A systematic study substituting polyether polyol with palm kernel oil based polyester polyol in rigid polyurethane foam

Industrial Crops and Products 66 (2015) 16–26

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A systematic study substituting polyether polyol with palm kernel oil based polyester polyol in rigid polyurethane foam Athanasia A. Septevani a,b , David A.C. Evans a , Celine Chaleat a , Darren J. Martin a,∗ , Pratheep K. Annamalai a,∗ a b

Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Queensland 4072, Australia Research Center for Chemistry, Indonesian Institute of Sciences, Serpong–Tangerang Selatan 15314, Indonesia

a r t i c l e

i n f o

Article history: Received 17 August 2014 Received in revised form 27 November 2014 Accepted 30 November 2014 Available online 24 December 2014 Keywords: Palm kernel oil polyol Rigid polyurethane foam Dimensional stability Mechanical properties Thermal conductivity Insulation

a b s t r a c t The future depletion of petroleum resources is driving development of sustainable alternatives based on biomaterials. This study is aimed at developing rigid polyurethane foam using high bio-based polyester polyol content without sacrificing the mechanical or thermal insulation performance associated with traditional polyether polyol based rigid polyurethane foam. In this paper, we quantify the properties of a model rigid polyurethane foam formulation based on a commercially available polyether polyol and then systematically substitute the polyether polyol with a commercially available palm kernel oil based polyester polyol. The influence of the palm kernel oil based polyester polyol content on reaction kinetics, structure, morphology and mechanical properties of rigid polyurethane foam were evaluated by cup test, Fourier transform infra-red spectroscopy, optical microscopy, and compression testing. Reaction rate was increased by the substation of polyether polyol with palm kernel based polyester polyol. Rigid polyurethane foams were successfully prepared by blending up to 50% of palm kernel oil based polyol with polyether polyol. Mechanical and thermal properties, as well as dimensional stability of rigid polyurethane foam with up to 30% palm kernel oil based polyester polyol gave comparable or better properties to the 100% polyether polyol based foams. Improved compressive strength without compromising thermal insulation was achieved at around 20% palm kernel oil based polyester polyol. This can be due to the formation of hard block segments of rigid urethane linkages composed of palm-based-polyols, into the discrete domains. In terms of the thermal conductivity, the improved thermal insulation properties were achieved at a composition of around 10% palm kernel oil based polyester polyol. Upon substitution of palm based polyols, while the onset degradation temperature was slightly reduced, stability (50% weight loss) above 350 ◦ C was improved. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Polyurethane is one of the most versatile plastics with a broad range of applications (Lee et al., 2005). The largest segment of the polyurethane market is polyurethane foam (Nikje et al., 2011) which has two major sub-segments. Flexible polyurethane foam is widely used in a variety of comfort applications such as bedding and seating in the furniture and transportation industries, while rigid polyurethane foam is an important thermal insulating material in domestic refrigerators and the construction and transportation sectors (Janik et al., 2014; Sonnenschein and Koonce, 2011).

∗ Corresponding authors. Tel.: +61 733463861. E-mail addresses: [email protected] (D.J. Martin), [email protected] (P.K. Annamalai). http://dx.doi.org/10.1016/j.indcrop.2014.11.053 0926-6690/© 2014 Elsevier B.V. All rights reserved.

To date, most of the rigid polyurethane foam raw materials are derived from petroleum-based precursors. Although current polyether polyols are often based on bio-material initiators, such as sucrose or glycerol, they all still utilize propylene oxide from the petrochemical industry to extend the polyol chain to the desired molecular weight. With an increasing awareness of the environment and the future availability of petroleum resources, it has become important to look for alternative processes and raw materials from renewable or sustainable resources. Thus, there have been many reports on synthesizing polyurethane foam using biobased polyols from various vegetable oils; such as palm (Pawlik and Prociak, 2011; Tanaka et al., 2008), soy (Campanella et al., 2009; Gu and Sain, 2013), rapeseed (Rojek and Prociak, 2012), castor (Mosiewicki et al., 2009; Wik et al., 2011), and other materials such as molasses (Hatakeyama et al., 2011), starch, potato, and wheat (David et al., 2009). However, challenges still remain in terms

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Table 1 Typical physical and chemical properties of polyols days. Property

Test method (ASTM)

Initiator Hydroxyl value, mgKOH/g Functionality pH value (25 ◦ C) Viscosity, cps (25 ◦ C) Water, max. % Color (ma Garner) Specific gravity (25 ◦ C), g/ml a

D4274

D4889 D4672 D4890 D4699

Voranol 446

Maskimiol PK-317

Sucrose/glycerine propoxylated 445.5 (456a ) 4.5 6–8 6511 0.1 (0.2a ) Colourless to brown 1.11

