http://www.e-polymers.org
e-Polymers 2012, no. 055
ISSN 1618-7229
Physical and mechanical properties of rigid polyurethane foams modified with polystyrene beads Elżbieta Malewska,* Anna Sabanowska, Jerzy Polaczek, Aleksander Prociak *
Cracow University of Technology, Department of Chemistry and Technology of Polymers, ul. Warszawska 24, 31-155 Cracow, Poland; e-mail:
[email protected] (Received: 11 April, 2011; published: 10 July, 2012) Abstract: This study presents a method of obtaining porous materials based on the rigid polyurethane foam (RPURF) as the matrix and containing thermoplastic polystyrene beads (EPS) as the fillers. In RPURF-EPS composites, the compact EPS were expanded after heating above a glass transition temperature of the EPS and gas vapouring incorporated inside the EPS, by using heat of exothermic reaction of polyol with isocyanate. The thermal conductivity, compressive strength, core density and cell structure of the new materials were investigated.
Introduction The increasing awareness of climate changes, legal duty of C02 emissions reduction and the number of buildings required to be renovated (to improve their thermal insulation characteristic) [1] influence on world insulation demand. It is forecast to increase 5% per year through 2014 [2]. Therefore a significant interest can be observed with regards to obtain a new insulation material for the building industry. That material should meet criteria of sustainable development, have properties requested by standards, and be economic. Principal aims of sustainable development could be realized by a requirement for energy reduction, thermal insulation improvement, application of ecological materials, environment-friendly production methods, and water saving [3]. Among all insulation materials rigid polyurethane foams (RPURFs) are considered as the best commercially available material for different applications due to their desirable physical and mechanical properties. Low thermal conductivity, low apparent density, excellent dimensional stability, high strength-to-weight ratio, low moisture permeability and low water absorption are the most important attributes [4]. Despite all the RPURFs advantages in Europe still the most popular are the expanded polystyrene foam (EPS), extruded polystyrene foam (XPS), and mineral wool. The EPS dominates all other thermal insulation materials and it controlled over 4550% of the Polish insulation market [5]. This situation is mainly caused by an attractive price of monomer (styrene) and polymer, satisfactory properties of products and easy processing. Although the RPURF has better insulating properties, it is more expensive than the expandable polystyrene. The higher price determines the lower market demand on the RPURF - despite the polyurethane insulation cost restore after only few years. In the 1990’s, investigations were carried out at the Cracow University of Technology to obtain a new composite containing the RPURF and EPS beads. The RPURF-EPS 1
material combines the advantages of its two components. According to PL patent [6], the filler of the RPURF matrix is a thermoplastic granulate with a blowing agent inside, and which has a softening point below 120 oC (e.g. EPS). The beads size can have diameters in the range of 0,2-4,0mm. The amount of the EPS beads added to the polyurethane reaction mixture should not be higher than 200% by weight. The idea of that process is the co-expanison of the EPS and RPURF and a full energy balance. The EPS is expanded by using the heat of exothermic polyol and isocyanine reaction. This technology allows to create new composite material based on well known and applied polymers. Since that time, the trials have been undertaken to obtain similar polymeric composites, which caused new patents to be registered. The American patents describe an analogical fabrication method of composite as the one originated in Poland. A mixture including isocyanate and polyol reactants, catalyst and blowing agent react to yield the RPURF. Expandable polymer beads made of a thermoplastic polymer (e.g. polystyrene or polyolefin) included in the mixture have a softening point below the maximum temperature reached during the exothermic reaction. The beads may be added in an unexpanded, partially or fully expanded form. The heat of the reaction causes the polymer beads to expand and sinter in polyurethane matrix. This creates gas-filled pockets in the foam. A particle size of EPS beads in the range of about 0,4-1,6mm are preferred for that invention. The amount of the expandable polymer should be about 5wt.