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An experimental method for evaluating isocyanate conversion and trimer formation in polyisocyanate–polyurethane foams
European Polymer Journal 37 (2001) 949±954
An experimental method for evaluating isocyanate conversion and trimer formation in polyisocyanate±polyurethane foams Michele Modesti *, Alessandra Lorenzetti Department of Chemical Process Engineering, Padova University, Via Marzolo 9, 35100 Padova, Italy Received 3 April 2000; accepted 31 August 2000
Abstract In this paper, we have developed simple though ecient test method for evaluating both isocyanurate formation and isocyanate degree of conversion using FT-IR analyses and have applied them to polyisocyanurate±polyurethane foams to verify the reliability of the results obtained and evaluate the in¯uence of isocyanate index on both these quantities. We have also characterised the foams from a physical±mechanical point of view and demonstrated that the dimensional stability and compression strength of polymers are closely related to isocyanurate content and therefore to isocyanate index. The results obtained have clearly revealed that an increase in isocyanate index leads to an increase in trimer content and consequently to an improvement of mechanical properties; on the other hand, an increase in the isocyanate index brings about a decrease in isocyanate conversion. Moreover, since isocyanurate content and free isocyanate amount aect the ®re behaviour of the foam we will apply the procedure proposed in this article to analyse the in¯uence of both factors on the ®re behaviour of such foams; the results of our research will be published in a future work. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Polyurethane; Polyisocyanurate; Isocyanate conversion; Physical±mechanical properties
1. Introduction Polyurethane (PUR) foams feature excellent mechanical properties in relation to their low density, as well as low thermal conductivity, which makes them generally suitable for use as thermal insulator materials. However, the use of these polymers at high temperatures is limited due to the low thermostability of the urethane groups. In order to increase their thermal stability, the PUR foams could be modi®ed by substituting urethane groups with more heat-resistant heterocyclic groups in the polymer chain, such as isocyanurate rings. The reactions that lead to polyurethane and polyisocyanurate (PIR) foams are described by the following expressions:
· Carbodiimide formation: · Isocyanurate formation:
· Urethane formation:
*
Corresponding author. Fax: +39-49-827-5555. E-mail address: [email protected] (M. Modesti).
It has been reported by several authors [1,2] that PIR± PUR foams have greater thermal stability, improved ®re
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 2 0 9 - 3
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M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
resistance and greater dimensional stability than PUR foams. Due to the fact that foams containing too few isocyanurate rings do not possess the desired properties, and foams containing too many isocyanurate rings are friable, it is necessary to evaluate the amount of isocyanurate groups formed. For this purpose, scienti®c literature provides relations [3] that enable a theoretical evaluation of trimer content. However, the theoretical values are most likely far from being realistic, since isocyanate reacts in a fairly complex way, generating a number of dierent products including, for example, urethane, urea, allophanate, biuret, carbodiimide, isocyanurate, and several others. For these reasons, experimental test methods need to be developed to con®rm the real trimerisation level of isocyanate to isocyanurate and to follow the degree of isocyanate conversion. In this paper, we describe the use of FT-IR analyses as a test method for evaluating the isocyanate conversion and the extent of isocyanurate formation.
of the sample. Therefore, we are able to evaluate the free isocyanate and the conversion degree a as: NCOf a 1 100 4 NCOo
2. Model development
3. Experimental
It is a known fact that chemical bonds absorb radiation at certain characteristic frequencies and that the intensity of absorbance is directly related to concentration, in accordance with Lambert±BeerÕs law:
3.1. Raw materials
A ebC
1
where A is absorbance, e, extinction coecient (mol mm/ l), b, optical path (mm), C, concentration (mol/l). Having determined IR absorbance from the spectrum, the eb product from the calibration curve and the optical path, the calculation of isocyanate concentration and therefore the conversion degree becomes a simple matter. However, because the spectrum is collected in the solid phase, we cannot assume that the optical paths of dierent samples are constant or perfectly known, which means that Eq. (1) is not applicable. To evaluate the concentration of a chemical bond without knowing the optical path of the sample, the absorbance of another group should be considered and taken as reference (indicated as ref. in the following expressions). The reference group should present a constant concentration in all foams and not react. Lambert±BeerÕs law for both groups could be written as: A ebC Aref ebref Cref
2
where a is isocyanate conversion degree, NCOf , ®nal % in weight of isocyanate, NCOo , initial % in weight of isocyanate. Unfortunately, Eq. (3) cannot be used for a quantitative evaluation of trimer formation, as the calibration curve could not be built up and therefore the eb product could not be calculated. Although an absolute estimation of isocyanurate formation is not possible, it is practicable to obtain comparative results by using the PIR±PUR ratio [4], de®ned as the ratio between isocyanurate and urethane absorbance. Though this ratio provides merely comparative results, it is nevertheless an index of the quantity of isocyanurate formed.
