Rigid polyurethane foams from a polyglycerol-based polyol

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Rigid polyurethane foams from a polyglycerol-based polyol

Łukasz Piszczyk

^a

, Michał Strankowski

^a,^⇑

, Magdalena Danowska

^b

, Aleksander Hejna

^a

, Józef T. Haponiuk

^a

aDepartment of Polymer Technology, Chemical Faculty, G. Narutowicza Str. 11/12, Gdansk University of Technology, G. 80-233 Gdansk, Poland

bDepartment of Solid State Physics, Faculty of Applied Physics and Mathematics, G. Narutowicza Str. 11/12, Gdansk University of Technology, G. 80-233 Gdansk, Poland

a r t i c l e i n f o

Article history:

Received 25 February 2014

Received in revised form 12 May 2014 Accepted 15 May 2014

Available online 29 May 2014

Keywords:

Polyurethane foam Polyglycerol-based polyol Thermal conductivity Thermal stability

a b s t r a c t

Rigid polyurethane foams (rPUs) were synthesized by replacing 35 and 70 wt.% of petrochemical polyol with polyglycerol. Two types of polyglycerol with different molecular weights and hydroxyl numbers were used to obtain new ‘‘green’’ polyurethane–polyglycerol foams. The foams were prepared by a single step method for the ratio of NCO/OH groups equal to 2. rPUs synthesized with polyglycerol showed regular cellular structure, with cell diameters comparable to those of reference foam. The apparent density and compressive strength of foams increased with increasing content of polyglycerols. The highest compressive strength of 240 kPa was observed for the foam containing 70 wt.% of polyglycerol PGK. In the case of foams comprising 35 wt.% of polyglycerols and the reference foam synthesized with petrochemical polyol only, the values of thermal stability, thermal conductivity (k value) and closed cell content were comparable.

1. Introduction

Nowadays, increasingly more information about the use of materials from renewable resources in the production of polyurethane is published. This is related to a decreasing stock of petroleum and law regulations, which stimulate researchers to look for ecological materials and novel tech- nical solutions. As demonstrated in many published studies, materials from renewable resources can almost fully substi- tute their petrochemical analogs without noticeable deteri- oration of the product’s properties. Moreover, due to constantly increasing price of petroleum, biomaterials will

become more and more competitive in the future because of their relatively low cost. Also, the accessibility of materials from renewable resources is obviously higher as they are regenerated all the time. Bio-based polyols are usually triglycerides of, predominantly, unsaturated fatty acids. Such compounds show lack of functional groups, however present in the structure unsaturated bonds can be effectively converted into hydroxyl groups, by e.g. epoxidation followed by oxirane ring-opening, hydroformylation and hydrogena- tion, thermal polymerization followed by transesterification or halogen addition and nucleophilic substitution. Research focusing on conversion of triglycerides showing rather low reactivity into polyols, that can be incorporated into polymer technology are very popular nowadays. Among the potential biopolyols, which are presently researched, one can find substances such as, glycerol, polyglycerol, modified and unmodified soybean or castor oils, chestnut oil, palm oils, and sunflower and linseed oils[1–11].

Soybean oils are mainly used in the United States, while palm and coconut oils are mostly applied in Asia. In

⇑Corresponding author. Present address: Department of Polymer Technology, Chemical Faculty, G. Narutowicza Str. 11/12, Gdansk University of Technology, G. 80-233 Gdansk, Poland. Tel.: +48 58 347 2434; fax: +48 58 347 2134.

E-mail addresses:[email protected](Ł. Piszczyk),michal.strankowski@

pg.gda.pl (M. Strankowski), [email protected] (M. Danowska), [email protected] (A. Hejna), [email protected] (J.T. Haponiuk).

Contents lists available atScienceDirect

European Polymer Journal

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e u r o p o l j

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Europe, the most important are polyols based on canola and sunflower oils. Glycerol and its polymeric form are relatively new materials; their incorporation is connected to another technology based on renewable resources, i.e. the production of biodiesel, which is a fuel based on fatty acid methyl esters (FAME), destined for diesel engines. Glycerol is a by-product of biodiesel production; 100 kg of glycerol is obtained per each tone of biodiesel produced. Definitely, such by-product can be quite easily and efficiently used in the production of polyurethane. According to prevailing trends, this would allow a gradual decrease in the amount of petrochemical materials used in polyurethane industry [12].