% or less [7,8,9]. The Korean patents describe the method of polyurethane foam panel manufacturing with dispersed polystyrene beads. Introducing of EPS beads provides reduction of the panel weight. In such panels the air-holes are created as the effect of polystyrene melting. The high pressure injection of the two-components polyurethane composition and the EPS takes place through separate nozzles onto a mold. The polystyrene beads have a particle size ranging from 1-5mm, and are added in the amount of about 30wt.% [10,11]. This paper presents the process of the RPURF-EPS composites obtaining according to the Polish method. Properties like thermal conductivity, compressive strength, core density and cellular structure were investigated. Results and discussion Thermal effects of the process of obtaining the RPURF-EPS composites In this investigation two-component (A and B) polyurethane (PUR) system was applied. The reaction between the PUR components starts immediately after mixing of polyol premix with isocyanete. The quite complex reactions taking place can be represented by two reactions. In the first reaction, isocyanate reacts with polyol to form polyurethane. The second reaction is called blowing reaction. Isocyanate reacts with the water contained in component A and forms carbamic acid as an intermediate product. Carbamic acid decomposes to amine and free carbon dioxide. Under consumption of additional isocyanate, the amine reacts to urea [12]. All these reactions are highly exothermic, the core temperature in the middle of the reference RPURF (R1) increased up to 155 °C. In the RPURF-EPS composites, the heat of mentioned reactions was utilized to heat the EPS beads over the glass transition temperature and expand them by vaporizing the blowing agent (mixture of pentane isomers) entrapped in the EPS pellet. These processes 2
connected with the EPS filler were endothermic, which was confirmed by the DSC analysis of the EPS beads. Two endothermic peaks can be seen on the DSC curve (Fig. 1). 0,5
Heat flow [mW]
0 -0,5 -1 1) 59,8oC
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2) 84,8oC
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Fig. 1. Heat flow curve obtained by the DSC method for the EPS beads – Owipian 1325. The first one at temperature of ca. 60 °C and the second one ca. 85 °C were a consequence of the pentane isomers evaporating and the EPS phase changes respectively. Additionally, the heat generated in reaction of isocyanate with polyol was utilized to evaporate the physical blowing agent contained in the component A. That was why the maximum core temperature decreased from 150 oC for R1 to ca. 100 °C for the RPURF-EPS composites (P1-P6). 0%wt EPS 38%wt EPS 41%wt EPS 44%wt EPS 47%wt EPS 50%wt EPS
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Fig. 2. The core temperature of the RPURF-EPS composites vs. the process time, as a function of the EPS concentration.
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Fig. 2 shows the core temperature in the reference RPURF and the RPURF-EPS composites versus the time process. In general, the higher content of the EPS beads in the reaction mixture, the more heat of the reaction of polyol with isocyanate is needed to expand the EPS beads. In this study, one type of the RPURF system was used to receive all samples, so the exothermal effect of A and B components reaction was constant. Under conditions of receiving of the RPURF-EPS composites (used RPURF system, mold size, EPS fraction etc.), 44 and 47 wt.% concentrations of the EPS beads were the most optimal and the heat of exothermic reaction could be effectively utilized for the EPS expansion. The RPURF-EPS composite containing 50 wt.% EPS filler had visibly smaller diameters of expanded EPS pellet, because the heat of reaction was not enough. Partial melting of the expanded EPS beads could be observed below 44 wt.% of the EPS beads content. The introduction of only 35 wt.% EPS beads caused their complete melting after expansion (Fig. 3) because of too high process temperature.
Fig. 3. RPURF-EPS composites containing adequately: 35 wt.% (P1), 38 wt.% (P2), 41 wt.% (P3), 44 wt.% (P4), 47 wt.% (P5), 50 %wt (P6) of EPS beads Core apparent density of the RPURF-EPS composites Core density of the RPURF-EPS composites was related to the concentration of the EPS filler and thermal effect of the utilized RPURF system. Fig. 4 shows 4
the variation in the apparent density of the RPURF-EPS composites with the different amount of EPS beads concentration.
