = a0 dt

(7)

where u(B ) = f(s(B ))/(ds/dB ). By operating the separation of variables in Eq. (7) and integrating:



B

0

dB = u(B )

t

 dt a0

(8)

0



B

0

2 K/min 5 K/min 10 K/min

14 0.6

12 10 8 6 4

dρ /dt (Kg/(s*m ))

3

(5)

Or, alternatively, for a constant heating rate ˇ:

tc a0

dρB/dt (Kg/(s*m ))

(4)

0.4



440 450 460 470 480 490 500 temperature (K)

450

475

500

temperature (K) Fig. 6. Derivatives of TMA sintering curves of PLLA at 2, 5, and 10 K/min.

 dT a0

(9)

0

G1 = 0.0

T

According to Eqs. (8) and (9), the evolution of density during coalescence process only depends on the integral on the right-hand side, which is a function of material properties (,  , a0 ) and process parameters (the time t or temperature T) and represents an alternative form for G1 valid for any thermal history of the material:

0.2

2 0 425

dB = u(B )

0

t

 dt a0

(10)

In order to test the validity of Eqs. (8)–(10), the dimensionless number G1 was calculated at the different heating rates, assuming a surface tension of 0.045 N/m [30], an average radius of powders equal to 200 ␮m, an infinite viscosity below the endset of melting (428 K) and a finite viscosity following a VFT behavior with the parameters reported in Table 3. The values of G1 calculated for the heating rates of 2, 5, 10 and 20 K/min are reported in Fig. 7.

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A. Greco et al. / Thermochimica Acta 582 (2014) 59–67

Table 4 Characteristic temperatures of the sintering process of PLLA compounds at different heating rates. Heating rate (K/min)

max (kg/m3 )

PLLA

2 5 10 20

1250 1250 1250 1150

PLLA DEHP

PLLA PEG

± ± ± ±

Ton,coalescence (K) 434.3 435.8 437.6 439.9

± ± ± ±

0.3 0.5 0.4 0.4

2 5 10 20

430 431.2 432.6 434

± ± ± ±

0.2 0.4 0.3 0.5

2 5 10 20

430.5 432.1 433.9 434.9

± ± ± ±

0.3 0.2 0.4 0.3

46 32 38 29

Also, the experimental temperatures corresponding to the onset of coalescence are reported as the vertical black lines. As it can be observed, the vertical black lines intersect the curves at a constant value of G1 = 1.5 ± 0.041. Analogously, the vertical red lines, representatives of the temperatures corresponding to B = 900 kg/m3 , intersect the curves for a constant value of G1 = 4.55 ± 0.048. This confirms that in the initial stage of sintering, when the process is governed by coalescence, the evolution of density only depends on the dimensionless number G1 , regardless of the heating rate, which confirms the validity of Eqs. (8) and (9). Finally, the vertical blue lines are representatives of the temperatures corresponding to the endset of sintering, which roughly corresponds to B = 1100 kg/m3 . In this case, all the vertical lines intersect the curves at a value of G1 = 8.5 ± 0.058, which indicates that coalescence is completed when the time of the experiment becomes much higher than the time of coalescence. In analogy to the discussion for dimensionless number G1 , the void removal step can be studied by introducing a second dimensionless number, which represents the ratio between the characteristic time of the experiment and the characteristic time of diffusion, and in non-isothermal conditions, becomes [23]



G2 = 0

t

± ± ± ±

0.7 0.9 0.5 0.8

(11)

where D is the diffusivity, and L is the diffusion distance, which can be assumed equal to the average radius of powders (about 200 ␮m). In applying Eq. (11), it is assumed that the void removal process is mainly dependent on the diffusion of the gas entrapped in the molten polymer. Therefore, the effect of viscous forces on the dissolution of the bubbles is neglected, which is a valid assumption for the high viscosities characteristic of molten polymers [31]. In order to calculate the diffusivity and its dependence on temperature, it is possible to assume an Arrhenius behavior, taking the parameters from Ref. [32]:

 −E  D

RT

Tend,sintering (K)

Ton,degradation (K)

442 ± 0.6 465 ± 0.8 490 ± 0.5 –

498 510 516 529

± ± ± ±

0.5 0.8 0.7 0.6

occur when the time of the experiment becomes much higher than the characteristic time of diffusion. Finally, the third fundamental mechanisms occurring during TMA tests, i.e. degradation, can be analyzed by considering the kinetic model introduced in the previous work [23], characterized by a kinetic constant, kt0 and an activation energy, Et0 : d dt

   B max

 E   t0

= −kto exp −

RT

f

1−

B max



(15)

Operating the separation of variables and integrating, it is possible to obtain:



B

B,0

d(B /max ) = f (1 − (B /max ))



t

 E  t0

kt0 exp − 0

RT

dt

(16)

which is an equivalent form to Eq. (8). Therefore, the evolution of density during the degradation step can be studied by considering the integral on the right-hand side, which represents the dimensionless number G3 , given as



