Green synthesis of flexible polyurethane foams from liquefied lignin

Green synthesis of flexible polyurethane foams from liquefied lignin

Accepted Manuscript Green synthesis of flexible polyurethane foams from liquefied lignin Patrizia Cinelli, Irene Anguillesi, Andrea Lazzeri PII: DOI: ...

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Accepted Manuscript Green synthesis of flexible polyurethane foams from liquefied lignin Patrizia Cinelli, Irene Anguillesi, Andrea Lazzeri PII: DOI: Reference:

S0014-3057(13)00174-2 http://dx.doi.org/10.1016/j.eurpolymj.2013.04.005 EPJ 6051

To appear in:

European Polymer Journal

Received Date: Revised Date: Accepted Date:

14 August 2012 25 March 2013 4 April 2013

Please cite this article as: Cinelli, P., Anguillesi, I., Lazzeri, A., Green synthesis of flexible polyurethane foams from liquefied lignin, European Polymer Journal (2013), doi: http://dx.doi.org/10.1016/j.eurpolymj.2013.04.005

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Green synthesis of flexible polyurethane foams from liquefied lignin Patrizia Cinelli, Irene Anguillesi, Andrea Lazzeri* Department of Industrial Chemistry, Chemical Engineering and Materials Science, University of Pisa, Via Diotisalvi, 2, 56126, Pisa, Italy, [email protected], Tel:+390502217807, Fax: +390502217866

ABSTRACT One of the targets of the research activity in the EC project FORBIOPLAST grant agreement no. 212239 was focused on the use of by-products from wood as raw materials for the production of soft polyurethane foams by innovative sustainable synthetic processes with reduced energy consumption. The present paper reports the green synthesis of soft foams from Kraft lignin. In an attempt to produce flexible foams, two types of chain extenders were used in combination with liquefied lignin: polypropylene glycol triol and castor oil. The samples were produced with the "one shot" technique and the only blowing agent used was water. All samples were produced with a NCO/OH ratio less than one, because it is well known that it significantly reduces the degree of crosslinking, resulting in higher flexibility of the material. Samples were produced in free and controlled rise expansion. Lowering the glass transition temperature of the polyol phase was determined to be the most important issue in order to increase the content of liquefied lignin in foam formulation, and ultimately achieving the goal of making soft foam with liquefied lignin. Keywords: polyurethane, lignin, soft foam, microwave, renewable resources

1.

Introduction

Polyurethanes (PU) are a broad class of polymers having urethane as a common group. They are known for their versatility, but one of the problems related to the production of PU nowadays is their dependence on petroleum-derived products. Foams represent one of the most important commercial products of PU. These foams are commonly classified as flexible, semi-rigid, or rigid, depending on their mechanical performance and core densities. The main field of polyurethane application is in the furniture industry followed by automotive applications for flexible and semi-flexible polyurethanes (seat cushioning, bumpers, sound insulation, etc.). Flexible PU foams are block copolymers that owe their elastic properties to the phase separation of so-called “hard blocks” and “soft blocks.” Hard blocks are rigid structures that are physically cross-linked and give the polymer its firmness; soft blocks are stretchable chains that give the polymer its elasticity. By adapting the composition and the ratio of the hard and the soft blocks, PU can be customized to its application. Polyurethanes are prepared from the polyol component and isocyanates. At present, for foam applications, only the production of polyol from renewable resources is reported. Although aliphatic di-isocyanates from dimerized fatty acids are commercial, they do not have sufficient reactivity for application in foams, but they could be used for coatings and other applications [1]. Thus, isocyanates for foams must be aromatic. On the other hand, vegetable oil polyols have an excellent