Core apparent density [kg/m3]
70 60 50 40 30 20 10 0 30
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Concentration of EPS beads [wt.%]
Fig. 4. The core apparent density of the RPURF-EPS composites vs. concentration of the EPS beads All RPURF-EPS composites had a similar core density ca. 50 kg/m3, higher than the core density of the reference RPURF (ca. 36 kg/m3). The reference RPURF and EPS beads fully expanded in hot water have a similar core density. In possibly beneficial conditions, core density of the RPURF-EPS composite should be comparable to EPS and RPURF. Unfortunately, the co-expansion process had limitations and it took place in a short period range between the time of the EPS phase change and the rise time of the RPURF. Above the rise time, expansion of the EPS beads at the RPURF caused the destruction of its structure; high volume increase of the RPURF-EPS composites was not observed. Volume of the EPS phase in the RPURF-EPS composites There are three parameters in stereology that can be measured: the fraction volume of structures [Vv], the fraction area of surface [Sv] and fraction the length of lines [Lv] . The relationship (1) between the three-dimensional structure and the appearance of the two-dimensional image makes the volume measurement possible [13]. The volume fraction of the phase is equal to the area fraction shown on the image. Vv=Sv=Lv
(1)
The only hypothesis required for validating all of these relations is that samples must be isotropic and random [13]. The obtained RPURF-EPS composites fit that criteria, therefore volume measurement was possible. Volume shares of the EPS and RPURF phases mostly depend on their expansion degree and the concentration of the EPS beads. The reference RPURF and EPS beads fully expanded in boiling water have a similar core density. Therefore in possible beneficial conditions of the co-expansion process, the percentage by weight and volume of the EPS phase should be equal in the RPURF-EPS composite. 5
(a)
(b)
Fig. 5. The picture of the RPURF-EPS composite section: (a) before the surface analysis in the IMAGE program, (b) after surface analysis in IMAGE
Volume share of the EPS beads [%]
Results presented below (fig. 6) show that volume share of the EPS phase was lower than its concentration for all RPURF-EPS composites. These tests confirmed that EPS expansion was limited. The composites with 35 and 38 wt.% of the EPS beads content had much lower volume share of the EPS phase as a result of melting and shrinking of the EPS filler.
50 45 40 35 30 25 20 15 10 5 0 30
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Concentration of the EPS beads [%wt]
Fig. 6. Volume share of the EPS beads versus its concentration in the RPURF-EPS composites. Compressive strength of the RPURF-EPS composites Introduction of the EPS beads as a filler to the RPURF affected composites mechanical properties - as compared with the reference RPURF. Compressive strength of the RPURF-EPS composites in the vertical direction to the rise direction was worse, but in the horizontal direction it was mostly better in the presence of the EPS pellet (Fig.7).
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Compressive strength [kPa]
300 250 200 150 100 50 0 0
35
38 vertical
41
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47 horizontal
50
PS
Concentration of the EPS beads [wt.%]
Fig. 7. Compressive strength of the RPURF-EPS composites versus concentration of the EPS beads. Disturbances of the RPURF cell structure were stronger at the horizontal direction to the rise direction than in the vertical direction - this can be seen in Fig. 8 and Fig. 9. The anisotropy factor was measured on the base of the vertical and horizontal Faret’s diameters. Anisotropy factors in the vertical and horizontal cross-sections to the rise direction were adequate for: e.g. the RPURF-EPS composite containing 44 wt.% beads of the EPS 2,93 and 0,56; references foam 1,90 and 0,93.
(a)
(b)
Fig. 8. Cell structure at the cross-section vertical to rise direction for: (a) RPURF-EPS composite, (b) reference RPURF. Thermal conductivity and the content of the closed cells of the RPURF-EPS composites Thermal conductivity coefficient (λ) of the RPURF-EPS composites was ca. 30,0 [mW/m·K)] and those values are more beneficial than in the case of EPS (ca. 35,0 [mW/ m·K]) but higher than in the case of RPURF (ca. 24,5 mW/ m·K]). 7
(a)
(b)
Fig. 9. Cell structure at the cross-section horizontal to the rise direction for: (a) the RPURF-EPS composite, (b) the reference RPURF. A good correlation was found between the thermal conductivity and the content of closed pores of the RPURF-EPS composites (Fig. 10). The higher content of closed cells, the better thermal conductivity of the RPURF-EPS composites containing over 38 wt.% of the EPS pellet. Composites with 35 and 38 wt.% of the EPS beads had not fulfilled that relation due to the presence of the defects in their structure caused by melting the EPS fillers. b
40
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Content of the closed cells [%]
Thermal conductivity [mW/m K]
a
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Concentration of the EPS beads [wt.%]