G3 =

 E  t0

t

kt0 exp − 0

D dt L2

D = D0 exp

Tend,colascence (K) 438 442 446 452

RT

dt

(17)

which is the ratio between the time of the experiment and the characteristic time of the degradation process. In order to obtain an estimation for G3 , the kinetic constant and activation energy were determined, taking f(1 − (B /max )) = (1 − (B /max ))n , which yields a more specific expression for Eq. (15): d dt

   B max

 E  t0

= −kt0 exp −

6

RT

1−

B max

n (18)

2 K/min 5 K/min 10 K/min

5 (12)

4 (13)

ED log10 D0 = 0.001 − 9.8 R

(14)

G2

ED = [6400 − 0.16(Tg − 298)2 ] R

Using a value of Tg = 332 K, according to the results of Table 1, ED = 51.7 kJ/mol and D0 = 2.8 × 10−4 m2 s−1 were calculated from Eqs. (13) and (14), respectively. G2 values calculated at the different heating rates are reported in Fig. 8, together with the experimental endset temperatures measured for the void removal stage, i.e. Tend,sintering in Table 4. The results of Fig. 8 show that at the heating rates of 2, 5 and 10 K/min, the endset of bubble removal corresponds to a value of G2 = 4.2 ± 0.026, which indicates that void removal can

3 2 1 0 420

440

460

480

500

temperature (K) Fig. 8. Evolution of dimensionless number G2 at different heating rates.

A. Greco et al. / Thermochimica Acta 582 (2014) 59–67

65

1.00 10 K/min

Ton,degradation a0=200 μm Ton,coalescence

540

0.95 2 K/min 5 K/min 20 K/min

0.90

temperature (K)

ρB/ρB,max

5 K/min

0.85

Tend,sintering a0=100 μm

510

Ton,coalescence Tend,sintering

480 450

0.80 480

500

520

540

560

420 100

580

1000

temperature (K) Fig. 9. Kinetic modeling of degradation of PLLA.

Fig. 11. CHT plot for an average powder radius of 100 and 200 ␮m.

Fitting the last portion of TMA curves of Fig. 5 with Eq. (18), yielded a value of Et0 = 167 kJ/mol, kt0 = 3.18 × 1012 s−1 and n = 0.113. The results of non-linear curve fitting are reported in Fig. 9. The values of G3 calculated according to Eq. (17) for the tests at different heating rates are reported in Fig. 10, together with the corresponding values of Ton,degradation , reported in Table 4. At every heating rate, degradation begins when G3 reaches a critical value, which is about 3.3 × 10−3 ± 1.3 × 10−4 . Therefore, it is possible to assume that the whole process of density evolution during a TMA experiment is governed by the three phenomena of coalescence, void removal, and degradation. Each phenomenon occurs at a temperature TC , at which the corresponding dimensionless number reaches a critical value Gcr : Gi (TC ) = Gcr

(19)

Sintering begins with coalescence, starting at a temperature Ton,sintering , such that:

 G1 (Ton,sintering ) = 0

Ton,sintering

 dT = 1.5 0 exp(A/(T − Tend,melt ))a0 ˇ (20)

Being the critical value for the onset of coalescence equal to 1.5, according to the discussion to Fig. 7. Sintering is completed when void removal is completed, i.e. when G2 reaches the critical value equal to 4.2, according to the

0.005 0.004

2 K/min 5 K/min 10 K/min 20 K/min

G3

0.003 0.002 0.001 0.000 480

500

520

10000

time (s)

540

temperature (K) Fig. 10. Evolution of dimensionless number G3 at different heating rates.

discussion to Fig. 8:



T

G2 (Tend,sintering ) =

end, sintering 0

D0 exp(−ED /RT ) dT = 7.2 L2 ˇ (21)

Finally, polymer degradation begins when G3 reaches a critical value equal to 0.0033, according to the discussion to Fig. 10:



G3 (Ton,degradation ) =



T

on, degradation 0

× dT = 0.0033

kt0 Et0 exp − RT ˇ

 (22)

For any arbitrary heating rate it is possible to calculate the temperature and time of onset and endset of sintering, and the onset temperature of degradation by numerical integration of Eqs. (20)–(22) Therefore, it is possible to build a Continuous Heating Transformation (CHT) plot for the sintering process, in analogy with curing CHT of thermosetting polymers [33]. The results obtained with the numerical parameters determined in the present work and assuming a starting temperature of 303 K are reported in Fig. 11. In the previous work, the processing window, defined as the difference between the endset of sintering and the onset of degradation, was introduced, and it was concluded that a polymer suitable for rotational molding should have a processing window higher than the maximum temperature gradient attained during the process, which can be as high as 40 ◦ C [23,34] in any case depending on the thickness of the molded part. In view of the results reported in Fig. 11, and also accounting for the fact that thermal gradients during the rotational molding process decrease with decreasing heating rates, a low heating rate should be used during rotational molding of PLLA, in order to attain a full sintering, avoiding degradation phenomena. In facts, in Fig. 11, Tend,sintering = 483 K and Ton,degradation = 510 K can be estimated for an heating rate of 5 K/min, which yield a processing window equal to 27 K. On the other hand, for an heating rate of 10 K/min, Tend,sintering = 508 K and Ton,degradation = 519 K give a lower processing window. CHT plot can be also used to analyze the effect of other processing parameters, such as the powder size. As evidenced in Fig. 11, reduction of the average powder radius from 200 to 100 ␮m has a dramatic influence on the bubble removal stage, but is expected to have no influence on the degradation behavior of powders, and is therefore expected to significantly increase the process window. TMA was also used to study the sintering behavior of plasticized PLLA. In this case, however, the too low viscosity caused significant squeezing of material from the sample holder at high temperature. Therefore, only the onset temperature of powder coalescence was