Glycerol can be used directly as a polyol or as an excip- ient in the production of other polyols based on different plant oils. The possibilities to apply glycerol and polyglycerol in PU production are vast, but they still require more research. This paper is an example of such research focusing on the introduction of polyglycerol into the production of polyurethane foams.

2. Experimental 2.1. Materials

Rigid polyurethane foams were synthesized from Rokopol 551 (oxypropylenated sorbitol), a commercial polyol, which was then partially replaced with two kinds of polyglycerol, i.e. Pole and PGK, which are the products of thermo-catalytic polycondensation of waste glycerol. The properties of the aforementioned polyols are presented in Table 1. The chemical structure of petrochemical polyol and the general structure of polyglycerols are shown in Fig. 1.

Isocyanate used in the reaction was polymeric diph- enylmethane-4,4⁰-diisocyanate PMDI characterized by 31% content of NCO groups (Zakłady Chemiczne ZACHEM, Poland).

A 33 wt.%. solution of potassium acetate in ethylene glycol (K12) was used as a gelling catalyst, while 2-[2-(dimethylamino)ethoxy]ethanol (KAmin) was applied as a blowing catalyst. Both catalysts were purchased from Sigma Aldrich (Poland).

Niax Silicone SR-393 (Momentive, Czech Republic) was used as silicon-based surfactant. The physical blowing agent was n-pentane (anhydrous, 99%) from Chempur (Poland). Distilled water served as a chemical blowing agent. All chemicals were used as received.

2.2. Foam synthesis

Rigid polyurethane foams were produced on a labora- tory scale by a single step method from a two-component (A and B) system with the ratio of NCO/OH groups equal to 2. The component A (polyol mixture) consisting of the proper amounts of oligoether Rokopol RF 551 mixed with polyglycerol at various ratios (i.e. 35 and 70 wt.% of polyglycerol), catalysts, surfactant and water was weighed and placed in a 500 ml polypropylene cup. Next, the polyol mixture was homogenized with a mechanical stirrer at 1800 rpm for 50–70 s. Such prepared component A was mixed with component B (polyisocyanate, pMDI) at a pre- determined mass ratio and stirred at 3000 rpm for 10 s.

The resulting reaction mixture was poured into an open metal mould of dimensions 10010050 mm³. After demoulding, the obtained PUF samples were kept at 60°C for 24 h and then seasoned at room temperature for 24 h.Table 2contains the details of foam formulations.

2.3. Characterization

The following process parameters were observed and characterized: start time (time elapsed from the start of the process until the start of volume expansion); rise time (time elapsed from the start of volume expansion until the foam reaches its maximum height); and tack free time (time elapsed from the moment the foam reaches its maximum height until the surface of the foam stops being tacky to the touch). Changes in the concentration of NCO in the reaction mixture were measured. Samples (1 g) were collected every 10 s and placed in liquid nitrogen to stop the reaction. Next, samples were dissolved in the solution of dibutylamine and acetone, and titrated with 0.1 M HCl.

The concentration of unreacted isocyanate groups was determined according to Eq.(1):

pðNCOÞ ¼0:42ðV0V1Þ

m ð1Þ

wherepis isocyanate concentration; 0.42 is gram equiva- lent of isocyanate groups;V0is the volume (ml) of 0.1 M HCl used for blind trial; V1 is the volume (ml) of 0.1 M HCl used for titrating the sample; and mis the sample mass.

After seasoning, the properties of the produced foams were determined according to standard procedures.

In order to determine the foam structure, FT-IR spectro- photometric analysis was performed by means of the

Table 1

Comparison of polyol properties.

Polyol Petrochemical Polyglycerol Polyglycerol

polyol Rokopol RF 551 Pole PGK

OH number, mg KOH/g 420 190 290

Molecular weight, g/mol 800 3300 4520

Functionality 6.0 12.1 16.2

Viscosity, mPa s 5842 2800 19,000

Acid value, mg KOH/g 0.02 0.4 0.7

Water content, ppm 60 2500 1800

Manufacture/resource PCC Rokita ECO – Innowa ECO – Innowa

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Nicolet 8700 apparatus (Thermo Electron Corporation, USA) equipped with a snap-Gold State II, which allows for making measurements in the reﬂection conﬁguration mode. The resolution used was 4 cm¹.