The raw materials employed in the preparation of PIR±PUR foams are the following: · Polymeric methane diphenyl diisocyanate (MDI): Tedimon 385 (Enichem, Italy). NCO% 30:5; average functionality 2:8. · Polyester polyol: Glendion 9801 (Enichem, Italy). N°OH 351 mg KOH/g. · Polyester polyol obtained from 55% in weight of adipic acid, 42% in weight of diethylene glycol, and 3% in weight of trimethylolpropane: Diexter G-173 (Coim, Italy). N°OH 57 mg KOH/g. · Catalysts: the catalyst employed for cyclotrimerisation of isocyanate is DABCO K-15 (70% potassium octoate in diethylene glycol) (Air Product, The Netherlands), whereas the catalyst employed for urethane formation is Policat 5 (pentamethyl-diethylenetriamine) (Air Product, The Netherlands). · Surfactant agents: Tegostab B8469 (GoldschmidtItaly). · Blowing agent: water was used as a chemical blowing agent, as the reaction between water and isocyanate leads to the production of CO2 , according to the following reaction:
and therefore the concentration of isocyanate groups is represented by the equation: A ebref C Cref 3 Aref eb
3.2. Foam preparation
This relation makes it possible to calculate the amount of a chemical bond, independent from the optical path
The foam formulations are reported in Table 1. We prepared foams characterised by three dierent isocya-
R±NCO H2 O ! R±NH±CO±OH ! R±NH2 CO2 "
M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
nate indexes, which are by convention 100 times the ratio of free isocyanate groups to isocyanate reactive groups (hydroxyl, amine and water) [5]. Moreover, we employed water only as a blowing agent, thus avoiding the use of n-pentane (which is currently the most commonly used blowing agent) to study only the in¯uence of isocyanate index on trimer formation and mechanical properties. The foam density was normalised to about 40 kg/m3 by varying the dosage of water. In order to obtain a mean value of the properties evaluated to ensure reliability of the results, we prepared three samples of each kind of formulation and the results reported are the average values of the properties of the three samples. The components of the formulation were mixed by means of a handmixing technique for 15 s at room temperature and poured into an aluminium cube measuring 250 250 250 mm3 and containing a paper cup. During the reaction, we measured the cream time, which is the time of the beginning of reaction, and the gel time, namely the time needed for the mass to reach the gel point (Table 1). After that, the foams were placed in an oven at 70°C for 24 h before carrying out mechanical characterisation. FT-IR analyses were performed on samples at 24 h, at 5 days and at 15 days after foam preparation, keeping them in an oven at 70°C.
951
transmitted through them. The infrared spectrum was obtained from 64 scans between 4000 and 600 cm 1 with a resolution of 4 cm 1 . The data, collected as transmittance values, were converted into absorbance, which is directly related to the concentration of the functional group by Lambert±BeerÕs law. Frequency assignments for a typical PIR±PUR functional group are given in Table 2 [6]. The absorbance of the dierent groups was measured by employing a common baseline that crosses three anchor points at approximately 1470±1350±1160 cm 1 for the trimer calculation and 2480±2015±985 cm 1 for the free isocyanate calculation. For the quantitative free isocyanate calculation it was necessary to choose a reference group and determine a calibration curve for both the isocyanate and the reference. We assumed the phenyl group as a reference because phenyl (unlike urethane) absorption remains constant for a given formulation and is independent of the type of reaction. The calibration curves have been obtained by plotting the concentration of several solutions of MDI in tetracarbonchloride versus the absorbance of the isocyanate or the phenyl group. The curves are straight lines (meaning that Lambert±BeerÕs law is valid) (Figs. 1 and 2) and their slope represents the value of the eb product for the isocyanate and the phenyl group, respectively.
