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Polyurethane foams from liquefied orange peel wastes
Accepted Manuscript Title: Polyurethane foams from liquefied orange peel wastes Authors: Idalina Domingos, Jos´e Ferreira, Lu´ısa Cruz-Lopes, Bruno Esteves PII: DOI: Reference:
S0960-3085(18)30420-6 https://doi.org/10.1016/j.fbp.2019.04.002 FBP 1063
To appear in:
Food and Bioproducts Processing
Received date: Revised date: Accepted date:
3 July 2018 18 March 2019 1 April 2019
Please cite this article as: Domingos I, Ferreira J, Cruz-Lopes L, Esteves B, Polyurethane foams from liquefied orange peel wastes, Food and Bioproducts Processing (2019), https://doi.org/10.1016/j.fbp.2019.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyurethane foams from liquefied orange peel wastes Idalina Domingos1, José Ferreira2*, Luísa Cruz-Lopes3 , Bruno Esteves4 1
CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal. (Orcid/0000-0002-4308-1563)
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CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal.
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CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal. (Orcid/0000-0001-7596-8065)
CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal. (Orcid/0000-0001-6660-3128).
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Corresponding author: Phone: (Orcid/0000-0001-7596-8065)
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Polyalcohol liquefaction by acid catalysis is adequate to liquefy orange peel wastes High added value products can be obtained from the polyol A good optimization between isocyanate, catalyst, surfactant and blowing agent is necessary to produce high quality foams
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Waste conversion into value added materials is a growing subject due to environmental concerns. In the production of orange juice high amounts of orange peel waste are generated and although they are used for the extraction of some extractable compounds a new waste is generated not much different from before. The aim of this work was to determine the possibility of efficiently converting orange peel waste into a liquefied material for the production of more environmentally benign polyurethane foams and test the influence of the proportion of isocyanate, catalyst, surfactant and blowing agent in physical and mechanical properties of the foams. Dry orange peel was liquefied using a mixture of ethyleneglycol and glycerol (1:1) as solvents, catalysed by sulphuric acid at 180ºC for 60 min. A ratio of 9:1 solvent/lignocellulosic material was used and 3% of sulphuric acid was added based on the solvent mass. Density, compressive stress at 10% and young modulus were determined for each foam. The results show that a good liquefaction yield can be achieved by polyalcohol liquefaction of orange peel waste and that this material can successfully be converted into a polyurethane foam with satisfying properties. Moreover the results showed that the physical and mechanical properties of the foam could be tailored by a careful choice of the additives used in foam formation.
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Keywords: Liquefaction; orange peel; FTIR. 1. Introduction
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Orange peel is a waste produced in orange juice companies in high amounts each year. Even though there are several uses for this waste, juice companies would certainly benefit from the production of high value products from this waste. The replacement of products that are usually made with fossil oil derivatives by new products based in lignocellulosic materials (preferably lignocellulosic wastes) is, more than a necessity, an obligation. Polyalcohol liquefaction with a suitable catalyst has proven before to be a good way to liquefy several lignocellulosic materials, like wood (Kobayashi et al., 2004; Kurimoto and Tamura, 1999), wheat straw (Chen and Lu, 2009) , barks (Cruz-Lopes et al., 2016; D’Souza et al., 2015; Esteves et al., 2017b), corn bran (Lee et al., 2000), etc. This process may be an important way to convert several lignocellulosic wastes into value added products since it is not significantly affected by the impurities of the materials and can be done with crude glycerol (Hu et al., 2012; Kosmela et al., 2016). Liquefied material was used with success to produce added value products like polyurethane foams (Alma and Shiraishi, 1998; Esteves et al., 2017b; Gama et al., 2015), polyester and polyurethane films (Budija et al., 2009; Kurimoto et al., 2001), adhesives (Esteves et al., 2015) and even as fuel (dos Santos et al., 2016; Seljak et al., 2012). One of the most promising uses for liquefied material is polyurethane foams since it contains high amounts of hydroxyl functional groups which makes this material ideal to substitute the polyol in polyurethane foam production. Due to environmental concerns several work has been done in order to produce polyurethane foams from more environmental benign sources as for example from vegetable oils like olive, canola, grape seed, linseed, and castor oil or epoxidized soybean oil (Zhang et al., 2013;2015a;2015b). There have also been several successful attempts to produce polyurethane foams from liquefied lignocellulosic materials. For example with liquefied corn bran (Lee et al., 2000), cork (Esteves et al., 2017b; Gama et al., 2015), sugar bagasse (Hakim et al., 2011), wood (Ertaş et al., 2014), coffee grains (Soares et al., 2014), soy wool (Hu et al., 2012), wheat straw (Chen and Lu, 2009) or cork rich barks from Pseudotsuga menziesii and Quercus cerris (Cruz-Lopes et al., 2016; Esteves et al., 2017a). The main compounds in polyurethane production are the polyol and the isocyanate but other compounds are essential to customize foams properties, they are: the blowing agent, the catalyst and the surfactant. Each of these compounds has a specific function in the chemical reactions that occur during the formation of the foam. Mainly two competing reactions occur during foam formation. The gelling reaction, which is the reaction of isocyanate with the polyol forming the urethane group in the foam and the expansion reaction that is the reaction between isocyanate and water originating carbamic acid that decomposes into an amine and carbon dioxide which is the responsible for the expansion of the polymer (Mahmood et al., 2016). Nevertheless there are two secondary reactions competing with the gelling and expansion reactions. The first one is when the hydrogen next to the nitrogen in the urethane group reacts with isocyanate in excess to form an allophanate group, reaction that is favoured by high temperature (Mahmood et al., 2016). On the other end amines can also react with additional isocyanate forming distributed urea and leading to the formation of Biuret linkage (Mahmood et al., 2016). Therefore the amount and type of isocyanate used in the reaction of polyurethane formation is critical. The most used isocyanates are toluene di-isocyanate (TDI) and polymeric diphenyl-methane di-isocyanate (MDI) (Mahmood et al., 2016). Since isocyanate is known to give rigidity to the foam while the diol group gives plasticity (Thomson, 2005) a balanced proportion of each agent has to be used. The catalyst increases the velocity of the reaction. There are catalyst acting in the gelling reaction like organometallic compounds (bismuth, iron, mercury and cobalt) or for example 4-diazabicyclo [2, 2, 2] octane (DABCO), and there are other acting in the expansion reaction like tertiary amines
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(Yan et al., 2008). Some commercial catalyst act simultaneously in both reactions, as for example Polycat 34 from Evonik (Esteves et al., 2017b). There are mainly two types of expansion agents. The chemical blowing agents that react with other compound to form a gas, like for instance water that reacts with isocyanate leading to the formation of carbon dioxide or physical blowing agents that vaporize like n-pentane or dichloromethane (Gama et al., 2015; Soares et al., 2014). The surfactant, usually a silicon, is responsible for the emulsification of the components, stabilization of the cell structure and adjustment of the size of the cells (Mahmood et al., 2016). Most of the common uses for polyurethane foams rely on its behaviour in compression. The compression curves of this material have three regions: first an elastic region where the deformation is directly proportional to the applied force and the cell walls bend, secondly a plastic region where the foam deforms due to the collapse of the cells and then a densification region where the density increases reducing the porosity of the foam (Gama et al., 2015). In some cases the second and third regions are indistinguishable therefore compression strength is commonly determined as compressive stress at 10% deformation. The aim of this work was to determine the possibility of efficiently converting orange peel waste in a liquefied material for the production of more environmentally benign polyurethane foams and test the influence of the proportion of isocyanate, catalyst, surfactant and blowing agent in physical and mechanical properties of the foams.
