Polyurethane–polyisocyanurate foams modified with hydroxyl derivatives of rapeseed oil

Industrial Crops and Products 74 (2015) 849–857

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Polyurethane–polyisocyanurate foams modified with hydroxyl derivatives of rapeseed oil a,∗ ´ Maria Kuranska , Aleksander Prociak a , Mikelis Kirpluks b , Ugis Cabulis b a b

Cracow University of Technology, Department of Chemistry and Technology of Polymers, Warszawska 24, 31-155 Cracow, Poland Latvian State Institute of Wood Chemistry, Dzerbenes 27, LV-1006 Riga, Latvia

a r t i c l e

i n f o

Article history: Received 15 February 2015 Received in revised form 28 May 2015 Accepted 3 June 2015 Keywords: Bio-polyol Rapeseed oil Polyurethane–polyisocyanurate foams Cellular structure Mechanical properties Thermal properties

a b s t r a c t Three types of rapeseed oil-based polyols were used for synthesis of rigid polyurethane–polyisocyanurate (PUR–PIR) foams with three different isocyanate indices (150, 200 and 250). The bio-polyols were synthesized using epoxidation and opening of oxirane rings, transesterification with triethanolamine and transamidization with diethanolamine. Increasing isocyanate index gave rigid foams with increased thermal stability, improved mechanical properties and decreased flammability. Mechanical and thermal properties of rigid PUR–PIR foams depended also on type of bio-polyols. PUR–PIR systems modified with bio-polyol synthesized in the reaction of epoxidation and oxirane ring opening had the largest number of isotropic cells, what beneficially influenced on compressive strength and heat insulating properties of obtained foams. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Currently in the world, there is an increasing interest in modern technologies, which are based on renewable raw materials (Jumat et al., 2012; Montero de Espinosa and Meie, 2011; Sharma and Kundu, 2008). The introduction of plant components to polyurethane (PUR) systems meets all the ideas of sustainable development and is an important challenge for chemical ´ 2014; Prociak, 2007). PURs are companies (Datta and Głowinska, synthesized using two basic components polyols and isocyanates. In recent years, increasing interest of industry in polyols derived from renewable sources is noticed (Caillol et al., 2012; Jia et al., 2011; Vasconcelos Vieira Lopes et al., 2013;). These natural ingredients can be successfully used to obtain different types of PUR materials including PUR–PIR foams (Fridrihsone et al., 2013; Javni ´ ´ et al., 2004; Kuranska et al., 2013; Kuranska and Prociak, 2012). In the literature, only few scholars are involved in the modification of rigid PUR–PIR foams by using bio-polyols. Stirna et al. (2008, 2006),) examined the effect of concentration of the catalyst potassium oleate on trimerization reactions of isocyanate groups in rigid PIR–PUR foams modified with rapeseed oil-based polyols. The isocyanate index of the obtained foams ranged from 150 to 250. As a result of this study it was found that the most preferred functional properties of the foam are characterized by an index of 150–200,

∗ Corresponding author. ´ E-mail address: [email protected] (M. Kuranska). http://dx.doi.org/10.1016/j.indcrop.2015.06.006 0926-6690/© 2015 Elsevier B.V. All rights reserved.

and the concentration of the catalyst, which preferably affects the trimerization reaction is 0.6% by mass. The obtained materials were also characterized by good dimensional stability. Javni et al. (2004) have synthesized rigid PUR–PIR foams with polyol from soybean oil. The aim of this work was to analyze the impact of the structure of the polyol and isocyanate index on selected properties of rigid foams. Polyol based on propylene oxide was used for the synthesis of the reference materials. The isocyanate index of obtained rigid PUR–PIR foams ranged from 110 to 350. In this work, the included results show the possibility to obtain environmentally friendly and dimensionally stable PUR–PIR foams using polyols based on rapeseed oil (RO). The bio-polyols were synthesized using three methods: epoxidation and opening of oxirane rings (EP), transesterification with triethanolamine (TE) and transamidization with diethanolamine (DE). The different isocyanate indices were applied in order to evaluate their influence on thermal stability, flammability resistance as well as mechanical properties of the modified foams in comparison with the reference PUR–PIR foams based only on petrochemical polyol. 2. Materials Three different RO-based polyols were prepared: the first one (EP with the LOH ca. 280 mgKOH/g and water content of 0.36 wt.%) in the Department of Chemistry and Technology of Polymers of Cracow University of Technology and second one (TE with the LOH