Triglyceride-palm kernel oil 315–330 (301a ) 2–2.4 8–9 250–350 0.30 (0.31a ) Brown liquid 0.90 (min)

Experimentally tested values.

ited structure-property correlations for rigid polyurethane foam modified with palm kernel oil based polyester polyol. This paper describes the effect on polyether polyol based rigid polyurethane foam subject to a systematic substitution with palm kernel oil based polyester polyol (palm based polyol).

of properties and performance compared to polyurethane foam based on polyether polyols. For example, rigid polyurethane foam based on vegetable oil polyester polyols exhibit poor dimensional stability (excessive shrinkage) at higher loadings of the bio-based polyester polyols (Badri, 2012; Pawlik and Prociak, 2011), unfortunately negating the potential savings of these cheaper renewable polyols. Oil palm (Elaeis guineensis) is an abundant and renewable resource that is largely cultivated in Indonesia and Malaysia. The oil palm fruit produces two types of oils; palm oil (PO) derived from the fibrous mesocarp, and palm kernel oil derived from the seed of the palm fruit. According to Basiron et al. (2004), oil derived from palm fruit has advantages over other natural oils, as it is the most readily available and the lowest cost of all the common vegetable oils. Further, it is claimed that the palm plantations have low environmental impact (e.g., low fertilizer and pesticide usage as well as using a “zero burning replanting method”) and help as a netsequester of carbon dioxide in the atmosphere (Basiron and Weng, 2004). Palm kernel oil is a triglyceride of a complex mixture of fatty acids with lauric acid (saturated) and oleic acid (unsaturated) as the major fatty acids. Palm kernel oil based polyester polyol is commercially available from the reaction of palm kernel oil and polyhydric alcohol in the presence of catalyst and emulsifier (Badri et al., 2001; Hassan et al., 2011; Soi et al., 2009; Tanaka et al., 2008). Despite the ecological and economic advantages of palm kernel oil based polyester polyol, very few studies (Badri, 2012; Badri et al., 2001; Chuayjuljit et al., 2010; Badri and Ahmad, 2004) have been published investigating the use of palm kernel oil based polyester polyol in rigid polyurethane foam. As such there have been lim-

2. Materials and methods 2.1. Materials Petroleum based polyether polyol (Voranol 446® , a sucrose/glycerine initiated polyether polyol (The Dow Chemical Company, 2014)) was used after testing for hydroxyl value (456 mgKOH/g) and water content (0.2%). A palm kernel oil based polyester polyol (Maskimiol PK 317® ), was obtained from M/s Maskimi Polyol Sdn., Bhd., Malaysia. Its hydroxyl value and water content were found to be 301 mg KOH/g and 0.31%, respectively, will be further referred shortly as ‘palm based polyol’. ‘A summary of the typical physical properties of both polyols are summarised in Table 1. Polymeric methylene diphenyl diisocyanate (pMDI), DMCHA catalyst (dimethylcyclohexylamine), surfactant (Tegostab 8460), blowing agent (water and HFC M1) were used as received. 2.2. Preparation of polyol blends (Part B) and rigid polyurethane foam samples The rigid polyurethane foam control formulation P0 (Table 2) was a modified commercial formulation of Voranol 446 rigid of Dow Chemical Company (Dow Plastics, 2001).

Table 2 Formulations, reactivity and apparent density of rigid polyurethane foams. Formulations

P0 (control)

P10

Part B Maskimiol PK317 Voranol 446

0 100

Polyol blend (pphp) 10 90

P20

P30

P50

P70

20 80

30 70

50 50

70 30

Surfactant Tegostab 8460

2

2

2

2

2

2

Blowing agent Water HFC M1

1.5 30

1.5 30

1.5 30

1.5 30

1.5 30

1.5 30

1 4.5

1 4.3

1 4.0

1 3.8

1 3.4

1 2.9

Catalyst DMCHA Nominal functionality (estimated) Part A pMDI

103 3

Apparent core density (kg/m ) Reactivity (±2 s) Cream time Gelling time Free rise time Tack free time

35.5 ± 0.7 40 245 367 590

(nominal index) 103 36.6 ± 0.6 34 211 305 844

103 37.5 ± 0.7 33 188 277 839

103 36.6 ± 0.8 30 150 231 781

103 35.6 ± 1.2 27 103 194 601

103 – 26 89 156 596

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Fig. 1. Miscibility of polyol blend formulations P0 to P70 , showing their stability for 6 months.