3.3. FT-IR analysis The FT-IR analysis was performed on a Nicolet Nexus 670 using diuse re¯ectance. The transmission technique could not be employed, because PIR±PUR foams are non-translucent solids and energy cannot be
Table 1 Formulations, kinetic parameters and some physical properties of PIR±PUR foams produced
Table 2 Wave number of typical PIR±PUR absorbance Chemical bond
Wave number (cm 1 )
Chemical structure
Isocyanate Phenyl Isocyanurate Urethane
2277 1595 1415 1220
N@C@O Ar±H PIR ±C±O±
NCO index 200
250
300
Formulation Polyol [Glendion 9801] (g) Polyol [diexter G-173] (g) Water (g) Surfactant agent (g) Catalyst (g) Catalyst (g) Isocyanate (g)
50 50 4.0 2.5 0.2 1.5 222
50 50 5.0 3.0 0.2 1.5 316
50 50 6.6 3.0 0.3 2.8 453
Kinetic parameters Cream time (s) Gel time (s)
26 90
32 123
33 132
38 31.9
42 32.2
39 32.3
Physical properties Density (kg/m3 ) Thermal conductivity (mW/m K)
Fig. 1. Calibration curve for isocyanate group.
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M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
Fig. 3. Variation during the time of isocyanate conversion vs. isocyanate index.
Fig. 2. Calibration curve for phenyl group.
3.4. Physical±mechanical characterisation The thermal conductivity of the foams has been evaluated according to ISO 8301. The compression strength of the foams has been measured according to ISO 844. Owing to the fact that PIR±PUR foams and, in particular, free-rise foams are anisotropic materials, they normally display diering compression strength values along their parallel and perpendicular directions due to an elongation of the foam cells in the direction of rise. As a consequence, both values need to be determined. To test the dimensional stability of the foams, we measured the variation of the length and width of samples, under ®xed conditions, in accordance with ISO 2796 Standards. The results are expressed as percentage variations of the initial length or width, namely: Lf Lo DL% 100 Lo where DL% is percentage variation, Lo , initial length (or width) of the sample (mm), Lf , ®nal length (or width) of the sample (mm).
complete, even after a long time. Moreover, the ®nal conversion degree is closely linked to the isocyanate index; the higher the isocyanate index, the lower the ®nal degree of isocyanate conversion. This is most likely due to the faster initial rate of trimer formation for high isocyanate indexes, which leads to a highly crosslinked structure, thus making it more dicult for the trimerisation process to continue. As the amount of isocyanurate formed is lower, even the ®nal isocyanate conversion degree is consequently lower. 4.2. Trimer formation The results obtained reveal the fact that the higher the isocyanate index, the higher the PIR±PUR ratio, as expected (Fig. 4). Moreover, we noticed that the PIR± PUR ratio is not constant in time; at the beginning it decreases faster at higher index, after which it increases. This is most likely due to the fact that for foams with a high isocyanate index the initial rate of trimer formation is higher than urethane one and then the PIR±PUR ratio is fairly high. After that, as trimerisation leads to a highly crosslinked structure, it becomes more dicult
4. Results and discussion 4.1. Degree of isocyanate conversion The results obtained show that isocyanate conversion increases during the conditioning period (Fig. 3); in fact, as the trimerisation of isocyanate occurs slowly after the gel point because of the high crosslinking density of the polymeric matrix, the amount of free isocyanate decreases slowly too. Therefore, the increase in isocyanate degree of conversion requires a certain amount of time, although the ®nal conversion degree is far from being
Fig. 4. Variation during the time of PIR±PUR vs. isocyanate index.