850

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

Table 1 The formulations of rigid PUR–PIR foams. Raw materials (g)

EP 150

TE 150

DE 150

EP 200

TE 200

DE 200

EP 250

TE 250

DE 250

EP TE DE Lupranol 3422 Water Polycat 5 Potassium acetate 30% L6915 PMDI Total mass of components Content of bio-polyols in foam, wt.%

70 – – 30 4.1 1.5 1.5 1.5 215.8 324.4 21.6

– 70 – 30 3.7 1.0 1.0 1.5 227.3 334.5 20.9

– – 70 30 3.2 0.4 0.5 1.5 224.9 330.5 21.2

70 – – 30 6.3 1.5 2.5 1.5 353.0 464.8 15.1

– 70 – 30 5.4 1.0 1.5 1.5 353.5 462.9 15.1

– – 70 30 3.9 1.5 1.5 1.5 320.6 429.0 16.3

70 – – 30 7.9 2.0 3.5 1.5 500.5 615.4 11.4

– 70 – 30 7.2 1.0 2.0 1.5 508.6 620.3 11.3

– – 70 30 5.4 1.6 2.0 1.5 456.4 566.9 12.4

ca. 380 mgKOH/g and water content of 0.14 wt.%) and third one (DE with the LOH ca. 390 mgKOH/g and water content of 0.16 wt.%) in the Latvian State Institute of Wood Chemistry. These bio-polyols were obtained on the basis of RO, products of Kruszwica SA (EP) and Iecavnieks (TE and DE), respectively. The bio-polyols were synthesized using three methods: epoxidation of RO with opening oxirane rings (EP), transesterification of RO with triethanolamine (TEA) and transamidization of RO with diethanolamine (DEA) (Kirpluks et al., 2013). Polyetherol based on sorbitol with trade name Lupranol 3422 (further named LUP) having a molecular weight approximately 570 g/mol and hydroxyl number (LOH) ca. 490 mgKOH/g and water content of 0,10 wt.%, as well as polyetherol based on sorbitol with trade name Lupranol 3300 having a molecular weight approximately 420 g/mol and LOH ca. 400 mgKOH/g and water content of 0,10 wt.% were supplied by BASF. Polymeric methylene diphenyldiisocyanate (PMDI) was supplied by BASF. The average functionality of this PMDI was c.a. 2.8. Potassium acetate (PC CAT® TKA30) produced by Performance Chemicals and tertiary amine (Polycat® 5) produced by Air Products Europe Chemicals B.V. were used as catalysts. Niax Silicone (L-6915) produced by Momentive Performance Materials Inc. was used as a stabilizer of porous composite structure. Carbon dioxide generated in the reaction of water and isocyanate groups was used as a chemical blowing agent. 3. Experimental 3.1. Compounding and forming The PUR–PIR foams were obtained by mixing two components (A and B). The reference chemical compositions of component A consisted of polyols, catalysts, water and surfactant. This formulation was modified by replacing of petrochemical polyol with different bio-polyols (EP, TE and DE) (70 wt.%). The chemical compositions of component A and amount of component B used for the preparation of rigid polyurethane foams are shown in Table 1. Polymeric methylene diphenyldiisocyanate (PMDI) as a component B was added to the component A and the mixtures were stirred for 10 s with an overhead stirrer (isocyanate index was from 150 to 250). Then the reaction mixtures were poured into the open mould. Two types of moulds (so called vertical and horizontal) were used, what allowed to make free rise foaming, respectively, in vertical or horizontal directions. The rigid PUR–PIR foams were marked with respect to bio-polyol type and isocyanate index. 3.2. Measurement methods The obtained foams after preparation were conditioned at 22 ◦ C and 50% relative humidity for 24 h, before being cut. The apparent density of PUR–PIR foams was determined according to ISO 845. The compressive strength was determined according