The control formulation was systematically modified with increasing levels of palm based polyol. Table 2 summarises the formulations of rigid polyurethane foam (P0 , P10 , P20 , P30 , P50 and P70 ). Their chemical equivalences were calculated according to Pinto (2010). Rigid polyurethane foam samples were prepared in a twoshot method for all formulations. Polyol was blended with catalyst, surfactant, and blowing agent to obtain homogenous polyol blends (Part B) according to the formulations in Table 2. Part B was then mixed with pMDI (Part A) at an index of 103. The foam reactivity data (Table 2) was generated from a standard ‘cup-test’. All other data was generated on rigid polyurethane foam molded samples which were prepared in a preheated (50 ◦ C) aluminum mold (30 cm × 30 cm × 10 cm), de-molded after 24 h and allowed to postcure for one week prior to testing. 2.3. Characterization Attenuated total reflectance – Fourier transform infrared spectroscopy was performed on a Thermo-Nicolet 5700 fitted with a diamond attenuated total reflection accessory (Thermo electron Corp., Waltham, USA). Spectra were recorded in the wavenumber range of 500–4000 cm −1 over 64 scanswith a resolution of 4 cm−1 . The morphology of foams was studied using cryogenically fractured specimen and gold deposition with a scanning electron microscope, Neoscope JCM-5000 (JEOL 1011, Tokyo, Japan) operating at 10 kV. Closed cell content of the foams was measured using an air pycnometer following ASTM D2850 (specimen dimensions 50 mm × 50 mm × 25 mm) at Huntsman Pty., Ltd., Australia. Thermal conductivity was measured at 25 ◦ C using a heat flow meter (FOX200, LaserComp, USA) equipped with a cooling unit (Huber) according to ASTM C518-10 using rectangular specimens (sample size: 200 mm × 200 mm × 25 mm). Compressive mechanical properties (strength and Young’s modulus) were measured using an Instron model 5584 type universal testing machine fitted with a 5 kN load cell under compression mode. The specimens were cut in to rectangular shape according to ASTM D1621-10 with the following dimensions; 51 mm (length), 51 mm (width) and 40 mm (thickness). A minimum of five specimens were tested and the average value along with standard deviation were calculated. Dimensional stability at 70 ± 2 ◦ C and −73 ± 3 ◦ C was determined according to ASTM D 2126-09. The dimensions of foam

specimens (100 mm × 100 mm × 5 mm blocks) were measured with digital calipers before and after conditioning for one, seven and 14 days. The % linear changes in length, width and thickness were calculated. A TGA/DSC 1-thermogravitrimetric analyzer (Mettler Toledo, Australia) was utilized to investigate the thermal stability of rigid polyurethane foam. The weight loss was followed over a programmed heating cycle from 20 ◦ C to 500 ◦ C at a heating rate of 10 ◦ C/min. Thermo-mechanical property analysis of the foams was performed on cylindrical specimens (10 mm diameter) in compression mode on a dynamic mechanical analyzer (DMA) (simultaneous differential thermal analysis (SDTA) 861e , Mettler Toledo, GmbH, Switzerland) operated by STA-Re version 9.10 software. Specimens were tested at a frequency of 1 Hz over a temperature range between 20 ◦ C and 200 ◦ C with a heating rate of 2 ◦ C/min. 3. Results and discussion 3.1. Stability of polyol blends (Part B) The stability of polyol blends (miscibility) is important with respect to potential commercial utility, because during production polyol blends might be kept in storage for a period of time prior to foam production. Formulations P0 to P70 were tested visually for phase separation (Fig. 1) and it was observed that all polyol blends were stable for up to 6 months. The stability of the blended polyols can be related to the chemical similarity of polyols and the hydrogen bonding between the polyols. In addition, the surfactants which are composed of siloxane units and polyether units are miscible with both relatively more hydrophilic (petroleum based) polyether polyol and less hydrophilic (palm based) polyester polyol. It also suggests that the used surfactant is appropriate for better miscibility of polyols. 3.2. Reaction kinetics of foaming Foam reaction kinetics were quantified by the traditional “cup test” using a constant level of catalyst (Table 2). While increased levels of palm based polyol in the formulation accelerates the initial rate of nuclei formation (cream time) it has a far greater effect on

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Fig. 2. Fourier transform infra-red spectroscopy spectra of polyols Voranol 446 and Maskimiol PK 317.