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2. Material and Methods
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2.1. Materials Orange peel was dried at 105ºC for 24 hours and afterwards split and crushed in a mill, sieved in a to separate the orange peel into fractions of >40, 40-60, 60- 80 mesh and powder which is the fraction remaining in the bottom of the sieve. The catalyst, surfactant and isocyanate used were: Polycat 34 ® from Evonik (Essen, Germany) which is a tertiary amine catalyst that primarily promotes the urethane reaction in rigid foam formulations. Tegostab B8404 ® that is a non-hydrolysable polyether polydimethylsiloxane copolymer and MDI Voranate M229 ® acquired from Dow Chemical Company (Michigan, US).
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2.2. Liquefaction Orange peel from juice production was completely dried in an oven at 105ºC, milled in a knives mill, followed by sieving. The 40-60 Mesh fraction was used for the tests. Afterwards 10 g of the material was liquefied in a double shirt reactor with heated oil using a mixture of ethyleneglycol and glycerol (87%) 1:1 as solvents catalysed by sulphuric acid (3%) based on the solvent mass. A ratio of 9:1 solvent/lignocellulosic material was used. The liquefaction process was conducted at 180ºC for 60 min. The liquefied material was then dissolved in methanol and separated from the solid residue using a paper filter. The solid residue was washed again with water to remove any glycerol still found in the residue and later placed in a rotary evaporator to remove the water and methanol. In order to ensure all the water was removed from the liquefied material it was placed in an oven at 105º C overnight. The residue was dried in an oven at 105ºC overnight, removed and cooled in a desiccator and liquefaction yield was determined in accordance to equation 1. 𝑆𝑜𝑙𝑖𝑑 𝑟𝑒𝑠𝑖𝑑𝑢𝑒(𝑔)
Liquefaction yield (%) = (1 − 𝐷𝑟𝑦 𝑂𝑟𝑎𝑛𝑔𝑒 𝑝𝑒𝑒𝑙(𝑔)) × 100
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2.3. Preparation of foams Polyurethane foams were produced with the neutralized liquefied material. Approximately 3 g of the polyol was mixed in a polypropylene glass, with the catalyst (Polycat 34), the blowing
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agent (water) and the surfactant (Tegostab B8404). This mixture was stirred for about 30 s at 750 rpm, in an IKA Ost Basic mixer and then added the polymeric isocyanate (MDI Voranate M229). After the addition of the isocyanate, the mixture was agitated again, for a few seconds at 750 rpm until the chemical reaction starts. This procedure was repeated for the preparation of all the PU foams. Several amounts of each chemical were tested in order to determine the effect of different quantities of isocyanate, catalyst, surfactant and blowing agent on the properties of the foam.
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2.4. Physical and mechanical properties of foams The density of PU foams was determined by the ratio between the mass and the volume of a cylindrical sample. The mass was measured on a digital scale, and the linear dimensions of the sample were assessed with a digital caliper, Mitutoyo Absolute CD − 15DCX, with an uncertainty of ± 0.01 mm. The determination of the mechanical properties was held in a universal test machine Servosis I − 405/5 according to ISO 844 [25] standard with some modifications. Each PU foam slice (cylindrical shape) was placed on the machine and subjected to growing tension. Tests were made in triplicate. Since in most of the foams it is impossible to find a maximum compressive strength, compressive stress at 10% deformation was determined. Equation 1 presents the calculations made to determine compressive stress at 10% (σ_10)
∆𝐹⁄
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F10 is the applied force for 10% of the sample deformation (N); A0 is the area of the base of the cylindrical specimen (mm2). Equation 2 presents the calculations made to determine Young´s modulus.
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𝑌𝑜𝑢𝑛𝑔´𝑠 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 = ∆𝑥 × 1000 (kPa) 𝐴0 ∆𝐹⁄ ∆𝑥 is the slope of the linear zone of the stress vs. deformation curve (N/mm2); h0 is the average height of the cylindrical specimen (mm); A0 is the area of the base of the cylindrical sample (mm2).