to ISO 826. The compressive force was applied at speed of 2 mm/s, axially in the perpendicular direction to square surface. Compressive strength has been investigated in two directions: parallel and perpendicular to the direction of foam rise. The compressive strength was measured using the instrument Zwick 1445. Brittleness of foams was determined according to ASTM C-421-61. The thermal conductivity factors –  were determined using a Laser Comp Heat Flow Instrument Fox 200. The measurements were made at an average temperature of 10 ◦ C (temperature of cold Plate 0 ◦ C and warm Plate 20 ◦ C). Water absorption was determined according to ISO 2896. The thermal stability was studied by thermogravimetric analysis under nitrogen flow and a heating rate of 10 ◦ C from room temperature to 1000 ◦ C. During this test the following parameters were determined: thermal degradation onset temperature (Tonset ), the temperature at which thermal degradation reached 5, 25, 50% by weight of the sample (T5,25,50% ), the temperature at which occurs faster weight loss of the sample in the first, second, third, fourth or fifth stage of thermal degradation (T1–5 ). The morphology of cells was analyzed using scanning electron microscope (Hitachi S-4700). The samples were sputter-coated with graphite before testing to avoid charging. The cell structure was analyzed also with using optical microscope. The analysis of cellular structure was performed in three cross-sections (Fig. 1). Analysis of photographs was performed using the software AphelionTM , which allows to measure the cross-sectional area, height and width of the cells. The coefficient of anisotropy was calculated as the ratio of cells height and width. The statistical analysis was performed using software IBM SPSS Statistics. The behavior of rigid PUR–PIR foams under heat flux of 35 kW/m2 during 300 s was tested using FTT Dual Cone Calorimeter (Fire Testing Technology Ltd., UK). Tests were done according to ISO 5660-1 standard. The following testing parameters were chosen: surface area – 88.4 cm2 ; separation – 25 mm, orientation – horizontal. During the experiments, time required to initiate the reaction of combustion and thermokinetic parameter i.e. average heat rate release (HRR), total heat release (THR), total smoke released (TSR). The oxygen index (LOI) was determined according to ISO 4589-2. Burning process was analyzed with the use of thermovisual camera. For the burned samples the following parameters were determined: maximal temperature (Tmax ) and average temperature (Tav ) during combustion of a sample, as well as the time (T5 cm ) after which 5 cm of sample was burnt.

4. Results and discussion Firstly, the PUR–PIR foams were prepared using the mould, that allowed to make free rise foaming reaction mixture in horizontal direction. The basic physical-mechanical properties of these foams are shown in Table 2. The apparent density of prepared foams was comparable and any unequivocal trends were not noticed taking into account the changes in isocyanate index. Generally, the

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

851

Fig. 1. System of determination of cross-sections in the horizontal mould: x – parallel to the foam rise viewed from the side, y – perpendicular to the foam rise, z – parallel to the foam rise viewed from the top.

increase of isocyanate index results in creating more isocyanurate groups, affecting the higher thermal stability of rigid PUR–PIR foams. However, it is also the cause of increased brittleness of materials with higher content of isocyanurate rings (Table 2). Among the PUR–PIR materials with isocyanate index of 150 and contained bio-polyols, the lowest value of brittleness characterized foams with bio-polyol EP. For all materials, the water absorption was determined giving the favorable results of less than 1 vol.%. The increasing of isocyanate index also affects the increase in rigidity of the foamed materials. The compressive strength (Fig. 2) was determined for the foam materials at 10% deformation. Due to the anisotropic cell structure, compressive strength was determined in two directions: parallel (Fig. 2a) and perpendicular (Fig. 2b) to the direction of foam rise. In order to investigate the effect of the value of the isocyanate index and type of bio-polyol on the compressive strength of the material in the two selected axes for each of the two dependent variables (strength), a two-way analysis of variance (the factors were the type of the polyol used and the value of the index) was performed. The distribution of the obtained compressive strength values was determined and it was found that the deviations from the normal distribution are statistically insignificant or at the limit of statistical significance. Therefore, it was concluded that there was no need for an alternative non-parametric analysis of variance. For the compressive strength measured in the parallel direction, the main effect associated with the type of the bio-polyol used was highly statistically significant, F(2,99) = 145.971; p < 0.0001. In order to examine the significance of differences between pairs from among the three bio-polyols used, it was verified that the Table 2 Selected properties of rigid PUR–PIR foams. System EP150 TE150 DE150 EP200 TE200 DE200 EP250 TE250 DE250

Apparent density (kg/m3 ) 37.5 ± 0.4 34.7 ± 0.6 39.1 ± 2.5 35.0 ± 1.4 38.1 ± 2.1 34.9 ± 0.3 37.0 ± 1.2 35.3 ± 0.5 38.2 ± 2.2

Water absorbtion (vol.%) 0.84 ± 0.06 0.53 ± 0.06 0.57 ± 0.03 0.87 ± 0.02 0.46 ± 0.03 0.57 ± 0.06 0.99 ± 0.19 0.60 ± 0.07 0.45 ± 0.03