chance for competing with petrochemical polyols. The importance of the development of polymeric materials using renewable sources rises from the concern about raw material processing and the development of alternative synthesis routes that are less hazardous to the environment. The goals are to reduce the demand for non-renewable fossil fuels and to cut production of carbon dioxide “greenhouse gas” to lower global warming. From the point of view of the polyurethane industry, the use of polyols produced with a closed carbon cycle seems to be attractive, especially if we consider the low cost of these residues. Since the 1960s, a wide range of vegetable oils have been considered for the preparation of polyurethanes; the most important oils are highly unsaturated oils, where, by using various chemical reactions, the double bonds are transformed into hydroxyl groups including, sunflower, palm, rapeseed, but mainly castor and soybean oils. Thus several papers report on polyurethanes based on vegetable oils [1-3]. Depending on the reactant and process methods, the polyurethane polymer can be an elastomer, a foam or a plastic with a wide range of applications such as automotive seating, furniture, packaging and medical devices. Vegetable oil polyols have to satisfy some structural requirements in order to compete with petrochemical polyols, such as the right functionality and molecular weight. Functionality of most flexible polyols is around 3 and molecular weights between 3000 and 6000 (OH numbers≈56-28 mg KOH/g, respectively). Higher OH numbers usually increase the cost of the formulation due to higher isocyanate consumption. Vegetable oils have molecular weights below 1000 and cannot be easily transformed into triols of the given molecular weights. Hydroxyl groups are usually introduced at the positions of double bonds, but are not terminal as in petrochemical polyols. The present paper reports part of the results of the research activity performed in the EU project FORBIOPLAST, Grant Agreement no. 212239, that was focused on the use of by-products from wood as raw materials for the production of soft polyurethane foams through innovative sustainable synthetic processes with reduced energy consumption. Attention has been focused on the production of flexible foams by lignin with an environmentally-friendly pathway. The lignin isolated by known methods (physical, chemical or enzymatic treatments) is a mixture of degraded or solubilised lignin, but to enhance the industrial use of lignin, there is need for a continuous supply of lignin products with constant quality as related to purity, chemical composition, and functional properties [4]. Lignin is mainly isolated from wood pulping and papermaking operations where it is used as fuel, and only a small amount of lignin (1-2%) is separated and employed in other kind of products [5]. Industrial unmodified kraft lignin is characterized by a relatively high degree of purity. Kraft lignin contains 1.5–3.0% sulphur, some of it organically bound and some as elemental sulphur [6]. The total hydroxyl content of industrial kraft lignin has been reported at 1.2–1.27 groups per C9 unit, of which 56–60% are phenolic. Replacing polyols derived from petroleum with polyols from renewable resources can have a deep impact on the polyurethane industry. In addition, no success has been reported yet on flexible foams synthesized from polyols derived from lignin. In the literature the preparation of semi-rigid polyurethane foams by using lignin molasses and polyethylene glycol polyols was reported. The investigation carried out in Hatakeyama’s laboratory considered various unmodified lignin including Kraft, organosolv and sulphonate varieties which were dissolved into oligoether diols before mixing the solution with a multi-functional aromatic isocyanate [7]. The strategy for improving the reactivity of the lignin OH groups consisted of using the macrodiol as both the solvent for lignin and a co-monomer with the aim of reducing the stiffness of the networks by introducing flexible oligoether sequences as spacers among the rigid aromatic domains formed by

the condensation of lignin macromolecules with the isocyanate. The apparent density was controlled by changing the mixing ratio of lignin polyol and molasses polyol [8]. Several attempts are made to show how lignin can be integrated into material industry processes and to highlight that lignin can be used as a novel and appropriate renewable feedstock rather than a replacement for synthetic phenols, and to overcome the idea that lignin is a waste material or a low value by-product of pulping used as a fuel to fire the pulping boilers [9]. For example, the literature reports approaches for the liquefaction of wood and the use of the liquefied wood for the production of polyester derivatives [10]. The use of lignin fragments as such, or after suitable chemical modifications, as macro-monomers has also been extensively investigated through the implication of both their phenolic and aliphatic hydroxyl groups to prepare polyesters and polyurethanes [11], but none of these polymers have reached a sizable commercial stage, despite their interesting properties. Lignin used as a raw material for the manufacturing of polyurethanes could represent a superior utilisation of a bio-refinery waste.

2.

Experimental

2.1 Materials Lignin was Indulin® AT, provided by MeadWestvaco Corporation, a Kraft pine lignin, free of hemicelluloses, characterized by a relatively high degree of purity (97%). Polyethylene glycol 400 and glycerol from Aldrich were used as solvents to liquefy lignin. Polyols used in the present study were polypropylene glycol triol (PPG triol) and Castor oil (CO), both purchased from Sigma Chemicals, and used as received. PPG triol from Sigma Chemicals has a Molecular weight Mr 4800, functionality of 2.6, and viscosity of 850 mPa.s at 25°C. Isocyanate was ISO 116/1, a polymeric diphenylmethane diisocyanate (MDI) with 25.7% part by weight of NCO content and provided by ELASTOGRAN Italia Spa. Isocyanate in excess of that needed to react with the OH groups on the polyols reacted with distilled water to form CO2, which acted as the foam blowing agent. The catalysts for the production of polyurethane can be divided into gelling catalysts and blowing catalysts. Gelling and blowing must be kept in proper balance in order to obtain the desired product. The catalysts used in the present study were kindly provided by Air Products and belong to two different series: classic catalysts and new generation catalysts (NE), with a low emission of amine. The classic catalysts consisted of non-reactive amines that can be released from the foams once it is produced. They were DABCO 33LV (triethylenediamine 67% by weight, dipropylene glycol 33% by weight, gelling agent) and DABCO BL11 (bisdimethylamminoethylether 70% by weight, dipropylene glycol 30% by weight, blowing agent). The series of catalysts NE comprised: DABCO NE1070 (3 dimethylaminopropyl urea, gelling agent) and DABCO NE300 (N-[2[2(dimethylamino) ethoxy]ethyl]-N-methyl-1,3-propanediamine, blowing agent); this series of catalysts can reduce emissions because the molecules that act as catalysts can chemically bind themselves to the polyurethane matrix. DABCO DC2525, a silicone surfactant made from 70% by weight of polysiloxane, was used to ensure the uniformity of structure and enhance the cell opening. 2.2 Liquefaction of lignin

