Mechanical properties and dimensional stability of water-blown PU foams with various water levels

CHEN Tao, MI Yan, DU Haijing, GAO Zhenhua* College of Material science and Engineering

Northeast Forestry University, NEFU Harbin 150040, China

Abstract—Polyurethane foam (PUF) is an outstanding material for various applications, which is manufactured by propelling liquid isocyanate-polyol mixture to form foams in the presence of blowing agent. In order to reduce the ozone depletion and environmental pollution from organic blowing agents, water-blown PU foams were prepared with various water levels (3-11phr), and the effects of water level on mechanical properties and dimensional stability of the foams were investigated in this study. The results indicated that water level had important effects on microstructures of water-blown PU foams and therefore affected the physi-mechanical properties because water not only formed the gas for foaming but also involved with the chain-extension of PU matrix. The volumetric changes of PU foams with various water levels were below 0.5% after kept at either -25oC or 85oC for 30 days, indicating the excellent dimensional stabilities of resultant PU foams.

Keywords- polyurethane foam; water blowing; mechanical properties; dimensional stability; foam morphology;

I. INTRODUCTION

Polyurethane foam (PUF) is thought to be an outstanding material for various applications, such as insulation materials, cushioning, automotive, structural materials, marine equipment, packaging and many more due to its excellent physical and mechanical performance and variable molecular structure design [1-3]. Liquid mixture of polyisocyanate monomer or pre-polymer with polyol in the presence of both a blowing agent and catalyst with proper formulations can be immediately foamed then crosslinked and cured to form PU foam. The PU foaming can be carried out with a physical blowing agent, chemical blowing agent, or mixture of the two. Because of the low thermal conductivity of the blown gas and small closed cell structure, PUF prepared with physical blowing agent has moderate density and extremely low thermal conductivity that gives rigid PUF a broad prospect in insulation energy-saving domain [4]; however, the blowing agents for economical manufacture of commercial PUF are generally these substances that depletes ozone and/or pollute environment, such as chlorofluorocarbons(CFCs such as CFC-11, CFC-12 and CFC-114), hydrochlorofluorocarbons (HCFCs such as HCFC-22, HCFC-141b and HCFC-142b) and some alkanes with low boiling points (such as pentane and cyclopentane). Of these blowing agents, CFCs and HCFCs will be phased-out beginning in 2003 due to their strong ozone depleting. Compared to the physical blown agent, water has none ozone

consumption potential value, and therefore water is recognized the perfect substitute. There are, however, some drawbacks to use water as a blowing agent such as dimensional stability, formulation viscosity, friability, blowing agent solubility [5-6]. In order to improve the properties of water-blown PUF, many a study was carried out to investigate the effects of polyol, blowing agent level, MDI level, catalyst species and level, and foaming process on foam properties [7-8]. However, most reported water-blown PUF are generally rigid polyurethane foams with density ranged from 100 to 400kg/m3 and water level less than 3phr; and water level up to 5phr is rarely reported [9]. Therefore, the aims of this paper are to prepare water-blown PU foams with higher water levels (3-11phr) and to investigate the effects of water level on the mechanical performance and dimensional stability of the resultant PU foams.

II. EXPERIMENTAL

A. Materials

Polyoxyethylene diol (Mw 400 -600 ) and polyoxy-propylene polyol (Mw 1100 - 1500 ) were purchased from Tianjin Tiantai Chemical Plant (Tianjin, China)and Fushun Jiahua Chemical Plant, respectively. Polymeric MDI (p-MDI) with isocyanate content (NCO%) of 30.1 wt% was supported by Nippon Co., Japan. Commercial foam stabilizer 8882 and 9901 were purchased from Nanjing Dymatic Shichuang chemical Co., LTD. Other chemicals were reagents grade and used without further pre-treatments.