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The ability to convert orange peel waste into a usable polyol was tested by liquefaction with a mixture of glycerol and ethyleneglycol (1:1). An optimization of the time and temperature to achieve the best liquefaction yield was done and presented elsewhere (Duarte et al., 2017). A characterization of the initial residue, polyol and un-liquefied residue by FTIR-ATR was also presented. The main conclusions were that liquefaction yield increases with time and temperature although it is possible to achieve a high liquefaction yield even at low temperatures. The solid residue and the original material have similar spectra and both are different from the liquefied material. Results showed that the liquefaction yield was, on average, around 85%, for temperatures from 140ºC to 200ºC, reaching a maximum of 92% at 200ºC. Overall, a higher temperature lead to a higher liquefaction yield but good liquefaction yield could already be obtained for lower temperatures as for example at 140ºC. This allows a significant reduction in costs since higher temperatures imply higher processing costs. The polyol produced at 180ºC for 60 min was used in the production of the foams. The isocyanate used in the production of PU foams has a significant influence on foam properties. In accordance to Thomson (2005) isocyanate gives rigidity to the foam while the diol group gives plasticity. Figure 1 presents the density and compressive properties of the foams for different percentages of isocyanate used in the polymerization. The results show that density
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decreases slightly, from about 53 kg/m3 to 46 kg/m3 in the beginning until 250% isocyanate, being approximately constant until about 450% where density increases, reaching 64 kg/m3. Ertas et al (2014) reported a similar decrease on the density of polyurethane foams made from liquefied eucalyptus and pine woods while Chen and Lu (2009) reported that density decreased with the increase of isocyanate in the production of polyurethane foams made with liquefied wheat straw. Somewhat different results were reported before (Hakim et al., 2011). These authors reported an increase until 1.2 NCO index (from 50 kg/m3 to 55 kg/m3) followed by a decrease (to 45 kg/m3) which, in accordance to them, is due to the fact that isocyanate, when in excess, reacts with urethane and urea groups, forming allophanate and biuret which builds in new three-dimensional networks and hydrogen bonds in the foam decreasing the density. A small increase in density (from about 38 kg/m3 to 40 kg/m3 ) for NCO indexes ranging from 0.95 to 1.25 was also reported before (Yan et al., 2008) for liquefied cornstalk. The different results presented might be due to the competition between the polymerization and expansion reactions that depend, not only on the isocyanate index but also on the blowing agent and catalysts used. Compressive stress at 10% seems to follow a similar pattern to density with an initial decrease from 28 kPa to 20 kPa although there is an increase after 350% isocyanate reaching 33 kPa. The initial decrease is probably due to the decrease in density as reported before (Kim et al., 2008) while the final increase might also be due to the increase in density since the foam was not able to expand more for higher isocyanate indexes. Different results were presented by Yan et al (2008) with an increase until 1.15 NCO index followed by a decrease. The same was reported by Hakim et al (2011) and Ertas et al (2014). The σ10% values ranged from 20-35 kPa, a little lower than the reported by Hakim et al (2011) who obtained compressive stress from 80 kPa to 90 kPa with liquefied sugar-cane bagasse polyol or the somewhat higher values of 150 kPa for pine and 250 kPa for eucalypt based polyols reported by Ertas et al (2014). Likewise Chen and Lu (2009) reported similar compressive stress of PUFs made with liquefied wheat straw with 169- 212 kPa. Young’s modulus is very inconstant but it increases from the initial 132 kPa to 303 kPa which is in accordance to Kim et al (2008) that reported an increase in elasticity for higher isocyanate indexes due to more allophanate crosslinks.
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There are two competing reactions in the synthesis of PU foams. The gelling reaction (reaction of isocyanate with polyol) and the expansion reaction (reaction of isocyanate with water in the polyol), therefore the correct choice of catalyst is crucial to obtain a foam with the desired properties. Figure 2 presents the properties of the foams according to the percentage of catalyst used in the polymerization. The results show that density decreases from 58 kg/m3 at about 8% to 40 kg/m3 for 12% increasing afterwards. There must be an equilibrium between the speed of the gelling reaction that leads to the formation of the foam and the expansion reaction. In accordance to Choe et al.(2004) if the gelling reaction is much faster than the expansion reaction the foam shrinks but if the contrary happens the foam expands well, then blows. Polycat 34 ® from Evonik, the catalyst used here is, according to the producer, a balanced catalyst with a tertiary amine that primarily promotes the urethane reaction in rigid foam formulations. The amounts tested ranged between 8-16% based on polyol amount (php) but it was only possible to achieve a foam for over 7% catalyst while over 20% the reaction was too fast and there was no time to properly mix the compounds. Results presented before (Yan et al., 2008) showed that the increase in the percentage of expansion catalyst or gelling catalyst led to a decrease in density. This only happened until about 12%. Contrary results were reported by Choe et al (2004) and by Seo et al (2004) that concluded the increase in the percentage of expansion catalyst or increased percentage of gelling catalyst, had no significant influence on density. These last authors however concluded that the amounts of each catalyst influenced cream, gel and tack-free time. Higher amounts of blowing catalyst led to faster cream time, and higher amounts of gelling catalyst to faster gel time and tack-free time.
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Compressive stress at 10% deformation increases with the increase in catalyst from about 3.5 kPa to about 6.6 kPa. The increase was higher for smaller amounts of catalyst. Since the catalyst used catalyses both reactions, this results are difficult to compare with similar works. The results presented by Seo et al (2004) showed a small increase followed by a decrease in the compressive stress for higher amounts of gelling catalyst while no significant differences were observed for the blowing catalyst. Somewhat different results were presented by Choe et al(2004) that reported an increase in density and compressive stress for blowing catalyst increase but only for HFC/water blowing agent. Young’s modulus follows a similar pattern increasing from about 186 kPa to about 321 kPa although after 10% a small decrease is observed to about 300 kPa.
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Figure 3 presents the properties of the foams according to the percentage of blowing agent used in the polymerization. The results show that density decreases from 60 kg/m3 at about 5% to 45 kg/m3 for 17% increasing afterwards. The decrease is to be expected since higher amounts of blowing agent leads to a higher expansion of the foam and therefore lower density. This was stated before by several authors (Hakim et al., 2011; Seo et al., 2004; Yan et al., 2008). The results obtained for amounts over 20% water content are probably due to the limit of expansion being attained and for higher amounts of water the excess will prejudice the expansion of the foam. Nevertheless, different results were presented before by Maldas and Shiriashi (1996) who reported that with the increase in blowing agent, density of the foams increased. Compressive stress at 10% deformation decreases sharply with the increase in blowing agent from about 90 kPa to about 20 kPa, increasing very slightly for higher amounts of blowing agent. The slight increase for about 27% of water is probably due to the increase density of the foam due to the lower expansion. Young’s modulus follows a similar pattern decreasing from about 700 kPa to about 200 kPa. The reduction observed initially for density and compressive properties is due to the increased growing of the foam with higher amounts of blowing agent.