The first step for the production of the foams was the dissolution of lignin, previously dried for 24 hours in an oven with air circulation at a temperature of 80 °C. Samples of lignin with the addition of liquefying polyols (glycerol and PEG400), at predetermined weight ratio, were placed in glass flasks and subjected to microwave treatment in a microwave oven. Parameters such as heat power (180 W) working at a temperature of 135 °C, and the duration of heating, about 3 min, were adjusted. The number of OH values of liquefied lignin calculated with titration is reported in Table 1, while the composition used for the preparation of liquefied lignin are reported in the first line of Table 2 for sample prepared with PPG triol, and in the first line of Table 3 for samples prepared with castor oil. 2.3 Foam preparation After mixing the components reported in Table 2, when PPG triol is used, and in Table 3 when Castor Oil is used, the foaming mixture was deposited into an open container (85 x 85 x 55 mm) and allowed to rise freely (free-rise process). The foaming mixture was also deposited into a mould that was then closed thus forcing the foam to take on the shape of the mould (controlled-rise process). All the foam samples were allowed to cure at ambient conditions for a minimum of 2 days. 2.4 Methods With a procedure similar to ASTM D284969 “Methods of Testing Urethane Foam Polyol Raw Materials,” the hydroxyl numbers of liquefied lignin was determined as follows: 1 g liquefied lignin and 25 ml of phthalate reagent were heated for 20 min at 110 °C. This was followed by the addition of 50 ml of pure 1.4-dioxane and 25 ml of distilled water, and the mixture was titrated with a 1M sodium hydroxide solution to the equivalence point. The phthalate reagent consisted of a mixture of 150 g phthalic anhydride, 24.2 g imidazol and 1000 g dioxane. The hydroxyl number in mg KOH/g of the sample was calculated by the following equation: Hydroxyl number (OH) = [(B-A) x N] / W where: A is the volume of the sodium hydroxide solution required for titration of liquefied lignin sample (ml), B is the volume of blank solution (ml), N is the normality of the sodium hydroxide solution, and W is the weight of liquefied lignin (g). The amount of isocyanate required for the reaction is calculated using the following equation: NCO/OH= MNCO WNCO / [ MOH WOH + MAd WAd + (2⁄18) WH2O] where: MNCO is the number of isocyanate groups in one gram of isocyanate, WNCO is the weight of isocyanate (g), MOH is the number of hydroxyl groups contained in one gram of polyol (liquefied lignin and chain extender), WOH is the weight of polyol (g), MAd is the number of hydroxyl groups in one gram of additives (catalysts and surfactant), WAd is the weight of additives (g), and WH2O is the weight of water. Thermogravimetric analysis (TGA) measurements were carried out following the ASTM standard procedure D3850-94. The apparatus used was a Rheometric Scientific instrument and the experiments were performed under a nitrogen gas atmosphere. The samples were ground into a fine powder prior to measurement and were heated from room temperature to 900 °C at a rate of 10 °C/min under a nitrogen atmosphere. For dynamic mechanical analysis (DMTA), foams were cut into 20 x 20 x 20 mm cubes and tested under compression mode between two serrated parallel plates with a 25 mm diameter (Gabo

Eplexor 100 N). Storage modulus (E’) and tan δ were recorded at a frequency of 1 Hz over the temperature range from –150 to 120 °C. The temperature ramp rate was controlled at 2 °C/min. Based on ASTM D 3574-05, the density and 50% compression force deflection (CFDV) of flexible polyurethane foams, were measured. The ASTM D 3574-05 describes the standard test methods for flexible cellular materials including slab, bonded, and moulded urethane foams. The sample size was 30 mm x 30 mm x 30 mm. The result of dividing between weight (M) and volume (V) of the specimens was the density in the unit of kg/m3. The compression force deflection test was performed using Instron 1185 equipment according to the following procedure: pre-flex the specimen twice 75 to 80% of its original thickness at 240 mm/min; rest for 6 min; bring the compression foot into contact with the specimen and determine the thickness after applying a contact load of 140 Pa; compress the specimen 50% of this thickness at 50 mm/min, observe the final load after 60 s. At least three replicates were tested for each sample. Compression strength was evaluated as the ratio of the final force at 50% compression of thickness of the specimen (N) after 60 s and the cross section area of the specimen (mm2). A Thermo Scientific NicoletTM 380 FT-IR was used to characterize the functional groups in the flexible polyurethane foams previously ground and then dried in an oven at 105 °C for 4 h. The samples for FT-IR measurement were taken from the centre of foam buns, ground and dried at 105 °C for 2 hours to ensure that no water was adsorbed on the foam. A SEM (Jeol JSM5600LV) was used to study the microstructure of the PU foams. Gold sputtering was applied using an Edwards Sputter Coater prior to investigation. 3.