the polymerization rate (gel time). This may be attributed to the presence of primary hydroxyl groups in the palm based polyol as it is well known that primary hydroxyl groups react three times faster than secondary ones in polyurethane reactions (Fan et al., 2012; Tu et al., 2007; Tu et al., 2008). The presence of primary hydroxyl groups in the palm based polyol was confirmed by Fourier transform infra-red spectroscopy (Fig. 2). According to Smith (1999), primary hydroxyl groups exhibit C O stretching between 1075 and 1000 cm−1 , whereas for secondary alcohols, C O stretching appears from 1150 to 1075 cm−1 . Voranol 446 exhibits only one intense peak at 1089 cm−1 attributable to the secondary hydroxyl moiety, whereas Maskimiol PK 317 exhibits peaks for both primary and secondary hydroxyl groups at 1051 and 1150 cm−1 , respectively. The chemical structure of oil based polyol was further supported by 1 H NMR spectroscopy and its synthetic route (see A1 and A2 in Appendices). While the initial reactivity (cream and gelling) was faster with increased levels of palm based polyol it took a longer time to completely cure the foams, as evidenced by the long tack free time. It is conjectured that this effect can be attributed to the position of the hydroxyl groups. The hydroxyl groups of the polyether polyol (Voranol 446) are located at the end of the polymer chains, whereas for palm based polyol, they are randomly distributed in the middle of the polymer chain (Prociak et al., 2012). It is reasonable to expect the less sterically hindered terminal hydroxyl groups in the polyether polyol to contribute to a faster cure rate. Molded rigid polyurethane foam samples were also produced and found to be visually stable at up to 50% of palm based polyol content (formulations P0 to P50 in Table 2). The molded foam with 70% palm based polyol (formulation P70 ) had excessive shrinkage. Therefore, formulation P70 was excluded from further evaluation in this study. The molded density of the foam was controlled in the range of 41 ± 1 kg/m3 for all the formulations. The apparent core density values are summarized in Table 2. The core density of foam was measured from the specimens used for mechanical properties and dimensional stability testing. 3.3. The chemical structure of the rigid polyurethane foam samples Attenuated total reflectance – Fourier transform infrared spectroscopy was utilised to study the chemical structure of the rigid polyurethane foam samples (Fig. 3). The characteristic peaks indicating the formation of urethane linkages were observed. They

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Fig. 3. Fourier transform infra-red spectroscopy spectra of rigid polyurethane foam with varying content of palm kernel oil based polyester polyol.

are NH stretching and bending at 3320 cm−1 and 1510 cm−1 , respectively, urethane carbonyl (OC O) vibration at 1703 cm−1 , O CO stretching at 1220 cm−1 , and C O vibration at 1060 cm−1 (Ragauskas et al., 2010; Wik et al., 2011). Further, the characteristic peaks for the hydroxyl groups in Voranol 446 and Maskimiol PK317 (Fig. 3) between 3400 and 3500 cm−1 were not observed, indicating that the quantity of isocyanate used was enough to completely react with all of the OH groups in the polyol blend, as would be expected at an index of 103. The chemical structure of polyurethane has blocks of alternating segments linked together by covalent bonds; “hard segments” of high polarity associated with the diisocyanate (pMDI) and “soft segments” of low polarity associated with the polyol. Polyols of high functionality, such as Voranol 446TM , form a three-dimensional cross-linked polymer network whereas polyols of low functionality, such as the Maskimiol PK317, form essentially more linear polymer networks with minimal cross-linking. In addition to the primary structure, a secondary structure based on aggregation (phase separation) of the “hard segments” into the domains can occur due to hydrogen bonding. While highly cross-linked structures are typically found in rigid polyurethane foam and phase separated structures in flexible polyurethane foam, they are not mutually exclusive and depending on the choice of polyols used both structures could be present in rigid polyurethane foam to varying degrees. The primary difference between the structure of control formulation (P0 ) and the structure of the palm based polyol modified formulations (P10–70 ) is that the palm based polyol has a much lower functionality (Fig. 4). Therefore, the increased palm based polyol content decreases the cross-link density of the rigid polyurethane foam which will reduce the physical strength of the rigid polyurethane foam. The presence of pendant fatty acid alkyl chains on the palm based polyol may also cause plasticisation of the structure and reinforce this effect. The second difference, which is not self-evident from the schematic (Fig. 4), is that because of the presence of more reactive primary hydroxyl groups on the palm based polyol, and the smaller molecular weight of the palm based polyol, at low levels of palm based polyol (P10 and P20 ) it can act as a chain extender by selectively reacting with the MDI to produce smaller, more polar hard segments which are also less compatible with the polyether soft segments, thus actually enhancing phase separation into palm based polyol rich micro-domains. This would have the effect of improving the physical strength of the rigid polyurethane foam

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Fig. 4. Schematic representation of architecture of rigid polyurethane foam with and without palm kernel oil based polyester polyol.