The cellular structures of samples were characterized with a Philips-FEI XL 30 Environmental Scanning Electron Microscope (ESEM) using an acceleration of 25 kV. Samples were cut to the required dimensions at ambient temperature. SEM images were analyzed with ImageJ computer software in order to measure the size of cells.

In accordance with PN-EN ISO 845: 2000, the apparent density of PUF samples was calculated as a ratio of the sample weight to the sample volume. The volumes of cube-shaped samples were measured with a slide caliper with an accuracy of 0.1 mm. Samples were weighed using an electronic analytical balance with an accuracy of 0.1 mg.

The compressive strength of PUF samples was esti- mated in accordance with PN-EN ISO 604:2006. The samples of cubic shape and 505050 mm³ in size were measured with a slide caliper with an accuracy of 0.1 mm. The compression test was performed on a Zwick/

Roell tensile tester at a constant speed of 10 mm/min to reach a 20% deformation.

Thermal conductivity coefﬁcient (k, mW/m K) was esti- mated using Laser Comp Heat Flow Instrument Fox 200

apparatus. Analysis was performed in average temperature of 10°C (temperature of cold plate 0°C and hot plate 20°C).

The differential scanning calorimetry (DSC) was performed on a DSC 204 F1 Phoenix apparatus under nitrogen atmosphere in the temperature range from50 to 100°C, and at a heating rate of 10°C/min.

Thermogravimetric analysis (TGA) was performed on a NETZSCH TG 209 apparatus using 5-mg samples in the temperature range 100–600°C and under argon atmosphere, at a heating rate of 15°C/min.

3. Results and discussion 3.1. Kinetic proﬁle of foaming

The processing times of reference foam and polyurethane–polyglycerol foams are shown inFig. 2. The results indicate a slight increase in start time and rise time for the foams containing polyglycerol, which is related to the molecular weight of polyglycerols (Table 1) and their viscosity (mainly of polyglycerol PGK). Properties of petrochemical polyol and polyglycerols are presented inTable 1.

Changes in the concentration of unreacted isocyanate groups and the values of foam rise in dependence on Fig. 1.Polyol structure: petrochemical polyol (left) and a major component of polyglycerols (right).

Table 2

Foam formulations.

Weight of a chemical (pbw) Foam symbol

P0 PPole 35 PPole 70 PPGK 35 PPGK 70

Rokopol RF 551 100 65 30 65 30

Pole – 35 70 – –

PGK – – – 35 70

Water 4 4 4 4 4

K12 2 2 2 2 2

KAmin 2 2 2 2 2

Surfactant 4 4 4 4 4

n-Pentane 2 2 2 2 2

Isocyanate 270 270 270 270 270

Isocyanate index 2 2 2 2 2

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reaction time are illustrated inFig. 3. The biggest drop in the concentration of unreactedANCO groups was regis- tered during the start step of foam production, which is connected to the mixing of components A and B. The larg- est decrease in the concentration of unreacted isocyanate groups was observed for the foam systems produced by using 70 wt.% of polyglycerol; it was caused by higher water content of polyglycerols compared to commonly used petrochemical polyol (Table 1). Water contained in polyols is obviously reacting with freeANCO groups, and water content for polyglycerols Pole and PGK is respectively 41 and 30 times bigger comparing to Rokopol RF551. In the next stage, a relatively constant decrease in the concentration of unreactedANCO groups in the reaction mixture is observed.

3.2. Structure and physical properties

The resulting foam properties are summarized in Table 3. SEM images of rigid polyurethane–polyglycerol foams are shown inFig. 4.

Mechanical properties of rigid polyurethane foams are signiﬁcantly related to their apparent density; an increase in apparent density causes a corresponding increase in compressive strength[8]. The incorporation of polyglycerols, substituted for petrochemical polyol, lowered the volumetric expansion of reaction mixture during polymerization. The synthesized foams had higher apparent density compared to reference foam (Table 3), which led to an increase in compressive strength. Similar effects related to the incorporation of bio-based polyols were reported in other studies[13]. Replacing 35 and 70 wt.% of Rokopol RF 551 with polyglycerol Pole increased the apparent density by 39% and 66%, respectively. This, in turn, led to an increase in the respective values of compressive strength by 21% and 28% (from 140 to 170 kPa, and from 140 to 180 kPa) compared to reference foam. The incorporation of polyglycerol PGK inPPGK 35andPPGK 70foams increased their compressive strength by 17% and 20%, respectively. A decrease in volumetric expansion during polymerization and higher apparent density are related to the difference

in molecular weights of polyglycerols and Rokopol RF 551 (Table 1) and to the order of hydroxyl groups present in the structure of polyols (Fig. 1). As mentioned before, these factors signiﬁcantly affect the reactivity of reaction mixture.

