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Formulation, Preparation, and Characterization of Polyurethane Foams s L. Pinto Moise Department of Chemistry and Biochemistry, and CQB, Faculty of Sciences, University of Lisbon, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
[email protected] Polymer chemistry, although recognized as an important part of chemistry, does not find a significant presence in many undergraduate chemistry curricula. However, polymer chemistry should be given appropriate attention in undergraduate curricula because of its importance in the chemical industry and in understanding chemical principles (1). This article describes a simple and illustrative laboratory experiment with polyurethane foams that can be implemented in the undergraduate chemistry curricula. Polyurethanes are an important class of polymers used to produce a large number of products. The polyurethane applications as coatings, foams, fibers, and elastomers demonstrate the flexibility of urethane chemistry and the creativity of polymer scientists. However, experiments for producing polyurethanes in the laboratory, especially as foams, are not easily available. The experiment described here provides a simple procedure to synthesize polyurethane foams. This experiment has been used as second-year laboratory project for a technological chemistry course. The methodology and reagents are similar to those used in polyurethane industry, which provides students with the knowledge and skills required to produce polyurethanes. The student response to the experiment was positive since they acknowledged the direct relation of this laboratory experiment with the production methods used in industry. This aspect increased their interest and motivation because they recognize that they were acquiring skills that may be applied in real-world polymer production.
alcohol (OH) groups must be equal. In practice, a small isocyanate excess is used to compensate for the moisture (water) usually present in polyols, which also reacts with isocyanates. The so-called NCO index is a measure of the ratio between the amount of NCO and OH groups in a given formulation, that is, (amount of NCO)/(amount of OH). For example, a formulation with 1.05 NCO index has a 5% excess (in moles) of NCO groups than needed to completely react with the OH groups. To produce foams, a gas must be injected or formed at the same time that the polymerization occurs. Different methods can be used for this purpose. The use of low boiling point solvents that vaporize during polymerization, carbon dioxide injection in the molds, or adding water to the formulation are common methods. The use of Freon for this purpose has been banned because of environmental regulations. When water is used, the formation of carbon dioxide occurs by reaction with isocyanate:
Formation of Polyurethanes
The intermediate carbamic acid decomposes to carbon dioxide and an amine. The amine then reacts with another isocyanate to produce a urea
Industrially, polyurethanes are produced by the reaction between a polyisocyanate and a polyol (also called polyalcohol). A complete discussion of the chemistry and properties of polyurethanes is outside the scope of this article, but can be found in other publications (2-4). Nevertheless, a small introduction to the main chemical reactions will be given with a discussion on their implications for calculating the required reactant amounts. The polymerization reaction for producing polyurethanes involves the condensation of a polyisocyanate and a polyol, for example,
Thus, each mole of water consumes two isocyanate moles and generates 22.4 dm3 of carbon dioxide gas (considering standard pressure and temperature). A surfactant is usually used in the formulation to help the stabilization of the gas-liquid emulsion during the initial moments of foam formation; polyether-polysiloxane copolymers are commonly used. An instructive discussion on foam formation is presented elsewhere (5). Calculations
To have a complete polymerization of the monomers (polyol and polyisocyanate molecules), the amount of isocyanate (NCO) and 212
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To calculate the required amounts of reagents for a polyurethane formulation, all of the above reactions must be considered. The calculations can be used to show students that basic