Results and Discussion

3.1 Production of the Polyurethane Soft Foams The production of polyurethane foams requires reagents with low viscosity, thus the best solution to reach a high amount of lignin in the final material would be the dissolution of lignin directly with the reagents which are suitable to produce the flexible foams, but experimentally it was observed that lignin is insoluble in the selected chain extenders. In various articles reported in the literature, where the liquefaction of lignocellulosic materials [12-15] and the dissolution of lignin [16-17] were studied, the polyols resulted appropriate for obtaining flexible foam; however, they were not suitable solvents to liquefy the lignin. In the present study, polyethylene glycol with a molecular weight of 400 g/mol and glycerol were used as solvents [18, 19] to liquefy lignin. The liquefied lignin is rich in hydroxyl groups, which can be directly used as feedstock for making polymers without further separation or purification. The mixture obtained from the liquefaction of lignin is not suitable for the production of flexible polyurethanes for the excessive viscosity and a very high OH value. It is therefore necessary to add an additional compound capable of reducing the viscosity of the solution and increasing the flexibility of the structure, reducing the glass transition (T g) of the final material. Thus polyols with a low OH value were used as chain extenders such as polypropylene glycol triol (PPG) that has an average functionality of about 3 and a high molecular weight, which gives considerable flexibility to the polymer chain; and castor oil (CO) that is a compound derived from natural sources obtained by esterification of glycerol with various fatty acids. The optimization of liquefaction is aimed at reducing the amount of solvents used while maintaining a mixture with moderate viscosity capable of mixing with the chain extenders. The

solution obtained by mixing and dissolving the lignin and the chain extender can be used as a reagent after determination of the concentration of hydroxyl groups. There are several methods to determine the content of hydroxyl groups of the reagents, but the method reported in ASTM D 2849-69 provides the esterification of polyol with an excess of acetic anhydride or phthalic anhydride, the hydrolysis of un-reacted anhydride with the formation of a dicarboxylic acid, and the titration of the resulting solution with sodium hydroxide. The procedure requires a long time for esterification but, in the literature, similar methods are reported in which the reaction time is drastically reduced by using imidazole as an esterification catalyst [20, 21]. The method adopted in this study is proved to be quite simple and a fast way to get a good estimation of the OH groups in the liquefied lignin. Liquefied lignin was titrated with the procedure described in the methods section, and the results are summarized in Table 1. The values, reported in Table 1, are in agreement with those reported in the literature, for example the OH value for lignin Indulin AT reported in the literature is 6.89 mmol/g [22]. One of the most used techniques for obtaining polyurethanes is the one-shot technique, which consists of a very efficient mixing of all the raw materials involved in polyurethane production (isocyanate, oligo-polyol, chain extenders or crosslink silicon emulsifiers, blowing agents, catalysts) in only one step and in a short amount time. In order to simplify the procedure with the use of many components, a master-batch was prepared based on a mixture of the components that do not react with each other, (e.g., oligo-polyol, water, chain extender, catalysts, etc.). Then it was possible to use only two components: one is the polyolic component containing a mixture of all raw materials except for the isocyanate in the proportions needed, and the second component is the isocyanate. The polyurethane that results is a consequence of the very efficient contact between the isocyanate component and the polyolic component. After mixing, the foaming mixture was deposited into an open container and allowed to rise freely (free-rise process). The foaming mixture was also deposited into a mould that is then closed thus forcing the foam to take on the shape of the mould (controlled-rise process). In the case of close moulded systems, the density is controlled by the mass of polyurethane mixture in the mould since the volume is fixed. Consequently the physical properties of these foams can differ from the properties of free-rise foams. In Tables 2 and 3 the composition of the foams produced and the values of cream time and rise time measured are reported. It is important to measure the cream time and raise time because when water is added to react with the isocyanate in polyurethane production, this reaction produces carbamic acid that is unstable and decomposes to primary amine and carbon dioxide. This carbon dioxide diffuses to existing gas bubbles in the polyol and expands the mixture into foam. Control of the amount of air contained in the polyol raw material is one way for the manufacturers to control the number of nucleation sites in the reacting mixture. These initially small bubbles quickly grow through either gaining gas from the diffusing carbon dioxide or by coalescing with other bubbles. In the production of flexible polyurethane foams there are two main reactions: the blow reaction and the gelation reaction. If the gelation (cross-linking) reaction occurs too quickly, close-celled foam may result. If the blow (gasproducing) reaction occurs too quickly, the cells may open before the polymer has enough strength to maintain the cellular structure, resulting in collapse of the foam. These two reactions must be kept in proper balance in order to obtain the desired product. The kinetically faster reaction, waterisocyanate reaction, quickly forms polyurea segments and releases CO2 gas to expand the mixture. The hydroxyl-isocyanate reaction gradually polymerizes isocyanates and polyols building up molecular weight. At a critical conversion, the entire system crosses the thermodynamic boundary