Fig. 10. Thermal conductivity of the RPURF-EPS composites (a) and content of the closed cells in the RPURF-EPS composites (b) versus the concentration of the EPS beads. Conclusions The obtained results prove that modification of the RPURF with the EPS beads is possible and the received composites have desirable properties. The new RPURFEPS composite combined the best properties from the RPURF and EPS. 8
The RPURF ensures lower thermal conductivity. Additionally the improvement of compressive strength in the horizontal direction was promising. The EPS addition to RPURF allowed to achieve economic profit. Experimental part Materials A two-component polyurethane system Izopianol 22/33 OT/02® was supplied by Purinova Sp. Z o.o.. Component A (Izopianol 22/33 OT-P/02) is a mixture of polyols with some additives. Component B (Purocyn B) is a polymeric diphenylmethane 4, 4’diisocyanate. The EPS beads (Owipian® 1325) were supplied by Synthos Dwory Sp. z o.o.. A sieve fraction of 1.6-2.4 mm beads was used. It contains 6%wt pentane as a blowing agent. Preparation of the RPURF-EPS composites All of the RPURF-EPS composite samples [P1-P6] and reference RPURF foam (R1) were synthesized with a one-shot method at room temperature. The formulation used for composites preparation was differed by the concentration of the EPS beads. The R1 - reference foam contained 0 wt.% of EPS beads and P1-P6 RPURF-EPS composites respectively: 35, 38, 41,44, 47, 50 wt.% of the EPS beads. (These proportions resulted from previous experiments.) Component A was weighed into a PP cup and mixed with a weighted amount of the EPS beads. Then, an appropriate component B was added to the mixture and vigorously stirred at 1 200 rpm for 10 seconds. After mixing, the mixture was poured into a 150×150×100 mm metal mold heated to a temperature of 50 °C according to the DT8855 InfraRed&KTypeThermometer pyrometer, with a detachable lid to produce free-rise foam. The temperature inside the foams was measured during the process of obtaining the RPURF-EPS composites and the reference RPURF foam. The foam was cured in the mold for two hours at room temperature before being removed, and cut into appropriate specimen for testing. Measurements For all the foams produced, physical and mechanical properties were analysed. The core apparent density of the RPURF-EPS composites was measured according to the ISO 845 standard [14] using 50mm cubical samples. Mechanical properties were measured on a Zwick 1445 universal testing machine at room temperature. The measurement of compressive strength of the RPURF-EPS composites was performed according to the ISO 844 standard [15]. The force required for 10% deformation based on the original thickness had been taken as compression strength of the foams. The speed of crosshead movement was 5 mm/min. Tests were performed at the horizontal and vertical directions to the rise direction of the foams. Thermal conductivity of the RPURF-EPS composites was tested using the Laser Comp Heat Flow Instrument Fox 200 apparatus, designed in accordance with the ASTM C518-91 standard [16]. The volume of the EPS phase in the RPURF-EPS composites was assessed by the IMAGE programme, created especially for this task. The picture for that task were 9
taken using a digital camera. Structure analysis of the RPURF-EPS composites was carried out by Aphelion programme. The picture for that task was taken using a optical microscope with lens 2,5x. Content of the closed cells in the RPURF-EPS composites was determined according to the ISO 4590 standard [17]. Moreover, the differential scanning calorimetry (DSC) analysis of the EPS beads was carried out using a differential calorimeter - MettlerDSC 823 (temperature range: 0300 oC, heating rate: 10 oC/min.). References [1] Huet, B.; Godet, N. Global insulation magazine, May 2008, 15-20. [2] World insulation: Industry forecasts for 2014 and 2019, The Freedonia Group, January 2011. [3] Prociak, A. Poliuretanowe materiały termoizolacyjne nowej generacji”, Wydawnictwo Politechniki Krakowskiej, Kraków, 2008, 7. [4] Thirumal, M.; Khastgir, D.; Singha, N. K.; Manjunath, B. S.; Naik, Y. P. Journal of Applied Polymer Science, 2008, 108, 1810. [5] PMR Publications, report: Rynek materiałów izolacyjnych w Polsce 2010 Prognozy rozwoju na lata 2010-2012, 2010. [6] Patent RP 328594 B1, 1998. [7] Patent US 6605650, 2003. [8] Patent US 6727290, 2004. [9] Patent US 20030181536, 2004. [10] Patent KR 20060071009, 2006. [11] Patent KR 20060071440, 2006. [12] Geier, S.; Winkler, C.; Piesche, M. Chem. Eng. Technol., 2009, 32, 1438–1447. [13] Jurgen Buschow, K.H. Encyclopaedia of materials: science and technology, Elsevier Science Ltd., vol.1, 2001, 8852,1396. [14] ISO 845:2006 Cellular plastics and rubbers - Determination of apparent density. [15] ISO 844:2007 Rigid cellular plastics - Determination of compression properties. [16] ASTM, 1991, C518-91, Standard test method for steady-state heat flux measurements and thermal transmission properties by means of the heat flow meter apparatus. [17] ISO 4590:2002 Rigid cellular plastics -- Determination of the volume percentage of open cells and of closed cells.
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