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The results on the influence of surfactant amount on the properties of the foams, are presented in Figure 4. Density has small variations ranging from 44 kg/m3 to 48 kg/m3. Compressive stress seems to increase with higher surfactant amount from the initial 7% to a maximum of 26% achieved for 11% php. In relation to Young’s modulus the results were not conclusive. Yan et al (2008) and Hakim et al (2011) studied the effect of surfactant amount on compressive stress and concluded that it displayed nonlinear behaviour in relation to the increase of surfactant amount. In accordance to Lim et al. (2008), however an increase in surfactant decreases the foam density abruptly until a minimum is reached increasing afterwards. These authors stated that the increased amount of surfactant, reduces the size of the cells and surface tension and increases the number of closed cells leading to a decrease in thermal conductivity and increase in the resistance to compression.
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The changes occurring in the foam mechanical properties seem to be dependent on the starting point. For instance when looking at the increase in isocyanate, this leads simultaneously to the increase in the gelling reaction between isocyanate and the polyol and the expansion reaction with the blowing agent (water). Initially the increase in isocyanate seems to increase more the expansion reaction leading to a less dense foam and therefore with weaker mechanical properties. Afterwards, when there is no more water available to increase the expansion reaction the increase in the gelling reaction leads to a more rigid foam with higher density and stronger mechanical properties. So it is expected that when we use a higher amount of water the increase in isocyanate should results in a higher expansion of the foam and weaker mechanical properties. When the amount of water is lower an increase in isocyanate amount would lead to the increase of the gelling reaction resulting in a more rigid and dense foam. In relation to the catalyst used
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there seems to be a contradictory effect on density and mechanical properties, since density decreases and mechanical properties increase with the increased amount of catalyst. With the catalyst used the speed of both reactions is increased, but there should be a higher increase of the speed of the gelling reaction. However, since density decreases this means that the expansion reaction was faster. The increased mechanical properties with lower foam density might be due to the formation of crosslinks. Similar results were presented before by (Choe et al. 2004). An increase in water, increases the expansion reaction producing a less dense and mechanically weaker foam but when the amount of blowing agent is too much the foam blows as reported before (Choe et al. 2004). In accordance to Lim et al (2008) the excessive amount of water leads to a negative pressure gradient, which is due to the rapid diffusion of carbon dioxide through the cell wall leading to cell deformation. The surfactant, seems to have no significant effect on foam density, presenting just a small decrease followed by an equally small increase. Nevertheless higher amounts of surfactant lead to stronger foams which is probably due to the decrease of the cell size as reported before (Lim et al 2008). Comparing these results with a commercial rigid polyurethane foam, the density obtained here is somewhat higher (40-64 kg/m3) while the compressive stress at 10% (18-86 kPa) is significantly lower than the commercial foam (40 kg/m3, 200 kPa) (Mahmood et al 2016), however for some packaging products the achieved mechanical properties are good enough. The optimization is important in order to produce less expensive and more environmentally benign foams by using less additives. Nevertheless these results show that it is possible to attain a polyurethane foam with reasonable properties with liquefied orange peel that can compete with traditional commercial polyurethane foams for some applications using the additives studied in this work. Similar results are expected if additives of the same family are used. Nevertheless new studies are necessary to improve PUFs properties to compete with traditional commercial polyurethane foams. One of the possibilities is to test new catalysts that catalyse only the gelling or the expansion reactions to allow us a better tailoring of the foam properties. Also different surfactants should be tested, since it was proven that the surfactant affects the mechanical properties of the foams. In a near future polyurethane foams can be a good use for orange peel wastes and bring more added value to the food processing companies.
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4. Conclusions The results show that a good liquefaction yield can be achieved by polyalcohol liquefaction of orange peel waste and that this material can successfully be converted into a polyurethane foam. Moreover the results showed that the properties of the foam could be tailored by a careful choice of the additives used in foam formation. Overall higher amounts of isocyanate increase physical and mechanical properties of the foams leading to higher density, compressive strength and young modulus. The catalyst, initially decreases density increasing afterwards but increasing compressive strength and young modulus. The blowing agent decreased density, compressive strength and young modulus while the surfactant had no significant influence on physical and mechanical properties of the foams.
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Acknowledgments This work is financed by national funds through FCT - Fundação para a Ciência e Tecnologia, I.P., under the project UID/Multi/04016/2016. Furthermore we would like to thank the Instituto Politécnico de Viseu and CI&DETS for their support.