of a miscible system, and phase separation occurs [23, 24]. The resulting polymer is a segmented block copolymer with domains that are rich in either polyurea segments or polyol segments [25]. Within a polyurea-rich hard domain, further association of the segments can also occur through hydrogen bonding [26, 27]. Thus a common method to determine the reaction rate of polyurethane foam formation is to monitor the cream time, the gel time and the rise time [28]. The cream time is defined as the time for the polyol and the isocyanate to change from a clear colour to a creamy colour (liquefied lignin has a very dark colour, but it was possible to recognize a change in colour when cream time was reached); the gel time is the time needed for an infinite network to be formed; and the rise time is the time needed for the foam to fully expand. The FT-IR spectra of samples based on the same components were extremely similar. FT-IR spectra of two representative samples based on lignin and PPG are shown in Figure 1. The FT-IR spectrum confirmed qualitatively the presence of urethane linkages. They are well represented by the characteristic -NH stretching vibration region (3200-3500 cm-1) and the characteristic -CO vibration region (1700-1730 cm-1). The characteristic bonds in the material were the same even with a varying composition; only some small differences between samples can be found in the intensity of some absorption peaks. The un-reacted isocyanate (-NCO group) was shown by a peak centred at 2275 cm-1. Even if the NCO/OH ratio was less than one, there can be a small amount of un-reacted isocyanate due to steric hindrance of lignin structure. The different ratios between the intensity of absorption peaks at 1710 and 1640 cm-1 were an index of phase separation in the material [29]. The peak at 1640 cm-1, in fact, was typical of bi-dentate urea that interacted through hydrogen bonds with the surrounding molecules, and the peak at 1710 cm-1 indicated the presence of free urea. The hydrogen-bonded urea, including both mono-dentate and bi-dentate, were an indication of hard domain ordering. The FT-IR spectra of two representative samples of the foams produced with CO are shown in Figure 2 and are similar to those obtained from samples containing PPG, but are characterized by a decrease of the absorption peak at 1640 cm-1 (associated with the H-bonded carbonyl in ordered hard domains) that is an index of lower phase separation of these samples [30] than the ones obtained with PPG. 3.2 Thermo Gravimetric Analysis The thermo-degradation stages of a polyurethane network based on vegetable oil have been commonly studied through TGA [31], and it was demonstrated that these polyurethanes show two or three degradation phases. In samples based on lignin and PPG triol or CO, thermal degradation started at 240 °C and presented two degradation phases, indicating that the same degradation mechanism can occur independently from the nature of the reagents (Figure 3). The first degradation step was associated with the degradation of urethane bonds which are known to dissociate and re-associate simultaneously at about 160 °C and to have an irreversible degradation at around 200 °C, depending on the nature of the polymer. The two degradation phases at 355 °C and 440 °C were associated with the presence of two phases within the material, where the event at 355 °C is related to soft segments and the event at 440 °C to the thermal degradation of hard segments in which the lignin is inserted, as it is also reported in the literature [32, 33]. In the samples of foams made using CO, however, the distinction between the two degradations was less pronounced. This can anticipate an intense mixing of the two phases. The residue at 1000 °C was less than 40% and was higher for the free rise foam. This can be attributed to a difficult heat transfer in the sample, caused by a very regular and uniform cell structure [34].