M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
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than the urethane formation process, and consequently the PIR±PUR ratio decreases. In the ®nal polymerisation step the polyols have entirely reacted and trimerisation becomes the only reaction possible. Therefore, the PIR±PUR ratio increases until the reaction is almost completed, where it reaches a constant value. 4.3. Physical±mechanical properties The thermal conductivity of the foams is reported in Table 1. It is clear from this table that the values obtained are quite high with respect to the common thermal conductivity values of polyurethane foams and are very similar to air thermal conductivity, as expected. These results are not a consequence of the open cell structure (at SEM, we can see that the cells are substantially closed (Fig. 5)), but rather of the blowing agent used. Having used water as a blowing agent, the gas inside the cells is CO2 , that diuses out from cells rapidly and is replaced by a counter-diusion of air; hence the thermal conductivity of the foam (kf ) is quite similar to that of air. In fact, since the thermal conductivity of a foam (kf ) is well represented as the sum of gas phase (kg ), solid phase (ks ) and radiative conductivity, namely:
Fig. 6. Variation of compression strength vs. isocyanate index.
kf kg ks kr and for all blowing agents kg is more than one half of kf [7], the thermal conductivity of the foams is very similar to that of the gas inside the cell, which is, in this case, air. During mechanical characterisation, we notice that foam compression strength is strictly related to the isocyanate index and consequently to the trimer content (Fig. 6); compression strength increases with increasing trimer content, since the polymer matrix becomes more highly crosslinked and therefore more resistant. Instead, for isocyanate indexes greater than 250, compression
Fig. 7. Variation of linear deformation vs. isocyanate index.
strength decreases because of excessive polymer crosslinking that causes the foams to become friable. From the measurements of dimensional stability of the foams (Fig. 7), we notice that all foams shrink (DL% < 0), as the diusion rate of CO2 is one order of magnitude greater than that of air [8]. Therefore, the internal cell gas pressure diminishes and this causes the foam to shrink, although the shrinkage of the foams is quite limited, remaining always less than 1%. It is also evident that dimensional stability reveals the same trend as that of compression strength, that is, it increases with increasing density [9] and with increasing isocyanate index. Thus, the greater the density or the isocyanate index, the greater the polymer matrix stiness and, therefore, the lower the foam shrinkage.
5. Conclusions
Fig. 5. SEM image of PIR±PUR foams (65).
As it is a commonly known fact that the mechanical properties of PUR±PIR foams, such as compression strength and dimensional stability, are aected by polymer structure and particularly by isocyanurate content, the aim of this work has been to apply an
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M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
experimental test method capable of providing an evaluation of the real trimerisation of isocyanate to isocyanurate and therefore an evaluation of the trimer content. Moreover, a test method for the evaluation of isocyanate conversion degree has been treated, as the amount of free isocyanate, which is clearly related to isocyanate conversion, aects the ®re behaviour of the foams, which will be treated in a future work. The results of our research have demonstrated that the use of FT-IR analyses and the employment of special spectrum collection techniques (diuse re¯ectance) enable straightforward evaluation of both the real amount of formed isocyanurate and the real isocyanate conversion degree. It has been demonstrated that trimer content increases with increasing isocyanate index, although the real amount of isocyanurate formed is dierent from the theoretical quantity. As the isocyanate conversion is not complete, especially in the case of high isocyanate index, not all the isocyanate exceeding the polyols reacts to form isocyanurate and a part remains free. The results of the mechanical characterisation of the foams have con®rmed that compression strength and dimensional stability properties are closely related to the isocyanate index and to the isocyanurate content; both compression strength and dimensional stability increase with increasing isocyanate index. It has also been demonstrated that for high isocyanate indexes (>250), the compression strength of the foams decreases due to higher friability of the polymeric matrix. From the results obtained it is therefore evident that simple test method, like those proposed in this article, could be used for studying PIR±PUR foams, in order to optimise isocyanate index and the trimerisation catalyst
level to increase isocyanurate conversion and consequently improve the mechanical properties and ®re behaviour of the foams.
Acknowledgements This research was supported by the Italian Ministry of University and Scienti®c Technological Research (ex. 40%).