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Kurimoto, Y., Takeda, M., Doi, S., Tamura, Y., Ono, H., 2001. Network structures and thermal properties of polyurethane films prepared from liquefied wood. Bioresour. Technol. 77, 33–40. Kurimoto, Y., Tamura, Y., 1999. Species effects on wood-liquefaction in polyhydric alcohols. Holzforschung 53, 617–622. Lee, S.-H., Yoshioka, M., Shiraishi, N., 2000. Liquefaction of corn bran (CB) in the presence of alcohols and preparation of polyurethane foam from its liquefied polyol. J. Appl. Polym. Sci. 78, 319–325. Lim, H., Kim, S.H., Kim, B.K., 2008. Effects of silicon surfactant in rigid polyurethane foams. Express Polym. Lett. 2, 194–200. https://doi.org/10.3144/expresspolymlett.2008.24 Mahmood, N., Yuan, Z., Schmidt, J., Xu, C. (Charles), 2016. Depolymerization of lignins and their applications for the preparation of polyols and rigid polyurethane foams: A review. Renew. Sustain. Energy Rev. 60, 317–329. https://doi.org/10.1016/j.rser.2016.01.037 Maldas, D., Shiraishi, N., 1996. Liquefaction of wood in the presence of polyol using NaOH as a catalyst and its application to polyurethane foams. Int. J. Polym. Mater. 33, 61–71. Seljak, T., Oprešnik, S.R., Kunaver, M., Katrašnik, T., 2012. Wood, liquefied in polyhydroxy alcohols as a fuel for gas turbines. Appl. Energy 99, 40–49. Seo, W.J., Park, J.H., Sung, Y.T., Hwang, D.H., Kim, W.N., Lee, H.S., 2004. Properties of water-blown rigid polyurethane foams with reactivity of raw materials. J. Appl. Polym. Sci. 93, 2334–2342. Soares, B., Gama, N., Freire, C., Barros-Timmons, A., Brandão, I., Silva, R., Pascoal Neto, C., Ferreira, A., 2014. Ecopolyol production from industrial cork powder via acid liquefaction using polyhydric alcohols. ACS Sustain. Chem. Eng. 2, 846–854. Thomson, T., 2005. Polyurethanes as specialty chemicals: principles and applications. CRC Press, Boca Raton, Fla. Yan, Y., Pang, H., Yang, X., Zhang, R., Liao, B., 2008. Preparation and characterization of water‐ blown polyurethane foams from liquefied cornstalk polyol. J. Appl. Polym. Sci. 110, 1099–1111. Zhang, C., Xia, Y., Chen, R., Huh, S., Johnston, P., Kessler, M., 2013, Soy-castor oil based polyols prepared using a solvent-free and catalyst-free method and polyurethanes therefrom. Green Chemistry, 15(6), p. 1477. Zhang, C., Kessler, M. 2015. Bio-based polyurethane foam made from compatible blends of vegetable-oil-based polyol and petroleum-based polyol. ACS Sustainable Chemistry and Engineering, 3(4), 743-749. Zhang, C., Madbouly, S., Kessler, M. 2015. Biobased polyurethanes prepared from different vegetable oils. ACS applied materials and interfaces, 7(2), 1226-1233.
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S0960-3085(18)30420-6 https://doi.org/10.1016/j.fbp.2019.04.002 FBP 1063
To appear in:
Food and Bioproducts Processing
Received date: Revised date: Accepted date:
3 July 2018 18 March 2019 1 April 2019
Please cite this article as: Domingos I, Ferreira J, Cruz-Lopes L, Esteves B, Polyurethane foams from liquefied orange peel wastes, Food and Bioproducts Processing (2019), https://doi.org/10.1016/j.fbp.2019.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyurethane foams from liquefied orange peel wastes Idalina Domingos1, José Ferreira2*, Luísa Cruz-Lopes3 , Bruno Esteves4 1
CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal. (Orcid/0000-0002-4308-1563)
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CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal.
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CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal. (Orcid/0000-0001-7596-8065)
CERNAS / Polytechnic Institute of Viseu, Viseu, Portugal. (Orcid/0000-0001-6660-3128).
*
Corresponding author: Phone: (Orcid/0000-0001-7596-8065)
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Polyalcohol liquefaction by acid catalysis is adequate to liquefy orange peel wastes High added value products can be obtained from the polyol A good optimization between isocyanate, catalyst, surfactant and blowing agent is necessary to produce high quality foams
Abstract
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+351 232-424 651
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HIGHLIGHTS
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email: [email protected]
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Waste conversion into value added materials is a growing subject due to environmental concerns. In the production of orange juice high amounts of orange peel waste are generated and although they are used for the extraction of some extractable compounds a new waste is generated not much different from before. The aim of this work was to determine the possibility of efficiently converting orange peel waste into a liquefied material for the production of more environmentally benign polyurethane foams and test the influence of the proportion of isocyanate, catalyst, surfactant and blowing agent in physical and mechanical properties of the foams. Dry orange peel was liquefied using a mixture of ethyleneglycol and glycerol (1:1) as solvents, catalysed by sulphuric acid at 180ºC for 60 min. A ratio of 9:1 solvent/lignocellulosic material was used and 3% of sulphuric acid was added based on the solvent mass. Density, compressive stress at 10% and young modulus were determined for each foam. The results show that a good liquefaction yield can be achieved by polyalcohol liquefaction of orange peel waste and that this material can successfully be converted into a polyurethane foam with satisfying properties. Moreover the results showed that the physical and mechanical properties of the foam could be tailored by a careful choice of the additives used in foam formation.
