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Mechanical properties of rigid polyurethane foams at room and cryogenic temperatures

Journal of Cellular Plastics 47(4) 337–355 ß The Author(s) 2011 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021955X11398381 cel.sagepub.com

U Stirna1, I Beverte2, V Yakushin1 and U Cabulis1

Abstract Studies on the effect of the foams’ polymeric matrix’ properties on the tension and compression properties of pour rigid polyurethane (PUR) foams, apparent core density 65–70 kg/m3, at 296 and 77 K were carried out. PUR foams were produced by the hand mixing method from polyol systems that comprised polyether, polyester polyols, and chain extenders. To produce PUR foams, crude MDI was used, and Solkane 365 mfc/227 ea was used as a blowing agent. The molecular weight per branching unit (Mc) of the polymeric matrix of PUR foams was varied in the range 300–1150. Cohesion energy densities of the blocks forming the polymeric matrix were calculated. The effect of Mc on the formation of hydrogen bonds between the urethane groups was estimated from FTIR spectroscopy data and ratio NHbonded/NHfree. It has been found that, with increasing polymeric matrix’ Mc, the tensile strength and elongation at break of PUR foams at 296 and 77 K increases, while Young’s modulus decreases. The increase in the parameter Mc promotes the decrease in the compressive strength and modulus of elasticity of PUR foams at 296 K, while compressive strength indices at 77 K are higher for the foams, whose polymeric matrix has the highest Mc. With increasing polymeric matrix’ Mc, the concentration of the urethane groups bonded with hydrogen bonds increases. Structural and mechanical properties of layered spray polyurethane foams, apparent core density approx. 48 kg/m3, having two layers and polymeric matrix’ Mc ¼ 740 were investigated. Keywords polyurethane foams, cryogenic insulation, tension and compression properties, molecular weight per branching unit, polymer matrix’ parameters, spray foams, density distribution, structural elements 1 2

Latvian State Institute of Wood Chemistry, Dzerbenes 27, LV-1006, Riga, Latvia. Institute of Polymer Mechanics, University of Latvia, Aizkraukles 23, LV-1006, Riga, Latvia.

Corresponding author: I Beverte, Institute of Polymer Mechanics, University of Latvia, Aizkraukles 23, LV-1006, Riga, Latvia Email: [email protected]

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Introduction The issue of development of efficient and safe cryogenic insulation materials is still urgent. The choice of cryogenic insulation materials is based on the fact that the foams should have a sufficiently high safety coefficient, which characterizes the material’s ability to resist thermal stresses that emerge in the insulation’s cool side due to different thermal expansion coefficients of the insulated surface and the foams.1 Extended studies on the mechanical properties of polyurethane (PUR) foams at low temperatures have been carried out by Reed et al.2 In this investigation, results on the tensile strength and Young’s modulus indices of PUR and polystyrene foams with different densities have been analyzed at 76, 195, and 300 K. Spark and Arvidson3 have characterized the mechanical and thermal properties of PUR foams at cryogenic temperatures. Nadeau et al.4 have characterized the properties of PIR foams and their application potentialities in cryogenic technology. The properties of the Space Shuttle cryogenic thermal insulation PIR foams NCFI 24-124 and PUR foams BX-265 both at room and cryogenic temperatures are characterized in detail.5 The mechanical properties and applicability of PUR foams at the temperatures from 77 to 403 K have been described by Demharter.6 However, in the above-mentioned investigations, the parameters of the polymeric matrix of the tested PUR or PIR foams are not characterized, while only in some cases, there are references of general character to the type of the used polyols, polyisocyanate and the blowing agent. Yakushin et al.7 have carried out studies on the effect of the chemical structure of the polymeric matrix on the tensile properties of the PUR foams at 293 and 98 K. Foams were obtained from polyetherpolyols, and it has been elucidated that the highest tensile properties are for the samples having the polymeric matrix with the molecular weight per branching unit Mc around 700. Studies on the effect of the flame retardant trichlorethylphosphate on the PUR foams’ tensile properties at 293 and 98 K were carried out. It has been concluded that the tensile properties at 98 K are little affected by this flame retardant, while the thermal expansion coefficient is affected considerably.8 The physical and mechanical characteristics of spray-on rigid polyurethane foams at normal and cryogenic temperatures have been analysed.9 A detailed analysis of the stress–strain state in PUR foams when used as thermal insulation of cryogenic fuel tanks has revealed importance of high values of tensile strength and elongation at break perpendicular to foams’ rise direction1,5,6 in order to ensure durability and integrity of insulation. In the present article, results of the effect of parameters characterizing the polymeric matrix’ Mc in the range 300. . .1150 and of the chemical structure on pour and spray PUR foams’ tensile and compressive strength as well as on Young’s moduli at 77 K and 296 K are expounded. Spray PUR foams are often ensuring better thermal insulation when applied as a layered material. Therefore structural, physical, and mechanical properties of layered spray polyurethane foams (Mc ¼ 740 and apparent core density 48 kg/m3) having two layers were investigated.