References [1] Frisch KC, Sendijarevic V, et al. Proceeding of Polyurethane World Congress 1991, Nice, 1991. p. 850±5. [2] Chittolini C. Proceeding of Polyurethane World Congress 1997, Amsterdam, 1997. p. 655±9. [3] Hipchen DE. Proceedings of SPI Fourth International Cellular Plastics Conference, Montreal, 1976. p. 217. [4] Burns SB, Schimdt LE. Proceeding of Polyurethane World Congress 1993, Vancouver, 1993. p. 234±40. [5] Bailey FE. Handbook of polymeric foams and foams technology, Munich: Hanser; 1991. p. 61. [6] Dillon JG. Infrared Spectroscopic Atlas of Polyurethanes, Technomic Publishing, 1989. [7] Ferrero-Heredia M, Day J, Ward W. Polyurethane World Congress 1995, Chicago, 1995. p. 310. [8] Singh SN, Lynch JJ, Daems D. Polyurethane World Congress 1995, Chicago, 1995. p. 11±9. [9] Lorenzetti A. In¯uenza della gra®te espandibile sulla stabilita termica e reazione al fuoco di espansi poliisocianurici, Degree on Chemical Engineering, A.A. 1998/1999. p. 71±8.
An experimental method for evaluating isocyanate conversion and trimer formation in polyisocyanate±polyurethane foams Michele Modesti *, Alessandra Lorenzetti Department of Chemical Process Engineering, Padova University, Via Marzolo 9, 35100 Padova, Italy Received 3 April 2000; accepted 31 August 2000
Abstract In this paper, we have developed simple though ecient test method for evaluating both isocyanurate formation and isocyanate degree of conversion using FT-IR analyses and have applied them to polyisocyanurate±polyurethane foams to verify the reliability of the results obtained and evaluate the in¯uence of isocyanate index on both these quantities. We have also characterised the foams from a physical±mechanical point of view and demonstrated that the dimensional stability and compression strength of polymers are closely related to isocyanurate content and therefore to isocyanate index. The results obtained have clearly revealed that an increase in isocyanate index leads to an increase in trimer content and consequently to an improvement of mechanical properties; on the other hand, an increase in the isocyanate index brings about a decrease in isocyanate conversion. Moreover, since isocyanurate content and free isocyanate amount aect the ®re behaviour of the foam we will apply the procedure proposed in this article to analyse the in¯uence of both factors on the ®re behaviour of such foams; the results of our research will be published in a future work. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Polyurethane; Polyisocyanurate; Isocyanate conversion; Physical±mechanical properties
1. Introduction Polyurethane (PUR) foams feature excellent mechanical properties in relation to their low density, as well as low thermal conductivity, which makes them generally suitable for use as thermal insulator materials. However, the use of these polymers at high temperatures is limited due to the low thermostability of the urethane groups. In order to increase their thermal stability, the PUR foams could be modi®ed by substituting urethane groups with more heat-resistant heterocyclic groups in the polymer chain, such as isocyanurate rings. The reactions that lead to polyurethane and polyisocyanurate (PIR) foams are described by the following expressions:
· Carbodiimide formation: · Isocyanurate formation:
· Urethane formation:
*
Corresponding author. Fax: +39-49-827-5555. E-mail address: [email protected] (M. Modesti).
It has been reported by several authors [1,2] that PIR± PUR foams have greater thermal stability, improved ®re
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 2 0 9 - 3
950
M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
resistance and greater dimensional stability than PUR foams. Due to the fact that foams containing too few isocyanurate rings do not possess the desired properties, and foams containing too many isocyanurate rings are friable, it is necessary to evaluate the amount of isocyanurate groups formed. For this purpose, scienti®c literature provides relations [3] that enable a theoretical evaluation of trimer content. However, the theoretical values are most likely far from being realistic, since isocyanate reacts in a fairly complex way, generating a number of dierent products including, for example, urethane, urea, allophanate, biuret, carbodiimide, isocyanurate, and several others. For these reasons, experimental test methods need to be developed to con®rm the real trimerisation level of isocyanate to isocyanurate and to follow the degree of isocyanate conversion. In this paper, we describe the use of FT-IR analyses as a test method for evaluating the isocyanate conversion and the extent of isocyanurate formation.