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Keywords: Liquefaction; orange peel; FTIR. 1. Introduction
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Orange peel is a waste produced in orange juice companies in high amounts each year. Even though there are several uses for this waste, juice companies would certainly benefit from the production of high value products from this waste. The replacement of products that are usually made with fossil oil derivatives by new products based in lignocellulosic materials (preferably lignocellulosic wastes) is, more than a necessity, an obligation. Polyalcohol liquefaction with a suitable catalyst has proven before to be a good way to liquefy several lignocellulosic materials, like wood (Kobayashi et al., 2004; Kurimoto and Tamura, 1999), wheat straw (Chen and Lu, 2009) , barks (Cruz-Lopes et al., 2016; D’Souza et al., 2015; Esteves et al., 2017b), corn bran (Lee et al., 2000), etc. This process may be an important way to convert several lignocellulosic wastes into value added products since it is not significantly affected by the impurities of the materials and can be done with crude glycerol (Hu et al., 2012; Kosmela et al., 2016). Liquefied material was used with success to produce added value products like polyurethane foams (Alma and Shiraishi, 1998; Esteves et al., 2017b; Gama et al., 2015), polyester and polyurethane films (Budija et al., 2009; Kurimoto et al., 2001), adhesives (Esteves et al., 2015) and even as fuel (dos Santos et al., 2016; Seljak et al., 2012). One of the most promising uses for liquefied material is polyurethane foams since it contains high amounts of hydroxyl functional groups which makes this material ideal to substitute the polyol in polyurethane foam production. Due to environmental concerns several work has been done in order to produce polyurethane foams from more environmental benign sources as for example from vegetable oils like olive, canola, grape seed, linseed, and castor oil or epoxidized soybean oil (Zhang et al., 2013;2015a;2015b). There have also been several successful attempts to produce polyurethane foams from liquefied lignocellulosic materials. For example with liquefied corn bran (Lee et al., 2000), cork (Esteves et al., 2017b; Gama et al., 2015), sugar bagasse (Hakim et al., 2011), wood (Ertaş et al., 2014), coffee grains (Soares et al., 2014), soy wool (Hu et al., 2012), wheat straw (Chen and Lu, 2009) or cork rich barks from Pseudotsuga menziesii and Quercus cerris (Cruz-Lopes et al., 2016; Esteves et al., 2017a). The main compounds in polyurethane production are the polyol and the isocyanate but other compounds are essential to customize foams properties, they are: the blowing agent, the catalyst and the surfactant. Each of these compounds has a specific function in the chemical reactions that occur during the formation of the foam. Mainly two competing reactions occur during foam formation. The gelling reaction, which is the reaction of isocyanate with the polyol forming the urethane group in the foam and the expansion reaction that is the reaction between isocyanate and water originating carbamic acid that decomposes into an amine and carbon dioxide which is the responsible for the expansion of the polymer (Mahmood et al., 2016). Nevertheless there are two secondary reactions competing with the gelling and expansion reactions. The first one is when the hydrogen next to the nitrogen in the urethane group reacts with isocyanate in excess to form an allophanate group, reaction that is favoured by high temperature (Mahmood et al., 2016). On the other end amines can also react with additional isocyanate forming distributed urea and leading to the formation of Biuret linkage (Mahmood et al., 2016). Therefore the amount and type of isocyanate used in the reaction of polyurethane formation is critical. The most used isocyanates are toluene di-isocyanate (TDI) and polymeric diphenyl-methane di-isocyanate (MDI) (Mahmood et al., 2016). Since isocyanate is known to give rigidity to the foam while the diol group gives plasticity (Thomson, 2005) a balanced proportion of each agent has to be used. The catalyst increases the velocity of the reaction. There are catalyst acting in the gelling reaction like organometallic compounds (bismuth, iron, mercury and cobalt) or for example 4-diazabicyclo [2, 2, 2] octane (DABCO), and there are other acting in the expansion reaction like tertiary amines
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(Yan et al., 2008). Some commercial catalyst act simultaneously in both reactions, as for example Polycat 34 from Evonik (Esteves et al., 2017b). There are mainly two types of expansion agents. The chemical blowing agents that react with other compound to form a gas, like for instance water that reacts with isocyanate leading to the formation of carbon dioxide or physical blowing agents that vaporize like n-pentane or dichloromethane (Gama et al., 2015; Soares et al., 2014). The surfactant, usually a silicon, is responsible for the emulsification of the components, stabilization of the cell structure and adjustment of the size of the cells (Mahmood et al., 2016). Most of the common uses for polyurethane foams rely on its behaviour in compression. The compression curves of this material have three regions: first an elastic region where the deformation is directly proportional to the applied force and the cell walls bend, secondly a plastic region where the foam deforms due to the collapse of the cells and then a densification region where the density increases reducing the porosity of the foam (Gama et al., 2015). In some cases the second and third regions are indistinguishable therefore compression strength is commonly determined as compressive stress at 10% deformation. The aim of this work was to determine the possibility of efficiently converting orange peel waste in a liquefied material for the production of more environmentally benign polyurethane foams and test the influence of the proportion of isocyanate, catalyst, surfactant and blowing agent in physical and mechanical properties of the foams.
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2.1. Materials Orange peel was dried at 105ºC for 24 hours and afterwards split and crushed in a mill, sieved in a to separate the orange peel into fractions of >40, 40-60, 60- 80 mesh and powder which is the fraction remaining in the bottom of the sieve. The catalyst, surfactant and isocyanate used were: Polycat 34 ® from Evonik (Essen, Germany) which is a tertiary amine catalyst that primarily promotes the urethane reaction in rigid foam formulations. Tegostab B8404 ® that is a non-hydrolysable polyether polydimethylsiloxane copolymer and MDI Voranate M229 ® acquired from Dow Chemical Company (Michigan, US).
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2.2. Liquefaction Orange peel from juice production was completely dried in an oven at 105ºC, milled in a knives mill, followed by sieving. The 40-60 Mesh fraction was used for the tests. Afterwards 10 g of the material was liquefied in a double shirt reactor with heated oil using a mixture of ethyleneglycol and glycerol (87%) 1:1 as solvents catalysed by sulphuric acid (3%) based on the solvent mass. A ratio of 9:1 solvent/lignocellulosic material was used. The liquefaction process was conducted at 180ºC for 60 min. The liquefied material was then dissolved in methanol and separated from the solid residue using a paper filter. The solid residue was washed again with water to remove any glycerol still found in the residue and later placed in a rotary evaporator to remove the water and methanol. In order to ensure all the water was removed from the liquefied material it was placed in an oven at 105º C overnight. The residue was dried in an oven at 105ºC overnight, removed and cooled in a desiccator and liquefaction yield was determined in accordance to equation 1. 𝑆𝑜𝑙𝑖𝑑 𝑟𝑒𝑠𝑖𝑑𝑢𝑒(𝑔)
Liquefaction yield (%) = (1 − 𝐷𝑟𝑦 𝑂𝑟𝑎𝑛𝑔𝑒 𝑝𝑒𝑒𝑙(𝑔)) × 100
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2.3. Preparation of foams Polyurethane foams were produced with the neutralized liquefied material. Approximately 3 g of the polyol was mixed in a polypropylene glass, with the catalyst (Polycat 34), the blowing
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agent (water) and the surfactant (Tegostab B8404). This mixture was stirred for about 30 s at 750 rpm, in an IKA Ost Basic mixer and then added the polymeric isocyanate (MDI Voranate M229). After the addition of the isocyanate, the mixture was agitated again, for a few seconds at 750 rpm until the chemical reaction starts. This procedure was repeated for the preparation of all the PU foams. Several amounts of each chemical were tested in order to determine the effect of different quantities of isocyanate, catalyst, surfactant and blowing agent on the properties of the foam.