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Experimental Formulation and materials The characteristics of the polyols and chain extenders used for manufacturing PUR foams are listed in Table 1.

Manufacturing of PUR foams Pour foams’ panels were manufactured at T ¼ 22 2 C applying a hand mixing method, according to typical PUR foams’ formulation (Table 2). The polyol premix and polymeric MDI were foamed in an open rectangular mould with bottom size 200 250 mm2. The compositions were formulated so that to produce foams with the apparent core density 65–70 kg/m3. The volume of PUR foams’

Table 1. Characteristics of polyols and chain extenders Polyols

Molecular weight

OH value, mg KOH/g

Functionality

Characterization

Lupranol 3300 Lupranol 3422 Lupraphen 8007 N-Phenyldiethanol-amine Diethylene glycol Dipropylene glycol N-methyldiethanol-amine

420 570 470 181 106 134 119

400 490 240 619 1057 836 942

3.0 5.0 2.0 2.0 2.0 2.0 2.0

Glycerol polyether polyol Sorbitol polyether polyol Aromatic polyester polyol Chain extender Chain extender Chain extender Chain extender

Table 2. A typical polyurethane foams’ formulation Component

pbw

Chain extender Polyester polyol Lupraphen 8007 Lupranol 3300 or Lupranol 3422 Blowing agent Solkane 365mfc/227ea Catalyst – dimethylethanolamine or Polycat 5 Gel catalyst Kosmos 19 Water Silicone surfactant L6915 Polymeric MDI

10–70 10–80 0–70 15–20 1.0–2.0 0–0.2 0–1.0 1.0–2.0 With isocyanate index 110

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liquid composition was chosen such as to obtain foams’ panels with thickness 70–100 mm. For testing the mechanical properties, samples were cut out from the blown PUR panels no earlier than after a 3-day conditioning at room temperature. Spray foams’ panels, size 1000 1000 50 mm3, with polymeric matrix’ Mc ¼ 740, having two 25–30 mm thick layers were manufactured in situ on a flat, 4 mm thick aluminum board, at temperature T ¼ 22 2 C and overlap time 30 s.

Testing of mechanical and physical properties Pour foams. The samples for mechanical testing were cut out from the middle (with regard to all three dimensions) part of the foams’ panels (Figure 1). Apparent core density was determined for each sample. The PUR foams’ compressive properties were determined in direction O3 parallel to foams’ rise direction (RD), tensile properties - in direction O2 perpendicular to it. Cylindrical samples 20 mm in diameter and 22 mm in height were used for determination of compression properties. Ring samples 13–14 mm in height with the inner diameter 43 mm and the outer diameter 53 mm were used for determination of tensile characteristics, strain rate "0 ¼ 10%/min. The size and shape of these samples differs from the standard ones. However, it has been established in the previous studies7 that for foams with a fine cell structure (cell average dimension less than 200 mm) at such sizes and shapes of the samples, the influence of the scaling effect is insignificant. In most cases, the results of the tests of these samples coincide or differ by no more than 10% from the results of the tests of the standard samples. At the same time, the use of such samples makes it possible to investigate the heterogeneity of the mechanical properties of the foams in the panels as well as to carry out tests at cryogenic temperatures. Mechanical testing was performed on testing machines Zwick/Roell 500 N and Zwick/Roell Z100 (sensor 1000 N). A compact cryostat (cryogenic agent - liquid nitrogen LN2) with mechanical testing appliances was used to ensure low temperature during tests. Prior to testing, the samples were held in the cryostat at the 3 a b a

RD

2

O c

1

Figure 1. A PUR foams’ panel. Rise direction (RD); a – core (layers 1 and 2 in spray foams); b – internal skin (absent in pour foams); c – external bottom skin.