3.3 Dynamic Mechanical Thermal Analysis Figure 4 shows the modulus profile and tan δ of samples with lignin, PPG as a chain extender and standard catalyst, while Figure 5 shows the modulus profile and tan δ of samples with lignin, PPG as chain extender and an NE catalyst. In the modulus profiles two different transitions are shown, one related to the flexible domain and one to the hard domain, both domains having a distinct Tg and mechanical stiffness. The polyol-rich domains, also called soft domains, have a low glass transition temperature (Tg) that is usually between –50 °C to –70 °C. The low Tg domains give its visco-elastic properties to the polyurethane flexible foam and allow absorption and dissipation of energy. In comparison, the polyurea-rich hard domains have a much higher Tg, generally above 100 °C. The high Tg hard domains provide polyurethane flexible foam with its modulus and thermal stability. These hard domains are not generally covalently bound together but derive their cohesive strength from physical associations. However, since most hard segments will eventually be bound into the polyol phase via the gelation reaction, it is considered that the properties of these polymers depend on a network of covalent and physical cross-links. The presence of hard domains in segmented polyurethanes is very important for their mechanical properties. The tan δ curves of all foams exhibited peaks at the same temperature. The soft phase in all foams based on PPG presented a Tg at -50 °C. The Tg of the soft phase of all PPG foams determined from tan δ peak remains the same with increasing the content of lignin in the foams; however the tan δ peak heights were significantly reduced. The smaller peaks indicated that a large part of the soft segment was mixed with a higher Tg component (lignin polyol). In fact, the peak height decreased when lignin content increased. Thus it is reported that soft and hard segments can be partially miscible. For example, Lee and Tsai [35] studied the effect of different types of diisocyanates on the properties of polyurethane material with hard segment content of 40% produced by bulk polymerization of a poly(tetramethylene ether) glycol and 1.4-butanediol and various diisocyanates. Their data showed that the thermal transitions are influenced significantly by the diisocyanate structure. In the segmented polyurethane materials with aliphatic hard segment, the polyether soft segment resulted immiscible with the hard segment. However, in the segmented polyurethane materials with an aromatic hard segment, the soft segment was partially miscible with the hard segment. In the samples with CO as a chain extender (Figure 6), the foam showed several transitions. The peak of foams with CO and liquefied lignin had a broad shape, different from the foam with PPG, which may be explained by a wider distribution of crosslink density and a lower homogeneity of the structure of the foam, as can be seen from SEM images. The storage modulus E' drops first gradually then exhibits a relatively small drop around -90 ° C and another large drop of about 0 °C during the transition from the glassy to the rubbery states. The first small drop was attributed to the β-transition and the second to the glass transition. The β-transition is related to the rotation [36] or movement of the dangling chains of castor oil fatty acids [37]. From Figure 6 it is clear that the Tg decreased as the amount of castor oil increased and that the intensity of the peak decreases with increasing NCO/OH ratio; peaks of lesser intensity are then related to less flexible foams. 3.4 Mechanical Properties and Morphology

Table 4 shows the density of the samples prepared with lignin and the chain extenders. Series 1 was produced in free-rise expansion, while Series 2 was produced with controlled rise expansion. In the samples with PPG the density decreased as the amount of lignin was increased, and samples with standard catalysts showed lower density than the samples with NE catalysts. Samples with CO as a chain extender showed higher density probably due to a less regular structure. Compression force deflection test measures the force necessary to produce a 50% compression over the entire top surface area of the foam specimen. In Figures 7a and 7b, the values of compression force versus density for the value for samples based on lignin and a glycerol/PEG400 ratio 1/0.4/2 are reported. The polyol flexible segments impart flexibility to polyurethanes and are responsible for their high elongation at break, low temperature resistance and low Tg. Two important properties of polyols play a dominant role in the properties of the final polyurethane polymers. These properties are functionality and equivalent weight. The functionality of polyols can be defined as the average number of functional groups reacting to isocyanate per molecule of polyols and the equivalent weight of polyols can be defined as the ratio between molecular weight of the polyol and functionality. For example, Petrović and coworkers [38] studied the effect of the NCO/OH molar ratio on properties of polyurethane based on soy polyol that was derivative of the soy triglyceride having a molecular weight of 874 g mol-1 and functionality 3.6. In that paper it was found that with a high NCO/OH ratio it was rigid and brittle in nature, while with a low ratio, the samples became more and more flexible and from thermal analysis they observed that the Tg of the cast polyurethane decreased when the NCO/OH molar ratio was decreased from 64 °C for the NCO/OH ratio of 1.05 to below 0 °C for the NCO/OH ratio of 0.4. In the present study, all of the samples with PPG as a chain extender, after the preflex load and recover time of 6 min, showed a small decrease in thickness, circa 2% of its initial thickness. The value of compression force deflection increased linearly as the apparent density increased for the samples with PPG. Low functionality, low hydroxyl number, and secondary hydroxyl groups lead to a low reactivity for CO in direct utilisation for the production of rigid polyurethane foams. Although both CO and processed oil polyols are considered as natural oil polyols, their resulting elastomers differ in mechanical properties [39]. By mixing CO with polyols such as glycerol (for example 75% CO, and 25% glycerol) a higher hydroxyl number polyol mixture is obtained, which leads to rigid PU foams with good physical-mechanical properties [40]. Unlike other processed natural oil polyols, unprocessed CO containing hydroxyls has naturally been experimented as a potential polyol for the synthesis of an elastomer. In the present study the addition of CO was shown to lower Young’s modulus and improve elongation at break. The foams based on CO had a higher density when compared to PPG foam, but in this case the density and compression force deflection value cannot be directly correlated because the structure of the foam is less regular; thinner cell walls and larger foam cells cause the compressive strength to decrease even if the density is higher than in the samples with PPG. In Figure 8, the results for the samples with higher contents of lignin prepared with lignin/glycerol/PEG400 ratio of 1/0.2/1 are shown. The strength is proportional to the density and is related to the amount of lignin that because of its aromatic rings led to an increase in chain stiffness. A higher compression force deflection indicates that the foam is firmer resulting from either a higher crosslink density or a higher foam density or both.