B. Foaming process

The PU foams with various water levels were prepared at room temperature (ca. 22-25oC) by a process as follows. In a container, the mixture of polyols (100 phr), foaming catalyst (1.5 phr), stabilizer (4 phr) and various amounts of water (3, 5, 7, 9 and 11 phr) were well blended with mechanical stirrer. Then the stoichiometric p-MDI that resulted in a consistent isocyanate index of 1.05 was injected and followed 10-second violent mechanical stirring for well blending of p-MDI and the other mixtures. After that the resultant mixture was immediately poured into a mold with dimensional size 15cm×15cm×20cm for foaming, during which the cream time, gel time and take-free time were measured. Finally, the foam was kept in oven at 75oCfor 24h for post-curing.

Corresponding to Zhenhua Gao: gaozh1976@163.com

C. Density of the PU foams

The (apparent) densities of the PU foams were measured according to ASTM D 1622-08. Total five specimens with dimensional size 50 × 50 × 30 mm (length × width× thickness) were cut from the PU foam after moisture conditioned at 23oC, 50%RH for 24h. The density was calculated by M/V, where M refers to the mass of each specimen and V is the volume obtained from the products of length, width and thickness of specimen. The averaged density of five specimens was reported.

D. Mechanics properties

Compressive strength and modulus of the PU foams were measured according to ASTM D 1621-04 with compressive strain 10% and speed of crosshead movement of 2.5 mm/min. The size of the specimen was 50 × 50 × 30 mm (width × length× thickness) after moisture conditioned at 23oC, 50%RH for 24h. The averaged compressive strength and modulus of five specimens was reported.

E. Morphology of the PU foams

A QUANTA-200 scanning electron microscope (SEM, FEI Co., USA) was applied to observe the morphologies of the PU foams. All SEM samples were coated with approximately 10–20 nm of gold before SEM examination. A software, Nano-measurer (version 1.2), was employed to determine quantitatively the number of the cell (n) and its averaged diameter. Cell density was calculated using the following equations: VF = 1-ρF/ρp, N0 = (n/A)3/2/(1-VF) , where Vf is the void fraction, ρf is the density of the foamed samples, ρp

(1180kgm-3) is the density of the unfoamed samples (PU matrix) [10], n is the number of cells in the micrograph, and A is the area of the micrograph.

F. Dimensional stability

The dimensional stabilities of PU foams were determined according to ISO 2796-1986. The dimensional size of testing specimens were 50 mm × 50 mm × 30 mm after moisture conditioned at 23oC, 50%RH for 24h. The volumes (obtained from the products of lengths, widths and thicknesses) of specimen before and after kept at -25oC and 85oC, respectively, for 1, 2, 3, 4, 5, 6, 7 and 30days were measured. The dimensional stabilities characterized by volumetric swelling at 85oC and volumetric shrinkage at -25oC were calculated by Vx/V0 ×100%, where V0 denotes foam volume before heating or freezing treatment, and Vx refers to foam volume after kept at 85oC or -25oC for various hours.

III. RESULTS AND DISCUSSION

PU foam can be considered as a composite in which the air bobbles in the foam act as “reinforcement” for their incontinuous distribution and the PU resins acts as continual polymer matrix. The cellular structures of PU foam with air bobbles as reinforcement endow foam with very low density, excellent heat insulation and good buffering. The mechanical response of PU foams depend on the intrinsic properties of the polymer matrix in the cell wall and on their architecture which determined by the cell wall thickness, the size distribution and

the shape of the cells. As for the formation of typical water-blown PU foam, there are three main chemical reactions as illustrated by Eq. 1 in Figure 1. Reaction between poly-isocyanate and polyols forms long-chain polyurethane macromolecules via urethane linkages. Compatible blending of polyisocyanate and water lead to the reaction of water with isocyanate that produces a carbamic acid which decomposes yielding heat, carbon dioxide, and a highly reactive primary amine, as shown by Eq. 2 in Figure 1. The reactive primary amine formed by Eq. 2 will immediately reacts with isocyanate to form a disubstituted urea as show by Eq. 3. The reactions in Figure 1 apparently illustrated that that the more water levels in foaming system will result in more carbon dioxide, heat and polyurea structures in resultant PU foams. The heat and carbon dioxide contribute to gas–liquid phase separation in the reactive mixture and the expansion and solidification of the gas bubbles, and, therefore play important roles in the development of the foam’s cellular structure. It was also reported that PU resin contained more di-substituted urea linkages had better mechanical properties than these contain only urethane linkages [5]. All these indicated that water level had great effects on cellular structures and mechanical properties.