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specified temperature for 15–20 min. As a rule, five samples were tested for each experimental point. In the majority of cases, the coefficient of variation of foams’ characteristics was within 6–15%. To determine the volume content of the open and closed cells, Gas Pycnometer AccuPyc 1340 (Micrometrics Instrument Corporation) with a 100 cm3chamber was used. The structure of the PUR foams was investigated using a Scanning Electron Microscope (SEM) Tescan 5536M. The spectra of ground and KBr pellet pressed PUR foams’ samples were registered with a Perkin Elmer Spectrum One FTIR spectrometer.

Spray foams. Apparent core density rf was determined using ISO 88731-1:2006(E) ‘Rigid cellular plastics – Spray-applied polyurethane foam for thermal insulation – Part 1: Material specifications’, ISO 844:2007(E) and ISO 1926:2005(E). A detailed density distribution in dependence of height in a single spray PUR foams’ panel (Figure 1), having two layers, internal skin, and the external bottom skin was determined by the way of cutting the panel into slices of approximate thickness 1–2 mm. Structure of the layered foams was investigated with Light Microscope (LM), equipped with digital camera ‘Olympus BX51’, 50X, samples cut out from the central part (with regard to thickness) of the foams’ layers. Average length and width of polymeric struts and diameters of strut junctions – nods, were determined from LM photos, counting 50 elements. In order to determine mechanical properties five samples were tested for each average data point. Testing was carried out on the servo-hydraulic testing machine MTS 5T, equipped with Flex TEST SE data gathering system, at T ¼ 23 C and strain rate "0 ¼ 10%/min. Extensometers of MTS Model 632.11C-20 type with specially adapted extensions of legs and balances of the self-weight were used to avoid deformation of the foam material. Compression tests were carried out according to ISO 844:2007(E) ‘Rigid cellular plastics – Determination of compression properties’. Samples had dimensions 40 40 40 mm3 and comprised the internal skin. Two series of compression tests were made: (1) Load P directed parallel to rise direction; internal skin placed in the middle of samples height; (2) Load P directed perpendicular to rise direction; internal skin placed in the middle of samples width. Deformation parallel to the compressive load was determined by traverse of testing machine; deformation perpendicular to the load with an extensometer placed on the side of the sample, on the internal skin. Tension tests were carried out according to ISO 1926 ‘Rigid cellular plastics – Determination of tensile properties’. The longitudinal axis of samples was directed perpendicular to rise direction O3, parallel to axis O2. Each sample comprised the internal skin running along its length and situated approximately in the middle of the sample’s height h0 ¼ lo3. In order to comprise the slightly uneven internal skin, the samples had an increased height lo3 ¼ 15 mm. Deformation parallel to the external load P ¼ P2 was measured with an extensometer on the base l02 ¼ 50 mm.

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Deformation perpendicular to the external load P2, parallel to axis O1, was measured with another extensometer on the side l01 ¼ 25 mm of the sample, placing the extensometer on the internal skin. Tests of spray foams at 77 K were performed in the same mode as those for pour foams.

Results and discussion PUR polymeric matrix and its main parameters For production of conventional rigid PUR foams, mainly polyether- and/or polyester-polyols with the functionality 3 are used, while the polyisocyanates (polymeric MDI) with the functionality in the range from 2.3 to 3.0 are used.10 The mechanical properties of PUR foams depend considerably on the parameters of the polymeric matrix of the composition. The polymeric matrix of PUR foams is a covalently cross linked polymer network. The mechanical properties of the PUR foams are affected greatly by the two factors: the polymeric matrix’ cross-link density of covalent nature (Mc) and noncovalent interactions, which can be characterized by the cohesion energy density ecoh. If the PUR foams’ polymeric matrix’ Mc grows, then the possibility of the formation of noncovalent interaction between macromolecules increases. In the polymeric matrix of PUR foams, the noncovalent interaction between macromolecules is formed by hydrogen bonds and van der Waals forces. The urethane groups present in the polymeric matrix of PUR foams are capable of forming hydrogen bonds: urethane–urethane, urethane– ester carbonyl groups, urethane–ether group.11 For producing conventional rigid PUR, mainly polyether and/or polyester polyols are obtained with OHV, in most cases, in the range from 400 to 600 mg KOH/g, while the Mc values of the produced PUR foams are in the range from 300 to 600.10 To evaluate the effect of the parameters of the polymeric matrix on the mechanical properties of PUR foams at room and cryogenic temperatures, compositions were prepared with the Mc values varied in the range 300–1150. for formation of the polymeric matrix, which occurs as polyols react with the isocyanate component, polyols of different structure and functionality and chain extenders were used, whose characteristics is presented in Table 1. As a chain extender, for example, diethylene glycol reacts with the 4,40 -diphenylmethane diisocyanate (MDI) present in the polyisocyanate composition, blocks I with the following structure are formed (1):