Thermal conductivity is one of the most important physical properties of rigid polyurethane foams as it influ- ences their potential applicability. The value of thermal conductivity coefficient (k) is closely related to the density and morphological structure of a given foam. The total value of coefficient k consists ofkgas, ksolid, kradiation, and kconvection [14]. High-density foams have lower kradiation, while higher non-cellular PU portion leads to higherksolid. The coefficients of thermal conductivity of solid polyurethane,n-pentane, CO₂, and air are 220, 13.7, 15.3 and 24.9 mW/m K, respectively [15]. In the case of PU, the value ofkalso depends on the size and type of cells.

The conducted studies demonstrated that a change in pore diameter from 0.25 to 0.6 mm results in an increase in the value of thermal conductivity coefﬁcient by almost 50%, while the materials with open-cell structure are characterized by higher convection of gases compared to closed-cell foams [15,16]. Foams with a low coefﬁcient of thermal Fig. 2.Processing times of reference foam (P0) and polyurethane–

polyglycerol foams (PPole 35PPole 70,PPGK 35,PPGK 70).

Fig. 3.Changes in the concentration of unreacted isocyanate groups as a function of time: reference foam and foams with polyglycerol Pole (left), and reference foam and foams with polyglycerol PGK (right).

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conductivity and therefore low apparent density, small pore size and the large number of closed cells are consid- ered to have good insulation parameters.

The effect of the type and amount of incorporated polyglycerol on the value of coefﬁcientKis presented inTable 3.

The thermal conductivity coefficient of reference foam was 26.1 mW/m K. The modification of foam system with 35 wt.% of polyglycerol had a minimal effect on thermal insulation of the produced foams. Moreover, rigid polyurethane foams containing 35 wt.% of polyglycerol were characterized by similar structural properties as the foams obtained without the use of bio based materials. The analysis of SEM images did not detect any significant differences in pore diameters and the numbers of closed cells.

The replacement of petrochemical polyol RF 551 with 70 wt.% of polyglycerol resulted in the increased value of thermal conductivity coefﬁcient associated with worsened thermal insulation of the foam compared to reference sample. As a result of the incorporation of larger amount of polyglycerol and the concurrent appearance of bigger amount of water, the share of carbon dioxide in n-pentane/CO2 system increased. Carbon dioxide has higher coefﬁcientKthann-pentane, and it creates cellular structure with larger pore diameters. Based on the conducted research, it was observed that the incorporation of larger amounts (70 wt.%) of polyglycerol had an effect on the formation of bigger number of open cells. The analysis of SEM images showed that the mean pore diameter increased

from 110 to 121

l

m in the case ofPPole 70foam. The change in pore size resulted in a visible increase in thermal conductivity coefﬁcient from 26.1 to 32.6 mW/m K in comparison to reference foam. The worsening of thermal insulation properties of this foam sample was also caused by decreased closed-cell content (a decrease from 82% to 65%).

When summarizing the presented results, it should be stated that foam systems containing 35 wt.% of polyglycerols instead of petrochemical polyols displayed the most advantageous thermal insulation parameters. The incorporation of larger amounts of scrap-based raw materials neg- atively inﬂuenced the morphology of the obtained foams (large number of open cells, and larger pore diameters), which, in turn, increased the value of thermal conductivity coefﬁcient.

The pooled FT-IR spectra of the produced foams are presented inFig. 5. A signal (a) characteristic for NAH group was observed in the range 3345–3380 cm¹, which has conﬁrmed the presence of secondary amine groups. The absorption maxima (b) at 2855 and 2930 cm¹were attributable to the CAH stretching vibrations of CH2groups in aliphatic chains. Higher signal intensity in foams containing polyglycerols is related to the different chemical structure and molecular weight of polyglycerols versus petrochemical polyol. For CH₂ groups, a signal (d) characteristic for bending vibrations at 1596 cm¹was also noted. The absorption maxima (c) in the range 1710–

Table 3

Comparison of foam properties.