Brittleness (%) 27.1 ± 2.8 48.5 ± 2.8 35.7 ± 0.3 49.4 ± 3.5 42.6 ± 4.9 46.6 ± 5.5 75.0 ± 0.9 58.3 ± 0.4 46.1 ± 4.5

assumption of homogeneity of variance was met, F(2,105) = 2.683; p = 0.073, and the post-hoc Bonferroni test was applied. As a result of the test, statistically significant differences between the materials obtained with bio-polyol EP with regard to both polyol TE (p < 0.0001) and polyol DE (p < 0.0001) were confirmed. The materials obtained with polyols TE and DE do not differ in terms of the strength in the parallel direction in a statistically significant way (p = 0.595). In the case of the strength in the parallel direction, the main effect associated with the change of the isocyanate index was statistically significant F(2,99) = 3.715; p = 0.028, although the differences were much less pronounced than in the case of the type of the bio-polyol used. In order to examine the significance of differences between pairs of isocyanate indices, the assumption of homogeneity of variance was tested F(2,105) = 35.175; p ≤ 0.0001. Due to the failure to meet the assumption of homogeneity of variance, the post-hoc Tamhane’s test was used to analyze individual differences. This test, taking into account separate estimates of variance for each of the subgroups, did not show statistically significant differences between them. In the case of the parallel strength, the interactive effect of the two factors (the type of polyol and the value of the isocyanate index) also turned out to be highly statistically significant F(4,99) = 26,632; p < 0,00001. For polyol EP, on average, the strength decreases with an increasing isocyanate index whereas for polyol TE it increases with an increasing isocyanate index. For polyol DE it also increases, but not as much as in the case of EP. As far as the perpendicular strength is concerned, the main effect related to the type of the polyol used proved highly statistically significant F(2,99) = 61.379; p < 0.0001. To study the importance of the differences between pairs from among the three polyols used, the assumption of homogeneity of variance F(2,105) = 23.048; p < 0.0001 was tested. Given the fact that the homogeneity assumption was not met, the analysis of particular differences was done using the post-hoc Tomhane’s test. The test confirmed statistically significant differences between the materials made using polyol EP in the case of both TE (p < 0.0001) and DE (p < 0.0001). The materials made of TE and DE do not differ in a statistically significant way (p = 0637) in terms of their strength in the parallel direction. In the case of the strength in the perpendicular direction, the main effect associated with the value of the isocyanate index turned out to be statistically significant F(2,99) = 24.970; p < 0.00001. The

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

0.40

a

EP TE DE

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 150

200

250

Isocyanate index

0.40

Compressive strength, MPa

Compressive strength (MPa)

852

b

EP TE DE

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 150

200

250

Isocyanate index

Fig. 2. Compressive strength of rigid PUR–PIR modified bio-polyols with different isocyanate index in two directions parallel (a) and perpendicular (b) to the direction of foam rise.

differences were clearly more strongly pronounced than in the case of the parallel strength. In order to study the significance of differences between pairs of the isocyanate indices used, the homogeneity assumption of variance F(2,105) = 7132; p = 0001 was tested. The homogeneity assumption was not met so the post-hoc Tomhane’s test was applied in an analysis of particular differences. The test, taking into account separate estimations of variance for each of the subgroups, did not show any statistically significant differences between them. As a result of the conducted test, statistically significant differences were confirmed in the case of the materials made using an isocyanate index of 150 comparing to both the material obtained with an isocyanate index of 200 (p = 0013) and 250 (p < 0.00001). The materials produced using an isocyanate index of 200 and 250 did not differ from each other in a statistically significant way (p = 0504). In the case of the strength in the perpendicular direction, the interactive effect of the two factors (the type of polyol and the isocyanate index) also turned out to be highly statistically significant F(4,99) = 8798; p < 0.00001, although the strength of the effect was lower than in the case of the parallel strength. In the case of polyol EP, on average, the strength changed very little with an increasing isocyanate index reaching a maximum for an index of 200, in the case of polyol TE the strength was much more varied, the difference in terms of the average strength between an isocyanate index of 150 and 200 was greater than 50% and, again, a maximum was observed for an index of 200, and for polyol DE there was a monotonous rise with an increasing isocyanate index and the increase of the average strength for an index of 250 compared to an index of 200 was 30%. The study clearly shows that the cellular structure of the foam material affects its mechanical properties. Compressive strength values in the direction perpendicular (Fig. 2b) to foam rise are lower when compared to parallel direction (Fig. 2a). These differences are an effect of cell anisotropy (Fig. 3a). Similar effect was observed by other researchers (Modesti and Lorenzetti, 2003). The elongation of the cells in compressive directions improves the mechanical strength (parallel to foam rise direction) but causes a worsening this property in perpendicular direction. The isotropic cell structure (Fig. 3b) promotes more favorable compressive strength, similar in both directions.