O-CH2CH2-O-CH2CH2-O-C-HN O

CH2

NHC

(1)

O n

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The formation of blocks of such a type is used for the synthesis of PU elastomers. These blocks can form domains-type structures, which are characterized by a high ordering degree of the segments, and this factor promotes the formation of H-bonds between the urethane groups.12 As a macrodiol of the aromatic structure reacts with MDI, blocks II with the following structure are formed (2):

O

CH 2 CH 2 -OC

CO-CH 2 CH2 -O

O

CHN

CH 2

O m

O

NHC

(2)

O

The incorporation of rings of aromatic structure in the polymeric matrix of the PUR foams promotes the growth in the rigidity of the polymeric chain and increases the thermal resistance of the obtained material. As a result of the reaction of oxypropylated polyol and MDI, blocks III with the following structure are formed (3): O CH2CHO CH3

CHN n

CH2

O

NHCO

(3)

O

The formulae (1), (2) and (3) show blocks that are formed as the polyols or the chain extender react with the 4,40 -diphenylmethane diisocyanate (MDI) present in the isocyanate component. Since the crude MDI contains also isocynates with the functionality 3, then, as a result of this reaction, covalently cross linked macromolecules are formed. The characterization of the blocks, which participate in the formation of PUR foam’s polymeric matrix, is summarized in Table 3. The values of the solubility parameter () and the cohesion energy density (ecoh) parameters, summarized in Table 3, were calculated according to Fedors.13 It can be seen from the data listed in table 3 that the blocks I and II, forming polymeric matrix, are characterized by high values of and ecoh. This factor could promote the increase in the tensile strength index of the PUR foam. Also other chain extenders can be used for the formation of block I, while, in this case, the

Table 3. parameters of the blocks, forming the polymeric matrix of PUR foams Blocks forming polymeric matrix

Molecular weight

(J1/2/cm3/2)

ecoh (J/cm3)

Block I Block II Block III

356 720 400–516

24.9 25.6 22.3–23.6

619 653 497–557

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parameters and ecoh will have other values. The molecular weight per branching unit Mc of the polymeric matrix of PUR foams was determined according to methodology presented by Ionescu.14 The tested PUR foams can be divided into two groups. a. The first group PUR foams with the polymeric matrix’ Mc in the range 300– 600, obtained using the polyetherpolyols Lupranol 3300 and Lupranol 3422, and the macrodiol Lupraphen 8007. The chain extender content in the polyol mixture does not exceed 20 pbw. The parameters of the polymeric matrix of the PUR foams of this group are close to the conventional rigid PUR foams’ indices, and this foams’ polymeric matrix is formed mainly by the blocks II and III. b. The second group PUR foams with the polymeric matrix’ Mc in the range 600– 1150 are obtained using polyol systems, which consist of chain extenders such as dipropylene glycol, diethylene glycol, 1,6-hexanediol, N-phenyldiethanolamine, N-methyldiethanolamine, etc., and a macrodiol of aromatic structure. The amount of the polyols of the functionality 3 incorporated in such compositions does not exceed 30 pbw from the total amount of polyols, and the polymeric matrix is comprised mainly by the blocks I and II, characterized in Table 3. It can be seen from the data listed in Table 4 that the polymeric matrices of the PUR foams of the first and second groups are characterized by rather similar ecoh indices, but differ considerably according to Mc values.