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2.4. Physical and mechanical properties of foams The density of PU foams was determined by the ratio between the mass and the volume of a cylindrical sample. The mass was measured on a digital scale, and the linear dimensions of the sample were assessed with a digital caliper, Mitutoyo Absolute CD − 15DCX, with an uncertainty of ± 0.01 mm. The determination of the mechanical properties was held in a universal test machine Servosis I − 405/5 according to ISO 844 [25] standard with some modifications. Each PU foam slice (cylindrical shape) was placed on the machine and subjected to growing tension. Tests were made in triplicate. Since in most of the foams it is impossible to find a maximum compressive strength, compressive stress at 10% deformation was determined. Equation 1 presents the calculations made to determine compressive stress at 10% (σ_10)
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𝑌𝑜𝑢𝑛𝑔´𝑠 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 = ∆𝑥 × 1000 (kPa) 𝐴0 ∆𝐹⁄ ∆𝑥 is the slope of the linear zone of the stress vs. deformation curve (N/mm2); h0 is the average height of the cylindrical specimen (mm); A0 is the area of the base of the cylindrical sample (mm2).
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The ability to convert orange peel waste into a usable polyol was tested by liquefaction with a mixture of glycerol and ethyleneglycol (1:1). An optimization of the time and temperature to achieve the best liquefaction yield was done and presented elsewhere (Duarte et al., 2017). A characterization of the initial residue, polyol and un-liquefied residue by FTIR-ATR was also presented. The main conclusions were that liquefaction yield increases with time and temperature although it is possible to achieve a high liquefaction yield even at low temperatures. The solid residue and the original material have similar spectra and both are different from the liquefied material. Results showed that the liquefaction yield was, on average, around 85%, for temperatures from 140ºC to 200ºC, reaching a maximum of 92% at 200ºC. Overall, a higher temperature lead to a higher liquefaction yield but good liquefaction yield could already be obtained for lower temperatures as for example at 140ºC. This allows a significant reduction in costs since higher temperatures imply higher processing costs. The polyol produced at 180ºC for 60 min was used in the production of the foams. The isocyanate used in the production of PU foams has a significant influence on foam properties. In accordance to Thomson (2005) isocyanate gives rigidity to the foam while the diol group gives plasticity. Figure 1 presents the density and compressive properties of the foams for different percentages of isocyanate used in the polymerization. The results show that density
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decreases slightly, from about 53 kg/m3 to 46 kg/m3 in the beginning until 250% isocyanate, being approximately constant until about 450% where density increases, reaching 64 kg/m3. Ertas et al (2014) reported a similar decrease on the density of polyurethane foams made from liquefied eucalyptus and pine woods while Chen and Lu (2009) reported that density decreased with the increase of isocyanate in the production of polyurethane foams made with liquefied wheat straw. Somewhat different results were reported before (Hakim et al., 2011). These authors reported an increase until 1.2 NCO index (from 50 kg/m3 to 55 kg/m3) followed by a decrease (to 45 kg/m3) which, in accordance to them, is due to the fact that isocyanate, when in excess, reacts with urethane and urea groups, forming allophanate and biuret which builds in new three-dimensional networks and hydrogen bonds in the foam decreasing the density. A small increase in density (from about 38 kg/m3 to 40 kg/m3 ) for NCO indexes ranging from 0.95 to 1.25 was also reported before (Yan et al., 2008) for liquefied cornstalk. The different results presented might be due to the competition between the polymerization and expansion reactions that depend, not only on the isocyanate index but also on the blowing agent and catalysts used. Compressive stress at 10% seems to follow a similar pattern to density with an initial decrease from 28 kPa to 20 kPa although there is an increase after 350% isocyanate reaching 33 kPa. The initial decrease is probably due to the decrease in density as reported before (Kim et al., 2008) while the final increase might also be due to the increase in density since the foam was not able to expand more for higher isocyanate indexes. Different results were presented by Yan et al (2008) with an increase until 1.15 NCO index followed by a decrease. The same was reported by Hakim et al (2011) and Ertas et al (2014). The σ10% values ranged from 20-35 kPa, a little lower than the reported by Hakim et al (2011) who obtained compressive stress from 80 kPa to 90 kPa with liquefied sugar-cane bagasse polyol or the somewhat higher values of 150 kPa for pine and 250 kPa for eucalypt based polyols reported by Ertas et al (2014). Likewise Chen and Lu (2009) reported similar compressive stress of PUFs made with liquefied wheat straw with 169- 212 kPa. Young’s modulus is very inconstant but it increases from the initial 132 kPa to 303 kPa which is in accordance to Kim et al (2008) that reported an increase in elasticity for higher isocyanate indexes due to more allophanate crosslinks.
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There are two competing reactions in the synthesis of PU foams. The gelling reaction (reaction of isocyanate with polyol) and the expansion reaction (reaction of isocyanate with water in the polyol), therefore the correct choice of catalyst is crucial to obtain a foam with the desired properties. Figure 2 presents the properties of the foams according to the percentage of catalyst used in the polymerization. The results show that density decreases from 58 kg/m3 at about 8% to 40 kg/m3 for 12% increasing afterwards. There must be an equilibrium between the speed of the gelling reaction that leads to the formation of the foam and the expansion reaction. In accordance to Choe et al.(2004) if the gelling reaction is much faster than the expansion reaction the foam shrinks but if the contrary happens the foam expands well, then blows. Polycat 34 ® from Evonik, the catalyst used here is, according to the producer, a balanced catalyst with a tertiary amine that primarily promotes the urethane reaction in rigid foam formulations. The amounts tested ranged between 8-16% based on polyol amount (php) but it was only possible to achieve a foam for over 7% catalyst while over 20% the reaction was too fast and there was no time to properly mix the compounds. Results presented before (Yan et al., 2008) showed that the increase in the percentage of expansion catalyst or gelling catalyst led to a decrease in density. This only happened until about 12%. Contrary results were reported by Choe et al (2004) and by Seo et al (2004) that concluded the increase in the percentage of expansion catalyst or increased percentage of gelling catalyst, had no significant influence on density. These last authors however concluded that the amounts of each catalyst influenced cream, gel and tack-free time. Higher amounts of blowing catalyst led to faster cream time, and higher amounts of gelling catalyst to faster gel time and tack-free time.