at room temperature, but in absolute terms should be an order of magnitude smaller than the effect of the reduced cross-link density. This effect would not be expected to be perpetuated at high levels of palm based polyol (P30 and P50 ) because, in general, polyether polyols are less compatible with MDI than polyols. Thus, there is less phase separation with polyols and more of the hard segments are miscible throughout the soft segment domains. 3.4. Morphology of rigid polyurethane foam Foam morphology was found to be essentially a polygon closed cell foam structure (Fig. 5). It is interesting to note that the average cell size was initially found to decrease (formulations P10 and P20 ) with increasing levels of palm based polyol, but then increased at higher levels (formulations P30 and P50 ), while also exhibiting a more uniform size and shape. Surprisingly, the closed cell content (Table 3) of the rigid polyurethane foam with lower palm based polyol content (formulations P10 and P20 ), and the smaller cells, was slightly lower than the control foam formulation (P0 ), while it was higher for rigid polyurethane foam with higher palm based polyol content (formulations P30 and P50 ), and the larger cells. There are a number of factors known to affect the size, shape and uniformity of cells in polyurethane foam. Oleochemical-based polyester polyols in rigid polyurethane foam are known to improve the compatibility with blowing agents such as water or HCFCs (Wouden and Stijntjes, 1994), analogous to the effect of adding surfactants, leading to foams with smaller cell size and higher closed cell content due to decreasing surface tension in the polyol blend. Increased levels of blowing agent are known to increase the cell size (Singh and Jain, 2007), however blowing agent content was held constant in this study. Faster polymerization (in terms of gelling time) is known to yield foams with smaller average cell size (Parenti et al., 2012). Decreased viscosity of reactants is known to Table 3 Cell size and closed cell content of the obtained rigid polyurethane foam. Palm kernel oil based polyester polyol (%)

Average diameter of the cell (␮m)

0 10 20 30 50

516 448 480 739 748

± ± ± ± ±

165 75 63 150 128

Closed cell content (%) 91.1 ± 2.4 89.5 ± 4.3 90.1 ± 2.4 92.2 ± 1.8 95.6 ± 4.9

yield foams with larger average cell size (Chuayjuljit et al., 2010; Tan et al., 2011). Additionally, reducing viscosity may also lead to a more uniform size and shape distribution. Song et al. (2000) explain that at very high viscosity, the formation and the growth of bubbles are suppressed leading to heterogeneity in the morphology of foam. Thus, a reduced viscosity by adding palm based polyol might help to stabilize the suppressed growth of bubbles to form a more uniform distribution. Increased level of “hard blocks” in the cell walls of rigid polyurethane foam is known to promote rupture (cell opening) (Randall and Lee 2002) due to reduced extensional flow. While lower viscosity of reactants may explain the increase in cell size of formulations P30 and P50 , viscosity alone does not explain the cell size reduction for formulations P10 and P20 or their decreased closed cell content (Table 3). Similarly, while faster polymerization may explain the decreased cell size for formulations P10 and P20 , increased polymerization rate alone does not explain the increased cell size for formulations P30 and P50 or their increased closed cell content (Table 3). It was postulated that the cell size and closed cell content was a complex interaction between polymerization rate, hard block segregation and cross link density (strength of the foam cell walls) with the process depicted schematically (Fig. 6) for each of the different formulations P0 to P50 . Formulation P0 is a typical rigid polyurethane foam, essentially closed cell, and with high levels of cross linking within the polymer matrix of the cell walls. Formulations P10 and P20 have smaller cell sizes (Fig. 5) because of the faster polymerization rate, due to the presence of the palm based polyol, while the decreased closed cell content (Table 3) is due to the optimum hard block segregation within the cell walls causing some cell rupture. Formulations P30 and P50 are weakened by the lower cross link density in combination with a non-optimal degree of hard block segregation, and consequently suffer cell rupture and coalescence into larger closed cells. Fig. 7 compares the cross-sections of the foams with content of palmbased-polyol about 20 (below 30%) and 50% (above 30%), where the coalescence of the cells can be clearly seen for foam with 50% of palm-based polyol. Ultimately formulation P70 simply does not have enough strength, due to the low cross link density, and the resultant rigid polyurethane foam structure completely collapses. 3.5. Dimensional stability of rigid polyurethane foam The dimensional stability of rigid polyurethane foam indicates that there were no visually distorted specimens after exposure. The % linear changes in length, width and thickness after 1, 7 and 14 days

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Fig. 5. Scanning electron micrographs of prepared foams with various amounts of palm kernel oil based polyester polyol.

exposure at −73 ± 3 ◦ C and +70 ± 2 ◦ C are summarized in Table 4. There were no visually distorted specimens after exposure. In general the % linear changes in the lateral directions (length and width) were lower than that in the direction of foam rise (thickness), which is in agreement with other studies (Badri, 2012; Badri et al., 2001). According to appropriate industrial standards (BS4370: Part 1: 1988), the cold store panel should have less than 3% and 1% of linear change, respectively, when tested at 70 ◦ C and at −15 ◦ C for 24 h (Badri, 2012). While the dimensional stability deteriorated progressively with higher palm based polyol content, even at 50% palm based polyol, the dimensional stability is thus still considered to be mild and within commercially acceptable bounds. Fig. 8 shows the % linear changes per unit of mass (% linear change were normalized to the sample density). It can be clearly seen that generally, % linear change in each dimension (length, width and thickness) did not significantly vary with the addition of up to 30% palm based polyol in both temperature exposures. Only the foam with 50% palm based polyol showed a noticeable change.