Tensile properties of pour PUR foams For testing the mechanical properties, pour foams with the apparent core density 65–70 kg/m3 and closed cell content in the range 78–96% were used. The mechanical properties of PUR foams are influenced not only by the properties of the polymeric matrix, but also by foams’ morphology, that can be characterized by cell dimensions and the anisotropy degree A (A ¼ d3/d1, d1 – diameter of the cell perpendicular to rise direction, d3 – diameter of the cell parallel to rise direction). It has been found that, with decreasing polymeric matrix’ Mc the cell elongation in the foams’ rise direction increases for the obtained PUR foams. For PUR foams with Mc values 300 . . . 600, the cell anisotropy degree is in the

Table 4. The main parameters of the PUR foams’ polymeric matrix PUR foams group

Polyol system’s OHV (mg KOH/g)

ecoh (J/cm3)

Mc

The first The second

440–630 490–700

580–620 605–660

300–600 600–1150

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range 1.2–1.4, while, at Mc values 650 . . . 1150 in the range 0.9–1.2; Figure 2 (a) and (b). As can be seen from Figure 3, with increasing polymeric matrix’ Mc from 366 to T 1150, the foams’ tensile strength at 296 K 296 grows from 0.66 to 0.95 MPa, while T at 77 K 77 its values grow from 0.72 to 1.85 MPa, respectively. Relatively low T tensile strength 22, 77 indices at low Mc values (the first group PUR foams) could be connected with the fact that the cross-links of covalent nature hamper the intermolecular action of the polymeric chains, hence, also the formation of H-bonds. The formation of the H-bonds in the polymeric matrix of the PUR foams of the first and the second groups was studied by the FTIR spectroscopy method. The absorption band corresponding to the H-bonded groups NHb is observed (a)

(b) 3

O

Figure 2. SEM images of pour PUR foams structure at: (a) Mc ¼ 366, (b) Mc ¼ 813. Note: O3 – foams’ rise direction.

Tensile strength σT22, MPa

2.00

1.50

1.00

0.50

0.00 0

300

600

900 Mc

1200 296 K

1500 77 K

T Figure 3. tensile strength 22 of PUR foams at 296 and 77 K depending on polymeric matrix’ Mc.

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at 3300 cm1. In addition, there is an absorption band at 3410–3450 cm1 that is typical of Pus, containing free (nonassociated) groups NHfr. The degree of formation of NHb groups was estimated from the value R, determined as the ratio of absorption intensities R ¼ NHb/NHfr.15 As can be seen from Figure 4, with increasing polymeric matrix’ Mc from 366 to 813, the R values grow from 1.38 to 2.40. These results show that one of the main reasons for increase in tensile strength of PUR foams, with increasing Mc, is the greater ability of macromolecules to form H-bonds. Still another reason for the growth in R could be the fact that, in the polyol system of the PUR foams of the second group, chain extenders are used, which promote the formation of macromolecules with a higher degree of ordering. The formation of hard segments with a high ordering degree, using chain extenders, is one of the main methods also for obtaining PU elastomers with high tensile properties.12 Figure 5 shows that, with increasing polymeric matrix’ Mc from 366 to 1150, the tensile elongation at break at 296 K "T22, 296 grows from 5.7% to 21.5%, while "T22,77 from 2.1% to 8.5%, respectively. As can be seen from Figure 6, the polymeric matrix’ Mc has a great effect also on the Young’s modulus ET2, 296 and ET2, 77 values. Thus, with increasing Mc from 366 to 1150, the Young’s modulus at 296 and 77 K decreases from 17 to 8.5 MPa and from 31.8 to 19.5 MPa, respectively. It can be concluded from the obtained results that the PUR foams of the second group with Mc in the range from 650 to 1150, in comparison with the PUR foams of the first group, are characterized by higher tensile strength and elongation properties, but lower ET2, 296 and ET2, 77 . The ratio ET2, 77 =ET2, 296 , with increasing Mc from 366 to 813 varies in the limits of 1.9–2.2. According to the data of Reed et al.,2 this index for rigid PUR foams is higher, namely, 2.9 times.

3.00 2.50

R value

2.00 1.50 1.00 0.50 0.00 0

300

600

900

1200

Mc

Figure 4. Ratio R-values, depending on the polymeric matrix’ Mc of PUR foams.

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compressive properties of pour PUR foams The compressive strength of pour PUR foams was tested in the samples’ direction parallel to foams’ rise direction O3. As can be seen from Figure 7, with increasing C Mc from 300 to 1150, compressive strength 33, 296 decreases from 0.72 to 0.27 MPa, C while, in its turn, at 77 K, with increasing Mc from 360 to 813, 33, 77 grows from 1.03 to 1.28 MPa. as can be seen from Figure 8, with Mc values increasing from 360 to 813 Young’s C moduli EC 3, 296 and E3, 77 are decreasing considerably. Elongation at break εT22, %

30

20

10

0 0

300

600

900

1200 296 K

Mc

1500 77 K

Figure 5. Elongation at break "T22 of PUR foams at 296 and 77 K depending on the polymeric matrix’ Mc.