of the sample. Therefore, we are able to evaluate the free isocyanate and the conversion degree a as: NCOf a 1 100 4 NCOo
2. Model development
3. Experimental
It is a known fact that chemical bonds absorb radiation at certain characteristic frequencies and that the intensity of absorbance is directly related to concentration, in accordance with Lambert±BeerÕs law:
3.1. Raw materials
A ebC
1
where A is absorbance, e, extinction coecient (mol mm/ l), b, optical path (mm), C, concentration (mol/l). Having determined IR absorbance from the spectrum, the eb product from the calibration curve and the optical path, the calculation of isocyanate concentration and therefore the conversion degree becomes a simple matter. However, because the spectrum is collected in the solid phase, we cannot assume that the optical paths of dierent samples are constant or perfectly known, which means that Eq. (1) is not applicable. To evaluate the concentration of a chemical bond without knowing the optical path of the sample, the absorbance of another group should be considered and taken as reference (indicated as ref. in the following expressions). The reference group should present a constant concentration in all foams and not react. Lambert±BeerÕs law for both groups could be written as: A ebC Aref ebref Cref
2
where a is isocyanate conversion degree, NCOf , ®nal % in weight of isocyanate, NCOo , initial % in weight of isocyanate. Unfortunately, Eq. (3) cannot be used for a quantitative evaluation of trimer formation, as the calibration curve could not be built up and therefore the eb product could not be calculated. Although an absolute estimation of isocyanurate formation is not possible, it is practicable to obtain comparative results by using the PIR±PUR ratio [4], de®ned as the ratio between isocyanurate and urethane absorbance. Though this ratio provides merely comparative results, it is nevertheless an index of the quantity of isocyanurate formed.
The raw materials employed in the preparation of PIR±PUR foams are the following: · Polymeric methane diphenyl diisocyanate (MDI): Tedimon 385 (Enichem, Italy). NCO% 30:5; average functionality 2:8. · Polyester polyol: Glendion 9801 (Enichem, Italy). N°OH 351 mg KOH/g. · Polyester polyol obtained from 55% in weight of adipic acid, 42% in weight of diethylene glycol, and 3% in weight of trimethylolpropane: Diexter G-173 (Coim, Italy). N°OH 57 mg KOH/g. · Catalysts: the catalyst employed for cyclotrimerisation of isocyanate is DABCO K-15 (70% potassium octoate in diethylene glycol) (Air Product, The Netherlands), whereas the catalyst employed for urethane formation is Policat 5 (pentamethyl-diethylenetriamine) (Air Product, The Netherlands). · Surfactant agents: Tegostab B8469 (GoldschmidtItaly). · Blowing agent: water was used as a chemical blowing agent, as the reaction between water and isocyanate leads to the production of CO2 , according to the following reaction:
and therefore the concentration of isocyanate groups is represented by the equation: A ebref C Cref 3 Aref eb
3.2. Foam preparation
This relation makes it possible to calculate the amount of a chemical bond, independent from the optical path
The foam formulations are reported in Table 1. We prepared foams characterised by three dierent isocya-
R±NCO H2 O ! R±NH±CO±OH ! R±NH2 CO2 "
M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
nate indexes, which are by convention 100 times the ratio of free isocyanate groups to isocyanate reactive groups (hydroxyl, amine and water) [5]. Moreover, we employed water only as a blowing agent, thus avoiding the use of n-pentane (which is currently the most commonly used blowing agent) to study only the in¯uence of isocyanate index on trimer formation and mechanical properties. The foam density was normalised to about 40 kg/m3 by varying the dosage of water. In order to obtain a mean value of the properties evaluated to ensure reliability of the results, we prepared three samples of each kind of formulation and the results reported are the average values of the properties of the three samples. The components of the formulation were mixed by means of a handmixing technique for 15 s at room temperature and poured into an aluminium cube measuring 250 250 250 mm3 and containing a paper cup. During the reaction, we measured the cream time, which is the time of the beginning of reaction, and the gel time, namely the time needed for the mass to reach the gel point (Table 1). After that, the foams were placed in an oven at 70°C for 24 h before carrying out mechanical characterisation. FT-IR analyses were performed on samples at 24 h, at 5 days and at 15 days after foam preparation, keeping them in an oven at 70°C.