3.6. Thermal conductivity of rigid polyurethane foam Thermal conductivity is an important property of rigid polyurethane foam because of is main use as material for thermal insulation. Thermal conductivity is influenced by three factors; conduction through the polymer phase, conductivity through the gas trapped (Fan et al., 2012; Mosiewicki et al., 2009; Tu, 2008) within the closed cell structure, and the radiation between cells (Sonnenschein and Koonce, 2011). To put these three modes of heat transfer into perspective; for a typical rigid polyurethane foam at 32 kg/m3 foam using R11 as blowing agent conduction through the cells, conduction through the polymer struts/cell walls and radiation through the foam represent about 50%, 16% and 34%, respectively. In terms of conduction the polymer (struts and cell walls) are only around 3% by volume and only about 15–20% of the polymer is in the cell walls. In terms of radiation it varies directly with cell size because at a fixed foam density, as the cell diameter decreases the number of struts which inhibit the radiation is

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Fig. 6. Schematic representation of proposed cell morphology of obtained foams showing the influence of palm kernel oil based polyester polyol on cell structure. (a) 100% polyether polyol based foam. (b) Foam with 10–20% of palm based polyol where the cell expansion is limited and cell wall is reinforced with hard segments at optimal content of palm based polyol. (c) Foam with 30–50% of palm based polyol where cell coalescence is facilitated by the reaction kinetics and low viscosity and (d) foam above 50% of palm based polyol where the extensive coalescence of cells leads to dimensionally less stable foams.

increased at the expense of cell walls which are transparent to radiation (Glicksman, 1988). The thermal conductivity values for the rigid polyurethane foam samples prepared from formulations P0 to P50 are summarized in Fig. 9a, where an optimum value was determined for formulation P10 , closely followed by formulation P20 . This would be expected given that the cell size (Fig. 5) of rigid polyurethane foam is strongly correlated with thermal conductivity (Niyogi et al., 1999; Seo et al., 2003) and is clearly more important than higher closed cell content (Table 3) in terms of the relative contribution toward improving the overall thermal conductivity performance of the rigid polyurethane foam.

The long term retention of the thermal conductivity value is also an important property for rigid polyurethane foam. After the first 6 months of testing (Fig. 9b) it would appear that the control formulation P0 may have a better retention of thermal conductivity than the modified formulations P10–50 , however, the formulations P10 and P20 , which were shown to have the optimum hard block segregation to compensate for the lower cross link density, still have the lowest absolute thermal conductivity values. Again this would be expected given that the rate of diffusion of gases is determined by cellular foam structure (Kondapi et al., 1999). The smaller cell size (Fig. 5) and stronger cell walls, as reflected by the high compressive strength (Table 5) of the P10 formulation should reduce the rate of

Table 4 Influence of palm kernel oil based polyester polyol content on % linear change in dimensions of rigid polyurethane foam at −73 ± 3 ◦ C and +70 ± 2 ◦ C. Palm kernel oil based polyester polyol (%)

Duration (days)