Fig. 3. Examples of cellular structure: cross-section parallel to foam rise (a), crosssection perpendicular (b).

Fig. 4 shows the SEM micrographs of the rigid PUR–PIR foams structure, in cross-section parallel to the direction of foam rise. Cellular structure of foams modified with bio-polyol EP is characterized by most regular shape. Materials EP150, EP200 and EP250 are less anisotropic when compared to PUR foams modified with TE and DE polyols with corresponding isocyanate index. These features have a positive influence on the mechanical properties of materials containing bio-polyol EP. Rigid PUR–PIR foams are used mainly as thermal insulation materials. That is why it is also important to analyze the influence of different types of bio-polyols on the thermal conductivity of foams with increased isocyanate index. In order to determine the effect of bio-polyols on thermal insulation properties of foams, they were synthesized using a mould allowed the free rise of foams in the horizontal direction. It was similar as in the case of materials prepared in a form of panels. The influence of different bio-polyols on selected properties of rigid PUR–PIR foams synthesized with isocyanate index in the range of 150–250 is shown in Table 3. The value of thermal conductivity was determined after 24 h from foams preparing. Foams synthesized in the horizontal mould are characterized by the higher level of apparent densities as compared to the materials obtained in the vertical mould. The difference in the apparent density is caused by the fact that the apparent density of the materials obtained in the horizontal form was determined with the so-called “peel” occurring in the porous material at the contact surface of the foam with the mould surface, wherein the concentration of the material occurs. Sample images of the foam structure at the surface of contact with the mould and in the muldd core are shown in Fig. 5a and b, respectively. In the lower part of Fig. 5a the higher concentration of PUR–PIR solids can be observed. Such phenomena does not take place in the core of the material (Fig. 5b). The structure of the material has a significant influence on foam heat insulating properties. The smallest values of the thermal conductivity of materials modified with bio-polyols was noticed for EP150 foam. The difference in thermal conductivity of EP150 foam in comparison to the foams EP200 and EP250 is caused by the different dimensions of foam cells. The results in Table 4 shows the effect of the applied polyol and isocyanate index on the number of cells per mm2 , the average cross-sectional area of the cells and the anisotropy coefficient. The analysis was performed in three directions: parallel, seen from the side (x) and from above (z), as well as in the direction perpendicular to the direction of expansion (y). For each system, the average cross-sectional area (y) of the cell was characterized by the lowest value and the anisotropy coefficient is close to 1, what means the isotropic structure. Increasing the isocyanate index of PUR–PIR system results in the increase of the average cross-sectional area of cells.

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

853

Fig. 4. SEM images of foam cross-sections in parallel direction to foam rise direction.

Table 3 Selected properties of PUR–PIR foams. System EP150 TE150 DE150 EP200 TE200 DE200 EP250 TE250 DE250

Apparent density(kg/m3 ) 42.2 ± 1.4 45.2 ± 0.2 43.7 ± 1.3 43.3 ± 1.0 44.1 ± 0.1 42.2 ± 0.8 41.0 ± 2.5 45.0 ± 0.4 44.9 ± 0.1

Coefficient of thermal conductivity (mW/m · K) 23.1 ± 0.0 24.1 ± 0.3 24.5 ± 0.2 24.0 ± 0.0 24.1 ± 0.3 23.8 ± 0.1 23.9 ± 0.3 24.3 ± 0.2 24.2 ± 0.1

Fig. 5. PUR–PIR foam structure at the contact surface with the mould (a) and in the mould core (b).