40

ET2, MPa

30

20

10

0 0

300

600

900 Mc

Figure 6. Young’s modulus polymeric matrix’ Mc.

ET2

1200 296 K

1500 77 K

in tension of PUR foams at 296 and 77 K depending on the

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Compressive strength σC33, MPa

1.50

1.00

0.50

0.00 0

300

600

900

1200 296 K

Mc

1500 77 K

Figure 7. compressive strength C33 of PUR foams at 296 and 77 K depending on the polymeric matrix’ Mc.

30

EC3, MPa

25 20 15 10 5 0 0

300

600

900 Mc

1200 296 K

1500 77 K

Figure 8. Young’s modulus EC3 in compression of PUR foams at 296 and 77 K depending on the polymeric matrix’ Mc.

Figure 9 shows that the numerical values of this ratio are in the limits from 1.7 to 2.4, which is close to the above-described ET2, 77 =ET2, 296 values. The obtained data C on the ET2, 77 =ET2, 296 and EC 3, 77 =E3, 296 changes, depending on the polymeric matrix’ Mc, make it possible to forecast the expected ET2, 77 or EC 3, 77 for the foams of the same core density, because their indices at room temperature are known. The character of dependences presented in Figures 3, 5, 6, 7, and 8 can be explained by simultaneous changes of two PUR foams’ characteristics when Mc

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EC3, 77/EC3, 296

2.50 2.00 1.50 1.00 0.50 0.00 0

300

600

900

Mc

Figure 9. Ratio EC3, 77 =EC3, 296 depending on PUR foams’ polymeric matrix’ Mc.

changes: (1) Changes of the stiffness of the base polymer and (2) Changes of the structure of foams. The foams’ structure can be characterized by an angle y, formed by the longitudinal axis of a strut with the positive direction of axis O3, if the Cartesian coordinate system is placed in the centre of a polymeric nod. Struts having y in the limits 0 y < 45 or 135 y 180 are considered as mainly oriented in the direction O3, struts having y in the limits 45 y < 135 – as mainly oriented in the direction O1. Angular distribution of the struts’ projections on the plane 1O2 can be assumed as uniform for free rise foams. Microscopic studies have revealed that struts, mainly oriented in direction O3, are longer and slimmer than struts, mainly oriented in direction O1: L3/t3 > L1/t1, where L3, t3 (L1, t1) are the average length and average transversal dimension of the struts mainly oriented in direction O3 (O1). PUR foams with low Mc value (Mc & 360) have an expressed anisotropic structure: A & 1.4. The struts are mainly oriented in rise direction O3: N3 > N1, where N3, N1 is the average number of struts mainly oriented in direction O3 or O1 in a volume unit of anisotropic foams. PUR foams with high values of Mc (Mc & 813) have nearly isotropic structure: A & 1.0. Struts are distributed spatially uniformly: K3 & K1, where K3, K1 is the average number of struts mainly oriented in direction O3 or O1 in a volume unit of isotropic foams. The dimensions L, t of all struts are approximately equal. Judging from the results, acquired in tension perpendicular to the rise direction (Figures 3, 5, and 6), when Mc increases, the Young’s modulus of the base polymer E0 decreases: E01 > E02, where E01 and E02 are moduli at Mc ¼ 360 and Mc ¼ 813. It is revealed by the considerable increase of foams’ elongation at break "T22 T (&4 times) at relatively small increase of strength 22 (&1.3–1.7 times) both at 296 and 77 K. The character of the dependences depicted in Figures 3, 5, 6, 7, and 8 is considerably influenced also by the low temperature T ¼ 77 K. Thus, during compression struts are subjected both to normal strains and bending. The compression C strength 33 for pour foams was determined according to load maximum on the stress–strain curve. Depending on the structure of foams (slimness of struts) and

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the stiffness of the base polymer this maximum is caused either by buckling processes of struts: (1) Loss of stability and following break, (2) Loss of stability and no break; retaining of load bearing capacity or (3) Break of struts prior to loss of stability. According to the Euler’s formula the critical load Fcrit ¼ E0Imin/L2. At low temperature 77 K the stiffness of the base polymer increases, while the geometric characteristics of the struts are not significantly changed. Simultaneous influence of the three factors: (1) Changes of the stiffness of the base polymer when Mc increases, (2) Changes of the structure of foams when Mc increases and C (3) Low temperature 77 K leads to an increase of the strength 33 when Mc increases at 77 K (Figure 7). Experimental results on physical and mechanical properties for pour foams having Mc ¼ 450 (the first group foams) and Mc ¼ 740 (the second group foams) are summarized at the end of the article.