% Change at −73◦ ± 3 ◦ C

% Change at +70◦ ± 2 ◦ C

Length

Width

Thickness

Length

Width

Thickness

0

1 7 14

−0.01 ± 0.00 −0.03 ± 0.01 −0.05 ± 0.01

−0.02 ± 0.00 −0.04 ± 0.01 −0.05 ± 0.01

−0.42 ± 0.33 −1.49 ± 0.36 −1.78 ± 0.64

0.05 ± 0.01 0.12 ± 0.05 0.13 ± 0.05

0.03 ± 0.01 0.09 ± 0.02 0.11 ± 0.01

0.13 ± 0.15 0.32 ± 0.05 0.51 ± 0.01

10

1 7 14

−0.01 ± 0.00 −0.03 ± 0.00 −0.04 ± 0.00

−0.02 ± 0.00 −0.04 ± 0.01 −0.06 ± 0.01

−0.56 ± 0.04 −1.41 ± 0.72 −1.58 ± 0.75

0.06 ± 0.03 0.09 ± 0.03 0.10 ± 0.03

0.04 ± 0.01 0.07 ± 0.01 0.12 ± 0.04

0.24 ± 0.13 0.36 ± 0.20 0.60 ± 0.23

20

1 7 14

−0.01 ± 0.00 −0.05 ± 0.00 −0.06 ± 0.01

−0.01 ± 0.00 −0.05 ± 0.03 −0.06 ± 0.03

−0.44 ± 0.04 −1.39 ± 0.18 −1.62 ± 0.14

0.04 ± 0.00 0.08 ± 0.00 −0.06 ± 0.01

0.02 ± 0.02 0.06 ± 0.04 0.05 ± 0.01

0.20 ± 0.06 0.28 ± 0.13 0.53 ± 0.12

30

1 7 14

−0.01 ± 0.00 −0.05 ± 0.02 −0.06 ± 0.01

−0.02 ± 0.00 −0.07 ± 0.03 −0.09 ± 0.02

−0.52 ± 0.50 −1.49 ± 0.38 −1.66 ± 0.46

0.05 ± 0.00 0.09 ± 0.00 −0.03 ± 0.03

0.04 ± 0.03 0.09 ± 0.05 −0.02 ± 0.12

0.22 ± 0.10 0.43 ± 0.00 0.80 ± 0.10

50

1 7 14

-0.02 ± 0.00 -0.06 ± 0.00 -0.08 ± 0.01

−0.03 ± 0.01 −0.10 ± 0.01 −0.11 ± 0.01

−1.16 ± 0.34 −1.87 ± 0.47 −2.17 ± 0.60

−0.23 ± 0.05 −0.43 ± 0.16 −0.46 ± 0.14

−0.33 ± 0.06 −0.53 ± 0.13 −0.55 ± 0.14

−1.02 ± 0.52 −1.26 ± 1.16 −1.46 ± 1.17

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Table 5 Compressive strength and Young’s modulus of rigid polyurethane foam. Palm kernel oil based polyester polyol (%wt)

Compressive strength

Parallel to rise (MPa) 0 10 20 30 50

0.19 0.20 0.22 0.19 0.15

± ± ± ± ±

0.02 0.04 0.03 0.01 0.02

Young’s modulus

% Change/unit mass

Perpendicular to rise (MPa)

– 0.79 7.92 −2.54 −21.51

0.16 0.16 0.19 0.15 0.14

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

% Change/unit mass

Parallel to rise (MPa)

– 2.70 14.13 −4.00 −8.58

4.26 4.59 5.03 4.49 3.54

± ± ± ± ±

0.50 0.95 0.65 0.30 0.45

% Change/unit mass

Perpendicular to rise (MPa)

– 4.78 11.43 1.79 −18.25

3.30 3.43 3.98 3.38 3.30

± ± ± ± ±

0.10 0.15 0.36 0.10 0.13

% Change/unit mass – 0.96 14.21 −1.04 −1.60

higher, respectively, than the control formulation P0 (Table 5). In contrast, the compressive strength of the P50 formulation decreased significantly, both parallel and perpendicular to the direction of rise, compared to the control formulation P0 . Similar results were observed for the Young’s modulus data (Table 5). The decrease in the mechanical properties for formulation P50 compared to the control formulation P0 is the result of lower cross

Fig. 7. P20 and P50 foams showing coalesced cells.

diffusion of carbon dioxide (thermal conductivity of 0.0146 W/m K) outwards and air (thermal conductivity of 0.024 W/m K) inwards, and thus maximize the long term retention of thermal conductivity. 3.7. Mechanical properties of rigid polyurethane foam 3.7.1. Compressive strength and Young’s modulus The mechanical properties of rigid polyurethane foam are closely correlated to the density (Guan and Hanna, 2004; Mosiewicki et al., 2009). The compressive strength and Young’s modulus (determined under compression test) after normalization for density of P0 to P50 formulation are summarized in Table 5 and Fig. 10. There appears to be a fairly broad plateau for compressive strength from 0 to 30% palm based polyol content, however the compressive strength, both parallel and perpendicular to the direction of rise, for the P20 formulation was almost 8% and 14%

Fig. 8. Percentage linear change per unit of mass at 70 ± 2 ◦ C and −73 ± 3 ◦ C after exposure for: (a) 1 day, (b) 7 days, and (c) 14 days.

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Fig. 9. Thermal conductivity of rigid polyurethane foam (a) as a function of palm kernel oil based polyester polyol content (%), (b) during aging for 6 months.

link density in the polymer combined with the larger cell size and thus weaker cell walls observed with this formulation (Guan and Hanna, 2004). The increased mechanical properties of the P20 formulation compared to the control formulation P0 is due to the optimum hard block segregation compensating for the loss of crosslink density. 3.7.2. Thermo-mechanical properties Fig. 11 shows the thermo-mechanical properties (storage modulus E and tan ı) of the prepared rigid polyurethane foam samples. As can be seen in Fig. 11a, P10 and P20 formulations have a higher E over a broader temperature range indicating the aforementioned reinforcement effect of hard domains at low palm based polyol content. This can also indicate the above proposed morphology where the decreased cross-linking density by low-functionality of palm based polyol might be compensated by the hard segment segregation into domains. However, this reinforcement effect was not observed for rigid polyurethane foam with palm based polyol above 20%. A tan ı curve showing the transition of a foam sample from glassy state to a rubbery state denoted as a visible peak is displayed in Fig. 11b. Similarly, the glass transition temperatures of the rigid polyurethane foam samples prepared from formulations P0 to P50 support the proposed mechanism of cell growth. As expected the control formulation P0 with the highest cross-link density has the highest Tg value (Wu et al., 2008; Lim et al., 2008) but the Tg of