Content of closed cells (%) 87.9 ± 0.8 91.9 ± 2.9 88.1 ± 1.8 87.3 ± 2.7 94.0 ± 1.8 87.8 ± 1.7 88.7 ± 0.8 90.8 ± 0.9 86.8 ± 0.6

854

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

Table 4 Parameters of cellular structure of obtained foams. System

Number of cells/mm2

Cross section area, mm2 10−2

EP150

x y z

50 78 60

0.92 ± 0.13 0.65 ± 0.07 0.82 ± 0.03

0.49 ± 0.02 1.01 ± 0.03 0.58 ± 0.01

TE150

x y z

60 93 65

0.69 ± 0.03 0.37 ± 0.01 0.70 ± 0.05

0.75 ± 0.05 0.87 ± 0.08 0.68 ± 0.03

DE150

x y z

38 57 42

0.97 ± 0.09 0.57 ± 0.12 0.83 ± 0.05

0.76 ± 0.04 0.77 ± 0.03 0.63 ± 0.03

EP200

x y z

38 58 37

1.11 ± 0.17 0.65 ± 0.07 1.01 ± 0.09

0.46 ± 0.10 1.19 ± 0.06 0.46 ± 0.01

TE200

x y z

48 75 51

0.80 ± 0.06 0.45 ± 0.04 0.82 ± 0.01

0.71 ± 0.02 0.89 ± 0.01 0.67 ± 0.05

DE200

x y z

50 73 55

0.83 ± 0.18 0.46 ± 0.02 0.68 ± 0.02

0.89 ± 0.03 0.66 ± 0.02 0.70 ± 0.06

TE250

x y z

41 60 44

0.97 ± 0.08 0.53 ± 0.06 0.86 ± 0.07

0.71 ± 0.02 0.96 ± 0.07 0.68 ± 0.03

DE250

x y z

44 61 48

0.96 ± 0.15 0.58 ± 0.04 0.91 ± 0.18

0.84 ± 0.08 0.72 ± 0.02 0.72 ± 0.03

The thermal properties of rigid PUR–PIR foams depend mainly on the isocyanate index which characterizes the excess of isocyanate groups relative to OH groups. Using isocyanate index 150–300 it is possible to obtain isocyanurate rings in polyurethane matrix which are much more thermally stable than urethane groups. Table 5 shows thermal properties (analyzed under nitrogen atmosphere) of the rigid PUR–PIR foams obtained with different type of bio-polyols and values of isocyanate indcies. Thermal degradation of rigid PUR–PIR foams starts with the disintegration of urethane bonds in the range of 200–300 ◦ C (Fig. 6). At the next stages, the breakdown affects chains of polyols and isocyanurate groups (Jiao et al., 2013; Levchik and Weil, 2004). The degradation of the foam modified with the bio-polyol EP (isocyanate index 150) occurs in two stages as compared to materials involving bio-polyols DE or TE. In the case of materials with these bio-polyols the degradation takes at least three steps. In the case of foams with an isocyanate index of 250, there are three distinct stages of thermal degradation (Fig. 6b). The data presented in the Table 5 confirm that the increase in isocyanate index improves the temperature resistance of the modified foams. Thermal decomposition of rigid PUR–PIR foams synthesized with bio-polyols TE and DE occurs in a wider temperature range than the foams modified with bio-polyol EP. The first stage of degra-

Coefficient of anisotropy

dation in the temperature region of 225 and 270 ◦ C may be related to thermal degradation and evaporation of DEA and TEA because in the case of foams with bio-polyol EP this peak was not observed. In addition, the foams with an index of 250, in which the share of bio-polyols is the smallest, peaks in the temperature region of 225 and 270 ◦ C disappear, which may confirm that the first stage of degradation foams DE150 and TE150 is associated with the presence of bio-polyols containing amines. Fig. 7 also shows the DTG of TE110 foam (such foam with isocyanate index 110, typical for polyurethane materials was prepared additionally), which comprises the largest part of bio-polyol because of the smallest isocyanate index as compared to those TE150, TE200 and TE250. The rate of mass loss during the first stage of degradation is the largest in the case of TE110 foam and it decreases with increasing isocyanate index. Temperatures in which 25 and 50% of weight loss was noticed increase along with increasing isocyanate index. The solid residue after the thermal degradation also has a greater value with the increase of isocyanate index. Another important feature characterizing rigid PUR–PIR foams is their flammability. The analysis shows that materials containing bio-polyols TE and DE are characterized by higher values of LOI compared to materials contained bio-polyol EP (Table 6).