951
transmitted through them. The infrared spectrum was obtained from 64 scans between 4000 and 600 cm 1 with a resolution of 4 cm 1 . The data, collected as transmittance values, were converted into absorbance, which is directly related to the concentration of the functional group by Lambert±BeerÕs law. Frequency assignments for a typical PIR±PUR functional group are given in Table 2 [6]. The absorbance of the dierent groups was measured by employing a common baseline that crosses three anchor points at approximately 1470±1350±1160 cm 1 for the trimer calculation and 2480±2015±985 cm 1 for the free isocyanate calculation. For the quantitative free isocyanate calculation it was necessary to choose a reference group and determine a calibration curve for both the isocyanate and the reference. We assumed the phenyl group as a reference because phenyl (unlike urethane) absorption remains constant for a given formulation and is independent of the type of reaction. The calibration curves have been obtained by plotting the concentration of several solutions of MDI in tetracarbonchloride versus the absorbance of the isocyanate or the phenyl group. The curves are straight lines (meaning that Lambert±BeerÕs law is valid) (Figs. 1 and 2) and their slope represents the value of the eb product for the isocyanate and the phenyl group, respectively.
3.3. FT-IR analysis The FT-IR analysis was performed on a Nicolet Nexus 670 using diuse re¯ectance. The transmission technique could not be employed, because PIR±PUR foams are non-translucent solids and energy cannot be
Table 1 Formulations, kinetic parameters and some physical properties of PIR±PUR foams produced
Table 2 Wave number of typical PIR±PUR absorbance Chemical bond
Wave number (cm 1 )
Chemical structure
Isocyanate Phenyl Isocyanurate Urethane
2277 1595 1415 1220
N@C@O Ar±H PIR ±C±O±
NCO index 200
250
300
Formulation Polyol [Glendion 9801] (g) Polyol [diexter G-173] (g) Water (g) Surfactant agent (g) Catalyst (g) Catalyst (g) Isocyanate (g)
50 50 4.0 2.5 0.2 1.5 222
50 50 5.0 3.0 0.2 1.5 316
50 50 6.6 3.0 0.3 2.8 453
Kinetic parameters Cream time (s) Gel time (s)
26 90
32 123
33 132
38 31.9
42 32.2
39 32.3
Physical properties Density (kg/m3 ) Thermal conductivity (mW/m K)
Fig. 1. Calibration curve for isocyanate group.
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M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
Fig. 3. Variation during the time of isocyanate conversion vs. isocyanate index.
Fig. 2. Calibration curve for phenyl group.
3.4. Physical±mechanical characterisation The thermal conductivity of the foams has been evaluated according to ISO 8301. The compression strength of the foams has been measured according to ISO 844. Owing to the fact that PIR±PUR foams and, in particular, free-rise foams are anisotropic materials, they normally display diering compression strength values along their parallel and perpendicular directions due to an elongation of the foam cells in the direction of rise. As a consequence, both values need to be determined. To test the dimensional stability of the foams, we measured the variation of the length and width of samples, under ®xed conditions, in accordance with ISO 2796 Standards. The results are expressed as percentage variations of the initial length or width, namely: Lf Lo DL% 100 Lo where DL% is percentage variation, Lo , initial length (or width) of the sample (mm), Lf , ®nal length (or width) of the sample (mm).
complete, even after a long time. Moreover, the ®nal conversion degree is closely linked to the isocyanate index; the higher the isocyanate index, the lower the ®nal degree of isocyanate conversion. This is most likely due to the faster initial rate of trimer formation for high isocyanate indexes, which leads to a highly crosslinked structure, thus making it more dicult for the trimerisation process to continue. As the amount of isocyanurate formed is lower, even the ®nal isocyanate conversion degree is consequently lower. 4.2. Trimer formation The results obtained reveal the fact that the higher the isocyanate index, the higher the PIR±PUR ratio, as expected (Fig. 4). Moreover, we noticed that the PIR± PUR ratio is not constant in time; at the beginning it decreases faster at higher index, after which it increases. This is most likely due to the fact that for foams with a high isocyanate index the initial rate of trimer formation is higher than urethane one and then the PIR±PUR ratio is fairly high. After that, as trimerisation leads to a highly crosslinked structure, it becomes more dicult
4. Results and discussion 4.1. Degree of isocyanate conversion The results obtained show that isocyanate conversion increases during the conditioning period (Fig. 3); in fact, as the trimerisation of isocyanate occurs slowly after the gel point because of the high crosslinking density of the polymeric matrix, the amount of free isocyanate decreases slowly too. Therefore, the increase in isocyanate degree of conversion requires a certain amount of time, although the ®nal conversion degree is far from being
Fig. 4. Variation during the time of PIR±PUR vs. isocyanate index.