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0.00

900

-0.01

PLLA PLLA_DEHP PLLA_PEG

3

ρB(Kg/m )

-1

d(M/M0)/dT (K )

800 PLLA PLLA_DEHP PLLA_PEG

700 600 500

-0.02

-0.03

400 426

428

430 432 434 436 temperature (K)

438

450

440

500

550

600

650

700

temperature (K)

Fig. 12. TMA sintering curves of PLLA and plasticized PLLA at 20 K/min.

Fig. 14. TGA analysis of PLLA compounds.

measured for plasticized PLLA. The sintering curves of plasticized PLLA are reported in Fig. 12 for a heating rate of 20 K/min. For comparison purposes, the sintering curve of PLLA is also reported. Compared to neat PLLA, plasticized PLLA shows a reduced onset temperature of sintering, as a consequence of the reduced viscosity. Even in the case of TMA analysis, the experimental observations for the onset temperature of coalescence were compared with ANOVA prediction. Comparing PLLA and PLLA DEHP, a value of F = 252 was obtained, whereas comparing PLLA and PLLA PEG a value of F = 259 was obtained. Both values of F are much higher than the corresponding FCV (1,4) = 7.7 which indicates that the difference in the onset temperatures of coalescence between PLLA and plasticized PLLA is substantial. In contrast, the F value calculated from comparison between PLLA DEHP and PLLA PEG is equal to 5.6 which is lower than FCV , and does not allow to establish a correlation between the type of plasticizer and onset of coalescence. In Fig. 13, the values of G1 calculated for an heating rate of 20 K/min, for PLLA and plasticized PLLA, are reported, together with the measured onset temperatures of coalescence. As it can be observed, for all the materials the onset of coalescence occurs when G1 reaches a value about 1.5, which confirms the discussion to Fig. 7. Finally, the derivative TGA curve for PLLA compounds, reported in Fig. 14, shows that the plasticized PLLA show a significantly lower onset temperature of degradation compared to neat PLLA. In facts, though the main degradation step is the same for all the materials, showing a peak at 625 K, a secondary degradation step can be observed at lower temperatures for DEHP plasticized

PLLA. Such secondary step, which involves a weight loss of about 10%, is expected to compromise the quality of rotational molded products. Analogously, the PEG plasticized PLLA shows an onset of degradation significantly reduced compared to neat PLLA. Both PLLA compounds, plasticized either by DEHP or PEG, show an onset degradation temperature which is about 30 K lower than that of neat PLLA. Therefore, in both cases the addition of plasticizer can improve the sintering behavior of PLLA, reducing the coalescence process by 5–10 K. On the other hand, however, the reduction of the onset of degradation is expected to play a more relevant role. This suggests that plasticized PLLA is characterized by a narrower processing window compared to neat PLLA.

3 PLLA PLLA_DEHP PLLA_PEG

G1

2

1

0 426 428 430 432 434 436 438 440 442 444

4. Conclusions In this work, the sintering behavior of PLLA powders has been studied, in view of their potential use in rotational molding. Thermomechanical analysis indicated that density evolution during heating of PLLA powders results from three different phenomena: powder coalescence, void removal, and polymer degradation. The results showed that PLLA is characterized by a good sinterability, which allows to obtain a completely densified product, before degradation of the polymer begins. The different phenomena occurring during TMA experiments, i.e. powder coalescence, bubble removal, and degradation, have been studied by dimensionless analysis. The use of this tool allowed to build continuous heating transformation (CHT) curves, which are able to predict the temperatures at which each of the three phenomena can occur. The use of CHT represents a useful tool to predict the processing window of PLLA. In order to be suitable for the use in rotational molding, the polymer should be characterized by a processing window wider than the maximum temperature difference which can be encountered in the process. For PLLA, this is more likely to occur if low heating rates, which involve wider processing window, are used. The effect of the addition of two plasticizers was also reported. Addition of the plasticizer involves a significant reduction of the viscosity, which in turn involves an improvement of the sinterability of the material. On the other hand, a drastic reduction of the thermal stability is also observed, which involves a decreased of the onset temperature of degradation by almost 100 K in such conditions, it is expected that the processing window of plasticized PLLA is much narrower than neat PLLA.

temperature (K) References Fig. 13. Evolution of dimensionless number G1 for PLLA and plasticized PLLA at 20 K/min.

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