Figure1. Illustration of key reactions for polyurethane foaming

As one-shot process was used to produce PU foams, p-MDI, polyol and water were simultaneously mixed along with suitable catalyst and stabilizer. The reactions illustrated in Figure 1 began immediately with reaction rates depending upon the catalyst level, catalyst species, water level, isocyanate index, foaming temperature, and so on. In current study, all foaming parameters except water level were consistent. Table 1 presented the effects of water level on the foaming properties, physic-mechanical properties and cellular structures of PU foams. With the water level increased from 3phr to 11phr gradually, the cream time, gel time and tack-free time of reaction mixture increased accordingly. Because the amount of polyol was constant and isocyanate index (1.05, the amount of isocyanate used relative to the theoretical equivalent required to react with the polyol and water) were constant, the more water level in foaming mixture represented not only the more water content but also more p-MDI content and more NCO/OH mole ratio. For instance, foaming mixtures with water level increased from 3phr to 11phr, the water content increased from 1.36wt% to 3.07wt% while p-MDI content from 50.8wt% to 67.5wt% and NCO/OH mole ratio from 1.86 to 4.02. Due to much more mass increase of p-MDI than water, the water became less compatible with the hydrophobic p-MDI, and, therefore the water-isocyanate reaction rate was reduced as indicated by the longer cream time. Meanwhile, the higher NCO/OH ratio also led to the slowly crosslinking reaction, and then resulted in longer gel time and tack-free time. If the foaming time was sufficient, the more water levels in the forming system gave off the more carbon dioxide and produced PU foam with lower density [11], as shown by the foam densities in Table 1.

TABLE I. TABLE I. EFFECTS OF WATER LEVEL ON PROPERTIES OF FOAMS

Foam ID F3 F5 F7 F9 F11

water level(phr) 3 5 7 9 11

Cream time(s) 20 27 28 30 35

Gel time(s) 49 72 80 85 91

Tack-free time(s) 62 110 134 200 240

Density(kg·m-3) 46.3 33.3 27.1 24.6 24.8

Averaged cell diameter (µm) 380.1 439.0 498.0 461.2 458.3

Cell density (cells·mm-3) 701.4 648.1 558.4 652.2 656.5

Compressive strength (kPa) 330.2 227.4 181.6 161.0 164.3

Compressive modulus (MPa) 6.28 5.09 3.93 3.14 3.41

Volume shrinkage (%) @-25oC for 30days

0.12 0.18 0.16 0.13 0.11

Volume swelling (%) @85oC for 30days

0.11 0.32 0.23 0.25 0.24

However, the SEM analysis indicated that the cellular structures of obtained PU foams did not vary proportionally with the ever-increasing water levels. The cell density that reflects the cell counts per cubic millimeter were decreased initially with water level increased from 3phr to 7phr and then increased with further increase of water level; the averaged cell diameter showed inverse tendency but the biggest cell diameter came to water level of 7phr. The formations of these cellular structures were attributed to the equilibrium between gas generation and stabilization of gas bubbles. When low water level such as 3phr or below was used, the water content and p-MDI content were so low that they could be easily blended and compatible with each other for quick gas-generated reaction as indicated by the shortest cream time; meanwhile the NCO/OH mole ratio came to minimal for 3phr water level compared with other cases in this study, resulting in the faster increase of molecular weight of PU resin due to the faster MDI-polyol reaction, as indicated by the shortest gel time and tack-free time. Before the CO2 bubbles expanded to the maximal volume, the molecular weight of PU resin in bubble walls has sufficiently built up and solidified to well fix the bubbles. As a result, the foams obtained by 3phr of water level had highest cell density and lowest cell diameter. When water level was 11phr, water content, p-MDI content and NCO/OH mole ratio were highest; all these led to poor compatibility between water and hydrophobic p-MDI and slow increase of molecular weight of PU matrix. Therefore, the CO2 bubbles formed at the beginning of foaming could not be stabilized by low viscosity PU matrix and run away from foaming system. At the end of foaming, the continual MDI-polyol reaction yielded high enough molecular weight or viscosity of PU matrix for stabilizing CO2 bubbles, but CO2 generated at this stage could not afford the sufficient expansions for the bubbles. The escapes of CO2 gas led to the open cell of foam, and the obvious opening of cell walls observed at the PU foams produced with water levels of 9phr and 11phr, as showed by SEM micrographs in Figure 1. The unmatched rates between CO2 generation and molecular-weight increase led to lower cell density and cell diameter though highest water level. Results in