M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
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than the urethane formation process, and consequently the PIR±PUR ratio decreases. In the ®nal polymerisation step the polyols have entirely reacted and trimerisation becomes the only reaction possible. Therefore, the PIR±PUR ratio increases until the reaction is almost completed, where it reaches a constant value. 4.3. Physical±mechanical properties The thermal conductivity of the foams is reported in Table 1. It is clear from this table that the values obtained are quite high with respect to the common thermal conductivity values of polyurethane foams and are very similar to air thermal conductivity, as expected. These results are not a consequence of the open cell structure (at SEM, we can see that the cells are substantially closed (Fig. 5)), but rather of the blowing agent used. Having used water as a blowing agent, the gas inside the cells is CO2 , that diuses out from cells rapidly and is replaced by a counter-diusion of air; hence the thermal conductivity of the foam (kf ) is quite similar to that of air. In fact, since the thermal conductivity of a foam (kf ) is well represented as the sum of gas phase (kg ), solid phase (ks ) and radiative conductivity, namely:
Fig. 6. Variation of compression strength vs. isocyanate index.
kf kg ks kr and for all blowing agents kg is more than one half of kf [7], the thermal conductivity of the foams is very similar to that of the gas inside the cell, which is, in this case, air. During mechanical characterisation, we notice that foam compression strength is strictly related to the isocyanate index and consequently to the trimer content (Fig. 6); compression strength increases with increasing trimer content, since the polymer matrix becomes more highly crosslinked and therefore more resistant. Instead, for isocyanate indexes greater than 250, compression
Fig. 7. Variation of linear deformation vs. isocyanate index.
strength decreases because of excessive polymer crosslinking that causes the foams to become friable. From the measurements of dimensional stability of the foams (Fig. 7), we notice that all foams shrink (DL% < 0), as the diusion rate of CO2 is one order of magnitude greater than that of air [8]. Therefore, the internal cell gas pressure diminishes and this causes the foam to shrink, although the shrinkage of the foams is quite limited, remaining always less than 1%. It is also evident that dimensional stability reveals the same trend as that of compression strength, that is, it increases with increasing density [9] and with increasing isocyanate index. Thus, the greater the density or the isocyanate index, the greater the polymer matrix stiness and, therefore, the lower the foam shrinkage.
5. Conclusions
Fig. 5. SEM image of PIR±PUR foams (65).
As it is a commonly known fact that the mechanical properties of PUR±PIR foams, such as compression strength and dimensional stability, are aected by polymer structure and particularly by isocyanurate content, the aim of this work has been to apply an
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M. Modesti, A. Lorenzetti / European Polymer Journal 37 (2001) 949±954
experimental test method capable of providing an evaluation of the real trimerisation of isocyanate to isocyanurate and therefore an evaluation of the trimer content. Moreover, a test method for the evaluation of isocyanate conversion degree has been treated, as the amount of free isocyanate, which is clearly related to isocyanate conversion, aects the ®re behaviour of the foams, which will be treated in a future work. The results of our research have demonstrated that the use of FT-IR analyses and the employment of special spectrum collection techniques (diuse re¯ectance) enable straightforward evaluation of both the real amount of formed isocyanurate and the real isocyanate conversion degree. It has been demonstrated that trimer content increases with increasing isocyanate index, although the real amount of isocyanurate formed is dierent from the theoretical quantity. As the isocyanate conversion is not complete, especially in the case of high isocyanate index, not all the isocyanate exceeding the polyols reacts to form isocyanurate and a part remains free. The results of the mechanical characterisation of the foams have con®rmed that compression strength and dimensional stability properties are closely related to the isocyanate index and to the isocyanurate content; both compression strength and dimensional stability increase with increasing isocyanate index. It has also been demonstrated that for high isocyanate indexes (>250), the compression strength of the foams decreases due to higher friability of the polymeric matrix. From the results obtained it is therefore evident that simple test method, like those proposed in this article, could be used for studying PIR±PUR foams, in order to optimise isocyanate index and the trimerisation catalyst
level to increase isocyanurate conversion and consequently improve the mechanical properties and ®re behaviour of the foams.
Acknowledgements This research was supported by the Italian Ministry of University and Scienti®c Technological Research (ex. 40%).
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