Table. 5 Thermal properties of rigid PUR–PIR foams. System EP150 TE150 DE150 EP200 TE200 DE200 EP250 TE250 DE250 a b c

Tonset a (◦ C) 272 247 212 276 262 217 293 277 221

T5% b (◦ C) 283 269 243 290 282 266 292 291 275

T25% b (◦ C) 337 343 337 340 346 340 350 353 343

T50% b (◦ C) 408 418 424 411 417 423 420 421 426

Residue (%) 17.4 16.7 17.0 22.9 19.0 18.7 25.1 23.0 26.0

T1 c (◦ C) – 275 229 – 294 246 – – 257

T2 c (◦ C) 335 335 331 338 334 331 342 337 332

T3 c (◦ C) 384 430 388 393 389 393 394

Thermal degradation onset temperature. The temperature at which thermal degradation has been 5, 25, 50% by weight of the sample. The temperature at which occurs faster weight loss of the sample in the first, second, third, fourth or fifth stage of thermal degradation.

T4 c (◦ C) – 456 453 – – – – – –

T5 c (◦ C) 469 471 474 466 466 479 463 464

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

0.002

0.002

0.000

0.000

-0.002

DTG (%/min)

-0.002

DTG (%/min)

855

-0.004

-0.004

-0.006

-0.006

-0.008

-0.008

EP150 TE150 DE150

-0.010 -0.012

EP250 TE250 DE250

-0.010 -0.012 -0.014

-0.014 0

200

400

600

Temperature (°C)

800

0

1000

200

400

600

800

1000

Temperature (°C)

Fig. 6. DTG curves of PUR–PIR foams obtained with different type of bio-polyols and isocyanate index 150 (a) and 250 (b).

0.002 0.000

TE110 TE150 TE200 TE250

-0.002

DTG (%/min)

This may be due to the higher hydroxyl number of these polyols, and consequently an increased number of aromatic rings, derived from the isocyanate component characterized by a higher thermal stability. More beneficial LOI of rigid PUR–PIR foams with biopolyols TE and DE, in comparison to foams modified with bio-polyol EP, may also be the result of nitrogen presence in their structure. Rigid PUR–PIR foams only with the index 250 may be qualified to a group of low flammable materials, since only these foams are characterized by LOI > 21%. But it must be taken into account that investigated foams were obtained without any additional flame retardant. One of the most objective methods to assess the flammability is the study of polymeric materials using cone calorimeter. This device is based on Thornton rule, whereby the heat released during the combustion of organic substances per unit mass of oxygen is constant at approximately 13.1 kJ/g (Jurkowski and Rydarowski, 2012). Using the measurement of the instantaneous oxygen concentration in the exhaust gas going out of the calorimeter the amount of the heat emitted at a given time can be calculated with reference to unit area or unit mass of a sample and recorded in a chart as a function of time. Rigid PUR–PIR foams during combustion produce char layer which allows to obtain a flame retarded material. The char layer can significantly impede the further degradation of the polymer in inner layer by reducing the oxygen supply and the heat flux to polymer matrix (Chattopadhyay and Webster, 2009; Zheng et al., 2014), which reduces heat generation and toxic gases during combustion.

-0.004 -0.006 -0.008 -0.010 -0.012 -0.014

200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500

0

200

400

600

800

1000

Temperature (°C) Fig. 7. DTG curves of PUR–PIR foams obtained with EP bio-polyol and different isocyanate indices.

The increase in the isocyanate index of the rigid PUR–PIR foams synthesis increases the ability of foams to form a char layer on the surface of foamed material in the course of burning. Fig. 8 shows exemplary images of TE foam samples after the test with the use of a cone calorimeter. In the pictures of foams TE200 and TE250 (with isocyanate index of 200 and 250) a higher layer of char compared to materials with

Table 6 Influence of type of polyol, bio-polyol and isocyanate index on flammability of rigid PUR–PIR foams. System

LOIa (%)

THRb (MJ/m2 )

TSRc (m2 /m2 )

HRRav d (kW/m2 )

Tmax e (◦ C)

Tav f (◦ C)

T5 cm g (s)

EP150 TE150 DE150 EP200 TE200 DE200 EP250 TE250 DE250

19.6 20.5 20.3 20.2 20.7 20.7 20.4 21.7 21.1

19.8 18.0 16.5 16.2 14.4 16.6 15.4 18.6 15.4

560 522 585 476 495 556 404 485 526

64.9 59.1 55.8 54.7 48.9 54.5 51.1 60.8 52.3

681 729 688 648 647 659 638 669 631

606 591 594 597 574 545 580 599 586

9 11 12 11 16 10 14 11 12

a b c d e f g

Limiting oxygen index. Total heat release. Total smoke release. Average heat rate release. Max. temperature recorded by thermal imaging camera during combustion of the sample. Average temperature recorded by thermal imaging camera during combustion of the sample. Burning time 5 cm of sample (Tav ), time of 5 cm burnt of the sample (T5 cm ).