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Compressive stress at 10% deformation increases with the increase in catalyst from about 3.5 kPa to about 6.6 kPa. The increase was higher for smaller amounts of catalyst. Since the catalyst used catalyses both reactions, this results are difficult to compare with similar works. The results presented by Seo et al (2004) showed a small increase followed by a decrease in the compressive stress for higher amounts of gelling catalyst while no significant differences were observed for the blowing catalyst. Somewhat different results were presented by Choe et al(2004) that reported an increase in density and compressive stress for blowing catalyst increase but only for HFC/water blowing agent. Young’s modulus follows a similar pattern increasing from about 186 kPa to about 321 kPa although after 10% a small decrease is observed to about 300 kPa.
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Figure 3 presents the properties of the foams according to the percentage of blowing agent used in the polymerization. The results show that density decreases from 60 kg/m3 at about 5% to 45 kg/m3 for 17% increasing afterwards. The decrease is to be expected since higher amounts of blowing agent leads to a higher expansion of the foam and therefore lower density. This was stated before by several authors (Hakim et al., 2011; Seo et al., 2004; Yan et al., 2008). The results obtained for amounts over 20% water content are probably due to the limit of expansion being attained and for higher amounts of water the excess will prejudice the expansion of the foam. Nevertheless, different results were presented before by Maldas and Shiriashi (1996) who reported that with the increase in blowing agent, density of the foams increased. Compressive stress at 10% deformation decreases sharply with the increase in blowing agent from about 90 kPa to about 20 kPa, increasing very slightly for higher amounts of blowing agent. The slight increase for about 27% of water is probably due to the increase density of the foam due to the lower expansion. Young’s modulus follows a similar pattern decreasing from about 700 kPa to about 200 kPa. The reduction observed initially for density and compressive properties is due to the increased growing of the foam with higher amounts of blowing agent.
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The results on the influence of surfactant amount on the properties of the foams, are presented in Figure 4. Density has small variations ranging from 44 kg/m3 to 48 kg/m3. Compressive stress seems to increase with higher surfactant amount from the initial 7% to a maximum of 26% achieved for 11% php. In relation to Young’s modulus the results were not conclusive. Yan et al (2008) and Hakim et al (2011) studied the effect of surfactant amount on compressive stress and concluded that it displayed nonlinear behaviour in relation to the increase of surfactant amount. In accordance to Lim et al. (2008), however an increase in surfactant decreases the foam density abruptly until a minimum is reached increasing afterwards. These authors stated that the increased amount of surfactant, reduces the size of the cells and surface tension and increases the number of closed cells leading to a decrease in thermal conductivity and increase in the resistance to compression.
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there seems to be a contradictory effect on density and mechanical properties, since density decreases and mechanical properties increase with the increased amount of catalyst. With the catalyst used the speed of both reactions is increased, but there should be a higher increase of the speed of the gelling reaction. However, since density decreases this means that the expansion reaction was faster. The increased mechanical properties with lower foam density might be due to the formation of crosslinks. Similar results were presented before by (Choe et al. 2004). An increase in water, increases the expansion reaction producing a less dense and mechanically weaker foam but when the amount of blowing agent is too much the foam blows as reported before (Choe et al. 2004). In accordance to Lim et al (2008) the excessive amount of water leads to a negative pressure gradient, which is due to the rapid diffusion of carbon dioxide through the cell wall leading to cell deformation. The surfactant, seems to have no significant effect on foam density, presenting just a small decrease followed by an equally small increase. Nevertheless higher amounts of surfactant lead to stronger foams which is probably due to the decrease of the cell size as reported before (Lim et al 2008). Comparing these results with a commercial rigid polyurethane foam, the density obtained here is somewhat higher (40-64 kg/m3) while the compressive stress at 10% (18-86 kPa) is significantly lower than the commercial foam (40 kg/m3, 200 kPa) (Mahmood et al 2016), however for some packaging products the achieved mechanical properties are good enough. The optimization is important in order to produce less expensive and more environmentally benign foams by using less additives. Nevertheless these results show that it is possible to attain a polyurethane foam with reasonable properties with liquefied orange peel that can compete with traditional commercial polyurethane foams for some applications using the additives studied in this work. Similar results are expected if additives of the same family are used. Nevertheless new studies are necessary to improve PUFs properties to compete with traditional commercial polyurethane foams. One of the possibilities is to test new catalysts that catalyse only the gelling or the expansion reactions to allow us a better tailoring of the foam properties. Also different surfactants should be tested, since it was proven that the surfactant affects the mechanical properties of the foams. In a near future polyurethane foams can be a good use for orange peel wastes and bring more added value to the food processing companies.
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4. Conclusions The results show that a good liquefaction yield can be achieved by polyalcohol liquefaction of orange peel waste and that this material can successfully be converted into a polyurethane foam. Moreover the results showed that the properties of the foam could be tailored by a careful choice of the additives used in foam formation. Overall higher amounts of isocyanate increase physical and mechanical properties of the foams leading to higher density, compressive strength and young modulus. The catalyst, initially decreases density increasing afterwards but increasing compressive strength and young modulus. The blowing agent decreased density, compressive strength and young modulus while the surfactant had no significant influence on physical and mechanical properties of the foams.
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Acknowledgments This work is financed by national funds through FCT - Fundação para a Ciência e Tecnologia, I.P., under the project UID/Multi/04016/2016. Furthermore we would like to thank the Instituto Politécnico de Viseu and CI&DETS for their support.
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Fig. 1 Density, compressive stress (σ10) and Young´s modulus as a function of isocyanate percentage
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Fig. 2 Density, compressive strength (σ10) and Young´s modulus as a function of catalyst percentage.
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Fig. 3 Density, compressive strength (σ10) and Young´s modulus as a function of blowing agent percentage.
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Fig. 4 Density, compressive strength (σ10) and Young´s modulus as a function of surfactant percentage.
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