Properties Foam symbol

P0 PPole 35 PPole 70 PPGK 35 PPGK 70

Apparent density, kg/m³ 21.7 ± 1.2 30.3 ± 0.9 36.1 ± 1.3 25.5 ± 1.2 28.1 ± 1.1

Compressive strength at 20% deformation, kPa 140 ± 3 170 ± 4 180 ± 4 164 ± 3 168 ± 4

kvalue, mW/m K 26.1 ± 0.8 25.8 ± 0.7 32.6 ± 0.6 26.3 ± 0.8 28.1 ± 0.8

Closed cell content, % 82 83 65 82 77

Cell size,lm 110 ± 9 111 ± 8 121 ± 9 107 ± 8 109 ± 8

Fig. 4.SEM images of foams: (A)P0, (B)PPole 35, (C)PPole 70, (D)PPGK 35, and (E)PPGK 70.

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1720 cm¹, corresponding to the stretching vibrations of C@O bonds, and the signal (e) characteristic for CAN bonds at 1515 cm¹were observed. The aforementioned signals conﬁrm the presence of urethane bonds in the investigated material. The very high intensity of these signals points to high content of urethane bonds in the produced polyurethane foams. A signal (f) characteristic for isocyanurate rings was observed in the range 1414–1416 cm¹; the rings form during synthesis when the ratio of NCO/AOH groups is 2:1[17]. Lower intensity of this signal for foams containing polyglycerol (especiallyPPole 70) indicates lower content of isocyanurate rings in these foams, which has signiﬁcant impact on thermal stability of material. The band (g) at 1310 cm¹ was attributable to the bending vibrations of CH3 groups. The absorption bands (h) and (i) at 1215–1230 and 1089 cm¹resulted from the presence of

r

bonds between carbon and oxygen atoms (in the case of ether bonds) and between carbon atoms and hydroxyl groups. The intensity of speciﬁc signals observed during FTIR analysis conﬁrmed the high content of urethane bonds in the produced polyurethane foams and indi- cated some differences in the structure of polyglycerols and petrochemical polyol used.

3.3. Thermal properties

The results of DSC analysis are presented inTable 4. The highest glass-transition temperature (Tg) was observed for reference foamP₀. The incorporation of polyglycerols Pole and PGK caused a signiﬁcant decrease inTgfor all analyzed samples. A maximum decrease was observed in the case of foamP_{PGK 35}, i.e. the difference betweenT_gof the produced material andTgof reference foam was almost 15°C. The glass-transition temperature of the system consisting in

70 wt.% of polyglycerol PGK was slightly higher than that ofPPGK 35foam. In the case ofPPolefoams, an increase in the amount of polyglycerol from 35 to 70 wt.% resulted in a decrease of Tg by 14.4°C, which is probably related to plasticizing effect of residual reactants contained in polyglycerol after thermo-catalytic polycondensation of waste glycerol.

The glass transition occurs as a step increase in the heat capacity (Cp) of the sample during heating, which is due to an enhancement of molecular motion in the polymer [18,19]. For systems containing polyglycerol, DCp varied from 0.02 to 0.07 J/(g K). In the case of reference foamP0, the value ofDCpwas even 10 times higher, which is related to higher mobility of the polymer chains within the net- work node and higher density of cross-linking compared to polyurethane–polyglycerol foams [20]. These results correlate with the values of the apparent density of investigated systems, which shows that the incorporation of polyglycerol has increased the apparent density of the material.

The thermal degradation mechanism of polyurethanes is usually described as a very complex process that involves dissociation of the initial polyol and isocyanate components, followed by the thermal decomposition Fig. 5.FTIR spectra of polyurethane and polyurethane–polyglycerol foams.

Table 4

Glass-transition temperatures and changes in heat capacity of the produced foams.