Fig. 10. Effect of palm kernel oil based polyester polyol content on compressive strength of rigid polyurethane foam measured in the; (a) parallel direction to the rise, (b) perpendicular direction to the rise.

the rigid polyurethane foam samples based on formulation P10 and P20 were closer to the control formulation P0 than the P30 or P50 formulations because of the efficient segregation of hard segments into domains. Generally, the breadth of the tan delta curve is considered as a good indicator of network homogeneity, while the peak height corresponds to the elasticity of the sample (Tan et al., 2011; Zhang, 2008). Fig. 11b shows that the peak height of the P0–30 rigid polyurethane foam samples were lower than those of the P30 or P50 samples indicating less elasticity in the former samples. Finally, the peak distribution was slightly broader with the increasing palm based polyol content, indicating increased network inhomogeneity consistent with previous studies, where it was communicated that the substitution of natural polyols lead to a substantial broadening of the tan ı glass transition peak, presumably due to the heterogeneity of the vegetable oil polyol employed (Ribeiro da Silva et al., 2013; Tan et al., 2011). 3.8. Thermal degradation of rigid polyurethane foam Generally, decomposition of polyurethane foam occurs in two steps; first the degradation of the urethane linkages followed by polyol decomposition. The thermal stability with varying content of palm based polyol is shown in Fig. 12. Initially, increased palm based polyol levels caused higher initial rate loss of the rigid polyurethane foam in particularly the noticeable lower thermally stable rigid polyurethane foam at 50% palm based polyol.

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Fig. 11. Thermo-mechanical properties of rigid polyurethane foam with varying amount of palm kernel oil based polyester polyol; (a) storage modulus, (b) tan ı curve.

As shown in Fig. 12b, the onset decomposition of urethane bonds started at about 170 ◦ C. The lower thermal stability of the increasing palm based polyol content can be attributed to more unstable urethane groups due to a lower crosslink density. As discussed above that decreased cross-linking density by low-functionality of palm based polyol must be compensated by the hard segment segregation into domains. This reinforcement effect was however not observed for rigid polyurethane foam with palm based polyol 30% and above. Thus, the foam with higher palm based polyol particularly at 50% palm based polyol may induce thermally unstable urethane network. It can be further correlated via a classical degradation mechanism. The urethane linkage degradation involves three competing mechanisms; the dissociation of the original isocyanates and polyol precursors, the formation of carbamic acid and olefin with subsequent carbamic acid dissociation to primary amine and carbon dioxide and the formation of secondary amine and carbon dioxide (Javni et al., 2000; Szycher, 1999). These three reactions occur simultaneously and the prevalence of each process is dependent on the structure of the urethane and the reaction conditions. Hence Fig. 12b shows several small peaks in the DTGA curve between 150 ◦ C and 250 ◦ C indicating the competing degradation mechanisms. In the temperature range above 350 ◦ C, associated with depolycondensation and polyol degradation (Gaboriaud and Vantelon, 1982), the observed improved performance of the formulations

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Fig. 12. Effect of palm kernel oil based polyester polyol content on thermal stability; (a) TGA curve, (b) DTGA curve.

with higher palm kernel oil based polyol levels is in agreement with the known thermal stability of polyester >urea>urethane» polyether (Szycher, 1999). and consistent with other studies of polyether polyol based foams modified with various vegetable oil based polyester polyols (Javni et al., 2000). 4. Conclusions In summary, rigid polyurethane foam were successfully prepared by blending up to 50% of palm polyol with polyether polyol. All polyol blends were highly miscible and stable for several months. Their kinetics showed acceleration in foaming reactions with palm based polyols. Mechanical and thermal properties as well as dimensional stability of rigid polyurethane foam with up to 30% palm based polyol gave comparable or better properties to the 100% polyether polyol based foams. Enhanced compressive strength and thermal insulation were achieved at ∼20% and 10% of palm based polyol, respectively. The improved performance of the foams with a 10% and 20% of palm based polyol is the result of the efficient segregation of the hard segments into domains for these samples which compensates for the lower properties due to the lower cross link density. Rigid polyurethane foam samples with greater than 30% palm based polyol content showed inferior performance compared to the foams with 100% petroleum based polyol formulation. Upon substitution of palm based polyols, while the onset degradation temperature was slightly reduced, stability (50% weight loss) above 350 ◦ C was improved. Investigations on other methods (of

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