Properties of spray PUR foams The results of pour PUR foams’ investigations have showed that foams with polymeric matrix’ Mc > 700 exhibit mechanical properties appropriate for cryogenic insulation. Therefore foams with polymeric matrix’ Mc ¼ 740 were used for further investigations when being prepared as layered spray foams’ panels. Results of density determination in different parts of the spray foams’ panels are presented in Table 5 and Figure 10. Sample’s height lo3 is parallel to foams’ rise direction RD. It can be seen that density in the central part of layer and around interlayer differs for less than 2 kg/m3 ( < 4%). The space filling coefficient P1 ¼ f/0 of the foams was determined, assuming the density of the base polymer 0 ¼ 1200 kg/m3.16 The small value of coefficient of variations v 2% shows that the density is highly uniform at one level of height h in the spray foams’ panel. A detailed density distribution in dependence of height in single spray PUR foams’ panel is depicted in Figure 11. It can be seen that near the external and

Table 5. Density of the spray PUR foams N

lo1, lo2, lo3 (mm)

Characteristics

f (kg/m3)

1 2 3 4 5 6

50 75 15 50 75 15 50 150 15 50 150 15 40 40 40 40 40 40

L1C L2C IN, INþ, INþ, INþ,

47.06 1.08 49.56 0.63 51.46 1.25 47.83 0.67 48.39 0.34 48.75 0.23

EXB þ EXB EXBþ EXBþ (lo3 o RD)

P1 (%) (2%) (1.3%) (2%) (1%) (1%) (0%)

4.0 4.1 4.3 4.0 4.0 4.1

‘þ’/‘’ denote that the corresponding element is present/absent in the sample. It can be seen that the highest is density of the samples comprising only the external bottom skin EXB and near-by foams: f ¼ 51.46 kg/m3, the smallest – in the central part L1C of upper layer L1: f ¼ 47.06 kg/m3 (Figure 10).

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Stirna et al. Density ρf,kg/m3 52

50

48

h, mm

46 0 EXB

10

20 L2

30 IN

40 L1

50

Figure 10. Density in spray PUR foams’ panel, depending on height h.

Figure 11. Density in a single spray PUR foams’ panel, depending on height ‘h’.

the internal skins foams’ density increases due to the un-adiabatic character of foaming process. That promotes formation of finer cell structure and as a result – increased foams’ density. The microscopic investigations of structure revealed that foams are nearly isotropic in the central part of both layers: A & 1.00. Near external and internal skins foams are anisotropic, with finer cell size and polymer struts oriented mainly parallel to the rise direction. The obtained average values for strut length ‘L’, width ‘t’ and diameter of nods ‘d’ in the central part of each layer, Table 6, are in a good correspondence with the data reported by other authors.16 It has been showed17,18 that for low density foams having an expressed strut-like structure, methods of orientational averaging19 can be used to calculate the mechanical characteristics. Compression parallel to rise direction at room temperature resulted in the stress–strain diagrams of type ‘c’(ISO 844:2007(E)), having no force maximum.

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Failure in terms of a force maximum was not reached up to "33 ¼ 60%, when 10 % loading was stopped. Therefore the compressive stress 33 at relative deformation "33 ¼ 10% was determined. Cores of both layers exhibited bigger transversal deformation than the internal skin that lead to the ‘Dumbbell’ after-deformation shape of the samples, Figure 12. In compression perpendicular to rise direction at room temperature the stress– strain diagram type (ISO 844:2007(E)) was ‘b’, having a force maximum at "22 < 10%, followed by a further increase of load. At the beginning of compression the internal skin and the nearby foams having higher density than the cores (Figure 11) possibly act as a reinforcing plate. The plate looses stability that corresponds to the force maximum and a following decrease of load. After that force increases again due to the load being carried mainly by the foams outside the internal skin. It can be concluded that layered plastic foams is a composite construction, were foams in each layer as well as in internal skin can have different physical and mechanical properties. It can be spoken about the effective mechanical properties exhibited by the whole layered material. The experimental results of PUR foams’ physical and mechanical properties are presented in Table 7. At 296 K both compressive moduli and strength of spray foams are nearly equal C for loading parallel and perpendicular to rise direction: EC and 3 E2