Table 1 indicated that the matched rates came to the water level at 7phr.

Figure 2. SEM micrographs of PU foams with different water levels

Figure3. Volumetric shringkage at -25oC for 30days

Figure4. Volumetric swelling at 85oC for 30days

The results of compression test in Table 1 indicated that

compressive strength decreased from 330.2KPa to 164.3KPa and compressive modulus decreased from 6.28 to 3.41 MPa with water level increased from 3 to 11phr. It is well known

that compressive strength (E) of PUF has a significant relationship with density (ρ) , as lnE = lnA + n·lnρ, where A is a constant related to the temperature and physical properties of the PU resin and n is related to the deformation mechanics of cellular materials [11]. By regressing compressive strength against density in Table 1 resulted in lnE = 1.4885+1.1239lnρ (R2 = 0.9998), which showed the strong dependency of the compressive strength on density. Because the densities of PU foams also were related to the water levels used during foaming, it is possible to control both density and mechanical properties of water-blown PU foam by varying the water level.

Dimensional stability is also of great importance for the applications of PU foam in various fields. Figure 3 and Figure 4 presented the volumetric changes of PU foams prepared with various water levels at temperature of -25oC and 85 oC for up to 30days, respectively. When kept the foams (F3, F5, F7, F9 and F11) at -25oC for 7days, the volumetric shrinkages ranged from 0.11% (F3) to 0.16% (F7); and ranged from 0.12% (F5) to 0.16 (F7) for 30days. When kept the foams at 85oC for 7days, the volumetric expansions ranged from 0.09% (F3) to 0.15% (F11); and ranged from 0.13% (F3) to 0.33 (F5) for 30days. All these values were much smaller than the requirements of some commercial standards such as Chinese standard GB/T 21558-2008 (<1.5%), indicating the excellent dimensional stabilities of PU foams prepared with higher water levels. The above results also indicated that water level had slight effect on dimensional stability of PU foams prepared with high water levels because the foam framework of water-blown PU foams were strong enough to balance the pressure difference between the inside and outside of the foam during freeze and thermal statuses. Compared the results in Figure 3 with that in Figure 4, volumetric changes at 85oC for 30days of each foams except the one prepared with 3phr were about two times as much as that at -25oC, implied that the resultant PU foams had better freeze resistance than heat resistance.

IV. CLUSSIONS

Polyurethane foams with good mechanical properties, very low density and excellent dimensional stability could be prepared by fully water-blown technology with water levels from 3phr to 11phr. With water level increased from 3phr to 11phr, the cream time, gel time and tack-free time of foaming

mixtures increased accordingly, while the density and mechanical properties decreased gradually and the cell diameter initially decreased and then increased. Water level had slight effect on dimensional stability of PU foams because of the strong foam framework of water-blown PU foams.

ACKNOWLEDGMENT

The authors thank to financial supports by the Fundamental Research Funds for the Central Universities in China (DL09EB02-2) and the Young Top-notch Talent Support Program of Northeast Forestry University (YTTP-1011-21).

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