Foam symbol Glass-transition temperature (°C) DCp(J/(g K))

P0 3.9 0.23

PPole 35 2.5 0.05

PPole 70 9.9 0.02

PPGK 35 11.0 0.04

PPGK 70 9.2 0.07

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leading to the formation of amines, small transition components and carbon dioxide[19,21]. Thermal degradation of polyurethanes occurs in two main stages. The ﬁrst stage, related to the ﬁrst weight loss, is due to the degradation of the hard segments as a consequence of the relatively low thermal stability of the urethane groups whereas the sec- ond stage has been associated with the soft segment decomposition [22–24]. Gomes Lage and Kawano sug- gested that mass loss in every stage of decomposition can be related to the amount of hard and soft segments in polyurethane [25]. Using this approximation, the amount of hard segments in the prepared foams reached up to 70 wt.% forPPGK 70foam.

The results of thermogravimetric analysis are shown in Fig. 6andTable 5. Initial degradation temperatures (Tinit), corresponding to 5% mass loss, fell within the interval of 240–300°C. Under these conditions, the main process is the degradation of urethane bonds[21]. The incorporation of 35 wt.% of polyglycerol Pole increased Tinit by 15°C, while the addition of 70 wt.% of polyglycerol worsened thermal stability of polyurethane foams. For PPole 70,Tinit

decreased by 45°C. As in the case of DSC analysis, such effect is probably related to plasticizing the polyurethane matrix, by residual reactants present in polyglycerol Pole.

The worsened thermal stability of foams containing 70 wt.% of polyglycerol in comparison to reference foam is also related to the amount of isocyanurate rings present

in the structure of the foams. This amount can be determined from the intensity of the signal corresponding to isocyanurate rings, which was higher for foamP0than for polyglycerol foamsPPole 70andPPGK 70.

According to differential thermogravimetric curves, the temperature of the fastest degradation (Tmax 1) of reference foam was 347°C. The incorporation of polyglycerol increasedTmax 1even by 39°C forPPGK 70. InFig. 6the peak corresponding to the degradation of soft segments in the foams containing polyglycerol Pole can be observed in the interval from 425 to 475°C. For foamsPPGKand reference foam, no such peak is present due to different molecular weight of polyols. Polyglycerol Pole has 2.5 times higher molecular weight than Rokopol RF 551, and

Fig. 6.Mass loss and differential thermogravimetric curves as a function of temperature for polyurethane and polyglycerol-based polyurethane foams:

polyglycerol Pole (left), and polyglycerol PGK (right).

Table 5

Characteristics of thermal degradation in rigid polyurethane and polyglycerol–polyurethane foams.

Sample Mass loss (%) Tmax 1(°C) Tmax 2(°C)

5 10 50

Temperature (°C)

P0 285 315 531 347 –

PPole 35 300 332 559 361 460

PPole 70 240 283 451 370 451

PPGK 35 285 321 455 356 –

PPGK 70 269 313 433 386 –

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70% higher than polyglycerol PGK. This has a signiﬁcant inﬂuence on the content of soft segments in polyurethane foams.

4. Conclusions

The presented results demonstrate that the introduction of polyglycerol into the structure of rigid polyurethane foams allows for obtaining materials that are more ther- mally stable and have increased compressive strength, while their cellular structure and thermal insulation properties remain practically unchanged. The foams produced with the use of polyglycerol were characterized by increased apparent density and higher compressive strength, which is connected to the differences in molecular weight and chemical structure of polyglycerols and petrochemical polyols. These differences cause lower reactivity of polyglycerol-containing polyol mixtures which, in turn, results in lower volume expansion during polymerization. The incorporation of 70 wt.% of polyglycerols into polyol mixtures resulted in the lower number of closed cells in the structure of polyurethane foams. This, in turn, caused an increase in the thermal conductivity coefﬁcient of the material as well as worsened its thermal insulation characteristics.

The ratio of NCO/AOH groups used during synthesis resulted in the formation of isocyanurate rings, as con- ﬁrmed by spectroscopic analysis. Isocyanurate rings signif- icantly improve thermal stability of polyurethane foams.

The thermal degradation temperature of isocyanurate rings is ca. 300–340°C, while in the case of traditional polyurethanes, it does not exceed 250°C. The incorporation of 35 wt.% of polyglycerol Pole into rigid PU foam allowed obtaining material characterized by the initial temperature of degradation of 300°C.

To summarize, the presented results demonstrate that the incorporation of polyglycerol into polyurethane foams can improve the mechanical and thermal properties of foams without the use of additional modiﬁers. This obviously would simplify the production of polyurethane foams and lower its cost. At same time, the use of raw materials from renewable resources would positively affect the ecological aspects of the production.

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