856

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

Fig. 8. Samples of residue after cone calorimeter tests of PUR–PIR foams with isocyanate indexes: 110 (a), 150 (b), 200 (c), 250 (d).

320

TE110 TE150 TE200 TE250

280 240 200

2

HRR (kW/m )

lower isocyanate index (110 and 150) is clearly visible. A similar trend was also noticed for materials containing bio-polyols EP and DE. Fig. 9 shows the rate of heat release as a function of time during the combustion of rigid PUR–PIR foams with different isocyanate indices. In order to show the effect of isocyanurate ring presence, the changes of heat release rate during the combustion of the rigid PUR–PIR foams with the isocyanate index of 150, 200, 250 are compared to the HRR of PUR foam with isocyanate index of 110. It has been found that the use of the isocyanate index has a significant impact on the rate of heat release during combustion. From literature, it is known that the value of the maximum heat release rate for the unmodified rigid PUR–PIR foams is above 300 kW/m2 (Qian et al., 2014). In the presented results of our studies, the value greater than 300 kW/m2 was received only for TE110 material. Rigid PUR–PIR foams with isocyanate index of 250 are characterized by the lowest rate of emitted heat regardless of bio-polyol type used for the foams preparation. It is very important during a fire because the rate at which heat is released results in the speed of fire spread. Synthesized materials involving EP bio-polyol are

160 120 80 40 0 0

50

100

150

200

250

300

Time (s) Fig. 9. Influence of isocyanate index on HRR of foams modified with bio-polyol TE.

Fig. 10. Thermograms of rigid PUR–PIR foams with EP bio-polyol.

M. Kura´ nska et al. / Industrial Crops and Products 74 (2015) 849–857

characterized by the lowest rate of heat release. In the case of an isocyanate index of 200 and 250 the differences are less evident, because in these systems the share of bio-polyol is much smaller. Furthermore, the materials with higher isocyanate index have the ability to create char layer due to higher content of isocyanurate rings. One of the important parameter concerned with flammability of foams is a the amount of smoke released. From the practical point of view, the awareness of the amount of fumes emitted during the combustion of insulation materials used in construction is essential during an emergency. Table 6 shows the total smoke release of investigated PUR–PIR foams. It was observed that foams with the higher isocyanate index have lower tendency to emit fumes. Using the infrared camera during combustion of the samples having different isocyanate indexes allows to determine temperature distribution along materials being on fire. For the studied foams maximum temperature (Tmax ), and the average temperature during combustion (Tav ) were specified (Table 6). Rigid PUR–PIR foams with bio-polyol TE, regardless of the value of the isocyanate index were characterized by the comparative TSR during their combustion. The largest Tmax of all tested samples was observed in the case of TE150 foam (Fig. 10). In the case of materials contained bio-polyol EP, both TSR and Tmax tended to decrease (Fig. 10 and Table 6) as a result of isocyanate index. For all the tested foams it can be clearly stated that the increase in isocyanate index affects the prolongation and slack of foams burning. The longest combustion times were noticed for TE200 and TE250 materials (Table 6). 5. Conclusions Rapeseed oil is a renewable and cheap raw material for synthesis hydroxyl derivatives, which are very good substitutions of petrochemical polyols in manufacturing bio-based PUR–PIR foams. The modification of PUR–PIR systems by replacing of 70 wt.% of petrochemical polyol with bio-polyols based on rapeseed oil have beneficial influence on the properties that are important taking into account heat insulating application of rigid foams. The increase of isocyanate index in the foam systems modified with bio-polyols derived from rapeseed oil increased oxygen index of prepared porous materials. PUR–PIR foams with isocyanate index of 250 and modified with bio-polyols obtained by transesterification (TE) and transamidization (DE) reactions can be qualified as low flammability materials (IO > 21). The increase of isocyanate index allows to gradually decrease the rate of the first degradation stage of the foams produced with TE and DE bio-polyols. On the other hand, PUR–PIR systems modified with bio-polyol synthesized in the reaction of epoxidation and oxirane ring opening had the largest number of isotropic cells, what can improve compressive strength and heat insulating properties of such foams. Acknowledgment The research leading to these results has received funding from the National Centre for Research and Development in Poland and Latvian Academy of Sciences in the frame ERA-Net MATERA project BBPM “Bio-Based Polyurethane Materials”.

857

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