A combination of both the foam gas bubbles, or cells, and the polymer phase morphology contributes to the final properties of flexible foams. The cellular structure can be described as a collection of tetrakaidecahedral shaped cells, which influences mechanical properties of the foams via a number of parameters. The foam modulus is most influenced by foam density, typically described by a power law relationship [41]. At the micro-structural level, polymer phase morphology varies at different length scales. The important parameters that affect the mechanical properties of foams are thickness and length of cell strut. The hypothesis of open cell structure is confirmed by the morphological analysis performed by scanning electron microscopy. Figure 9 shows SEM images of foams produced with PPG triol as a chain extender, while Figure 10 shows micrographs of foams produced with castor oil as a chain extender. The cells shape of the foam is polyhedral and shows preferential orientation associated with the polyurethane rise direction. Samples produced in the free expansion, labelled as Sample 2, had slightly larger cells than those produced in controlled rise (Series 1), and in the controlled rise samples some of the intercellular membranes are not completely broken. As it can be seen, the foam designed to be an open-celled structure has very few closed cells. Cell walls are thinner for the samples with PPG, and the shape of the cells is more regular than in the samples with CO as a chain extender. Samples with CO as a chain extender in general showed less regular structure; the cell shape is bigger than samples with PPG and a larger number of untouched intercellular membranes can be found. 4.

Conclusions

The present paper reports the successful green synthesis of soft foams from Kraft lignin. The process is environmentally friendly, based on the use of microwave technology with water as a blowing agent. Two types of chain extenders, polypropylene glycol triol and castor oil, were used in combination with liquefied lignin in order to produce flexible foams. All samples were produced with a NCO/OH ratio less than one because it significantly reduces the degree of crosslinking, resulting in higher flexibility of the material. Samples were produced in free and controlled rise expansion. Lowering the glass transition temperature of the polyol phase was determined to be the most important issue in order to increase the content of liquefied lignin in foam formulation, and ultimately achieving the goal of making soft foam with liquefied lignin. Properties of these foams can be tailored by industrial producers, such as partners of the FORBIOPLAST project according to the desired application. The addition of efficient chain extenders, thus introducing flexible chains in the macromolecular structure, can reduce the glass transition temperature of the materials and generate foams with higher flexibility. It has been demonstrated that controlling the phase mixing will be a key to improve the material performance in various fields of application. The properties of the foams are compatible with applications in packaging, such as packaging of furniture, and for the interior parts of a car seat.

Acknowledgements The authors wish to acknowledge the support from FP7 – KBBE project n° 212239 Forbioplast (Forest Resource Sustainability through Bio-Based Composite Development) to carry out this research.

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Figure Captions Figure 1. FT-IR spectra for samples produced with lignin and Polypropylene Glycol triol (PPG) as a chain extender Figure 2. FT-IR spectra for the samples produced with lignin and Castor Oil (CO) as a chain extender. Figure 3. TGA curves for free rise samples produced with lignin and respectively (a) PPG, and (b) CO as a chain extender Figure 4. DMTA results of samples with lignin, PPG as a chain extender and standard catalyst with different degrees of expansion: (A) samples with 6.7% of lignin, (B) samples with 10.0% of lignin, (C) samples with 11.9% of lignin, (D) samples with 13.3% of lignin. Figure 5 DMTA results of samples with lignin, PPG as a chain extender and NE catalyst with different degrees of expansion: (A) samples with 6.7% of lignin, (B) samples with 10% of lignin, (C) samples with 11.8% of lignin. Figure 6. DMTA results of samples with lignin, CO as a chain extender with different degrees of expansion: (A) samples with 6.0 % of lignin and standard catalysts, (B) samples with 11.7% of lignin and standard catalysts, (C) samples with 6.4% of lignin and NE catalysts, (D) samples with 12.1% of lignin and NE catalysts. Figure 7. Compression force deflection value for samples with lignin and glycerol/PEG400 ratio 1/0.4/2: (a) samples with PPG as chain extender, (b) samples with castor oil as chain extender. Figure 8. Compression force deflection value for samples with lignin Indulin, PPG as a chain extender and glycerol/PEG400 ratio 1/0.2/1. Figure 9. SEM images (50X) of samples with lignin and PPG as a chain extender Figure 10. SEM images (50X) of samples with lignin and CO as a chain extender