Table 6. Dimensions of structural elements of spray PUR foams N

Layer

L (mm)

t (mm)

d (mm)

1 2

L1 L2

0.068 0.008 12% 0.132 0.018 14%

0.019 0.003 16% 0.035 0.005 14%

0.031 0.003 10% 0.042 0.004 9%

Figure 12. The after-deformation shape of samples; internal skin marked with dots.

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C C 33 10 % 22 due to nearly isotropic character of structure. The slightly higher values at compression perpendicular to rise direction could be explained by higher mechanical resistance of the internal skin and near-by foams, that act as a reinforcement. The relationship between Poisson’s coefficients 23 > 32 could be explained by different action of internal skin and surrounding foams in compression parallel and perpendicular to rise direction, as well as mode of measuring the transversal displacement.

Table 7. Pour and spray PUR foams’ properties parallel (//) and perpendicular (o) to rise direction N

Characteristics

Value

1 2 3 4 5 T ¼ 296 K 6 7 8 9 10 11 12 13 14 15 T ¼ 77 K 16 17 18 19 20 21 22 23 24

Manufacturing mode Mol. weight per branching unit Mc Apparent core density & f, kg/m3 Closed cell content, % Anisotropy degree A

Spray 740 48.0 96.9 1.00–1.05

Pour 450 65–70 93.0 1.20–1.30

740 65–70 93.0 1.00–1.05

C Compressive strength // 33 , MPa C Compressive strength o 22 , MPa Modulus in compression // EC3 , MPa Modulus in compression o EC2 , MPa Poisson’s ratio in compression // 32 Poisson’s ratio in compression o 23 Tensile strength o "T22 , MPa Modulus in tension o ET2 , MPa T Elongation at break o 22 ,% Inter-layer adhesion, MPa

0.16 0.17 4.21 4.61 0.15 0.29 0.41 7.41 23.1 0.38

0.60 – 12.00 – – – 0.65 13.00 12.00 –

0.50 – 10.00 – – – 0.75 10.00 12.00 –

C Compressive strength // 33 , MPa C Compressive strength o 22 , MPa Modulus in compression // EC3 , MPa Modulus in compression o EC2 , MPa T Tensile strength o 22 , MPa Modulus in tension o ET2 , MPa T Elongation at break o 22 % T T Ratio E2, 77 =E2, 296 Ratio EC3, 77 =EC3, 296

0.61 0.62 6.37 9.76 1.03 20.49 5.33 2.76 1.51

1.10 – 25.00 – 1.00 28.00 3.00 1.95 2.00

1.20 – 22.00 – 1.30 21.00 4.30 2.10 2.20

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Conclusions 1. Mechanical properties of rigid PUR foams have been investigated in dependence of polymer matrix’ parameter molecular weight per branching unit Mc With Mc values increasing from 360 to 1150, the PUR foams’ tensile strength perpendicular to rise direction and the corresponding elongation at break increases, while modulus in tension perpendicular to rise direction decreases both at temperatures 296 and 77 K. For PUR foams, core density 65–70 kg/m3, tensile strength perpendicular to rise direction and corresponding elongation at break at 77 K can be increased for 30–40% by increasing polymer matrix’ molecular weight per branching unit Mc value from 450 to 740. 2. With polymeric matrix’ Mc values increasing from 300 to 1150, the PUR foams’ compressive strength parallel to rise direction at 296 K decreases, but grows at 77 K. 3. With PUR foams’ polymeric matrix’ Mc increasing, the intermolecular interaction increases, which is testified by the growth in the concentration of the urethane groups connected with H-bonds. This factor promotes the increase in the tensile strength perpendicular to rise direction of PUR foams. 4. Layered spray PUR foams with core density 48 kg/m3and Mc ¼ 740 exhibit competitive tensile strength and elongation at break at cryogenic temperature 77 K.

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