Table 1 OH values of liquefied lignin calculated with titration, Lignin/Glycerol/PEG400 (g)

OH value

Lignin

(mmol/g)

(% by weight)

1/0.4/2

6.61

32.8

1/0.2/1

6.53

45.4

1/0.2/1.5

6.45

37.0

Table 2 Formulations (grams) used for synthesis of PUs with polypropylene glycol triol (PPG) as chain extender and values of cream time and rise time (s) I-PPG-A

I-PPG-B

I-PPG-C

I-PPG-D

I-PPG-E

I-PPG-F

I-PPG-G

1/0.4/2

1/0.4/2

1/0.2/1.5

1/0.2/1.5

1/0.2/1.5

1/0.2/1.5

1/0.2/1.5

Liquefied lignin

14.2

11.4

10.0

10.0

10.0

10.0

10.0

PPG triol

20.8

16.6

14.0

10.0

7.6

14.0

10.0

Water

0.6

0.7

0.66

0.66

0.66

0.65

0.65

DABCO33LV(gelling)

0.53

0.36

0.34

0.31

DABCOBL11(blowing)

0.53

0.35

0.31

0.31

DABCONE1070(gelling)

-

0.9

-

-

-

0.63

0.6

DABCONE300(blowing)

-

0.35

-

-

-

0.2

0.22

Surfactant

0.35

0.56

0.26

0.21

0.17

0.21

0.2

Isocyanate

25.0

23.0

20.0

16.8

15.0

20.0

16.9

Cream time (s)

15

20

25

25

30

40

50

Rise time (s)

15

45

40

45

50

120

135

Lignin (% by weight)

6.7

6.7

10.0

11.9

13.3

10.0

11.8

Lignin/Glycerol/PEG400

Table 3 Formulations (grams) used for synthesis of PUs with Castor Oil (CO) triol as chain extender and values of cream time and rise time (s) I-CO-A

I-CO-B

I-CO-C

I-CO-D

1/0.4/2

1/0.4/2

1/0.2/1

1/0.2/1

Liquefied lignin

17.0

17.0

23.0

23.0

Castor oil

27.5

27.5

23.5

23.5

Water

0.81

0.5

0.32

0.34

DABCO33LV(gelling)

0.67

-

0.41

-

DABCOBL11(blowing)

0.67

-

0.26

-

DABCONE1070(gelling)

-

1.24

-

0.74

DABCONE300(blowing)

-

0.25

-

0.16

Surfactant

0.45

0.66

0.29

0.3

Isocyanate

36.8

32.0

25.1

22.3

Cream time (s)

30

60

40

80

Rise time (s)

45

130

90

150

Lignin (% by weight)

6.0

6.3

11.7

12.1

Renewable resources (% by weight)

40.3

42.7

43.4

45.5

Lignin/Glycerol/PEG400

Table 4 Density of foams prepared in free rise expansion (1) and controlled rise expansion (2) Sample Apparent density (kg/m3) 1 2 86 130 I-PPG-A 12 145 I-PPG-B 121 144 I-CO-A 146 209 I-CO-B 55 80 I-PPG-C 59 62 I-PPG-D 64 69 I-PPG-E 75 93 I-PPG-F 73 101 I-PPG-G 140 183 I-CO-C 143 167 I-CO-D

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Figure(s)

Green synthesis of flexible polyurethane foams from liquefied lignin Patrizia Cinelli, Irene Anguillesi, Andrea Lazzeri, The present paper reports the innovative, sustainable, and green synthesis of soft foams from Kraft lignin. This research activity was performed in the EC project FORBIOPLAST grant agreement no. 212239 focused on the use valorisation of forest resources which include lignin as by-products of wood industries and of bioethanol production. In order to produce flexible foams two type of chain extender were used in combination with liquefied lignin: polypropilenglycol triol and castor oil. The samples were produced with the "one shot" technique and the only blowing agent used was water. All samples were produced with a ratio NCO/OH less than one, because it is well known that it significantly reduces the degree of crosslink, resulting in higher flexibility of the material. Samples were produced in free and controlled rise expansion. The properties of these foams can be modulated by industrial producer, such as partner of the FORBIOPLAST project, by the individuation of the most efficient chain extenders, thus introducing flexible chains in the macromolecular structure, can reduce the glass transition temperature of the materials and generates foams with higher flexibility. The qualityof the foams are compatible with application in packaging, such as packaging of furniture, and for the interior part of car seat.

HIGHLIGHTS

We report a green process for the synthesis of soft polyurethane by lignin The process is based on microwave technology for liquefaction of lignin Polypropylene glycol triol and castor oil are efficient for production of soft foams Controlling the phase mixing is a key to improve the material performance Properties of these foams can be tailored by industrial producers according to the desired application