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chemistry principles are essential in developing polyurethane formulations. It is helpful to introduce the concept of the equivalent number of reacting groups, neq, defined as the amount of reacting groups (OH or NCO) per mass of reagent (usually in units of mmol g-1). For the case of water, 2 mol of NCO groups reacts with 18.02 g of water (1 mol), which gives neq(H2O) = (2 mol)/(18.02 g) = 111.1 mmol g-1. The fundamental idea that follows is to ensure that the amount of isocyanate groups present in the reacting mixture is sufficient to react with all the alcohol groups of the polyol and also with the water. Considering this and also that, as discussed above, a small excess of polyisocyanate is usually used, the following equation can be written mðisoÞneq ðisoÞ ¼ ½mðH2 OÞneq ðH2 OÞ þ mðpolyÞneq ðpolyÞiNCO where m(iso), m(H2O), and m(poly) are the masses of polyisocyanate, water, and polyol, respectively, neq(iso), neq(H2O), and neq(poly) are the equivalent numbers of polyisocyanate, water, and polyol, respectively, and iNCO is the desired NCO index for the formulation. This equation can be applied to calculate the mass of polyisocyanate to be used with a given mass of water and polyol to produce a polyurethane foam. Sometimes difficulties arrive in knowing the neq(iso) and neq(poly) values for a particular polyol or polyisocyanate because most suppliers do not characterize their products with the equivalent number. Polyols are normally characterized by their OH content in mass equivalent of KOH per mass of polyol: mg(KOH) g-1. This value can be converted to neq by simply dividing by the KOH molar mass (56.11 g mol-1). The percentage of NCO in the polyisocyanate (in mass of NCO per total mass) is usually used to characterize the polyisocyanates and this can be converted to neq taking into account the NCO molar mass (42.02 g mol-1). The number of OH or NCO groups in the molecule, that is, the functionality of the molecule (polyisocyanate or polyol), can also be used to obtain the neq required for the calculations presented above. Additional details are available in the supporting information. Experiment Overview The experimental details to produce and determine the apparent bulk densities of foams are given in a later section. They should be regarded as guidelines to the activities that may be developed in the laboratory with polyurethane foam synthesis. Students should be challenged to propose new formulations, based on known formulations, and make the correspondent calculations for the quantities of reagents to use. These new formulations are tested during their work in the laboratory to support some conclusions about the changes they made. This approach compels the students to understand the function of each component in the formulation by observing the effect of their variation in the reaction and final properties of the foam materials. Reagents The experimental procedure is similar to standard production procedures used in industry but at laboratory scale. Usually the first step is to “formulate” the polyol by mixing all additives (catalysts, surfactants, and blowing agents) with the polyol (or polyols) to obtain a homogeneous mixture called formulated polyol. The second step is to mix the necessary amount of the
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Table 1. Quantities of MDI and Water in the Formulation of Polyurethane Foams Foama
MDI/g
Water/cm3
1
11.5
0.60
2
12.3
0.65
3
13.1
0.70
4
13.9
0.75
5
14.7
0.80
a
Fixed quantities of glycerol propoxylate (20.0 g), DBTL (0.2 g), and silicone oil (0.4 g) were used.
polyisocyanate with the formulated polyol. Some chemical companies sell polyols already formulated to be used by mixing with the polyisocyanate, which are called two-component systems. Therefore, the procedure that follows gives students a good idea of what happens in the polyurethane industry, from the formulation step until its production and use. Several formulations could be tested, but for clarity, only one representative series is presented. For the formulated polyol, glycerol propoxylate (Aldrich, Mn ≈ 3600, 41 mg(KOH) g-1) is used as the triol, dibutyltin dilaurate (DBTL) (Merck, >97%) is used as the catalyst, and silicone oil (Dow Corning, 193 Surfactant) is used as the foam stabilizer. The polyisocyanate is diphenylmethane 4,40 -diisocyanate (MDI) (Merck, mixture of di- and triisocyanates for synthesis). The component quantities of the formulations are listed in Table 1. The main difference among the foams is the water content used to obtain foams with different densities. The polyisocyanate must also be varied accordingly, as can be seen in Table 1, to compensate for the additional amount of OH groups in the reaction mixture and to maintain the NCO index. Foam Preparation The required quantities (see Table 1) of polyol, distilled water, silicone oil, and catalyst are added in a flat-bottom polyethylene beaker (500 mL). A balance with 0.1 g precision is used to measure the component quantities; however, the volume of water is measured with a graduated pipet. The components are mixed vigorously for 1 min with a mechanical stirrer to obtain the formulated polyol. The isocyanate is then added to the same beaker and rigorously stirred for 15 s. The resulting mixture should be left undisturbed for 1 min, which allows the formation and growth of the foam. A small increase in the beaker temperature is noticed, as polymerization reactions are exothermic. It is not necessary to control the reaction temperature and the vessel should be opened to the surroundings, which allows the generated heat to be released. Under these conditions, the temperature increase (ΔT) in the vessel walls is between 30 and 35 °C, depending on the reagent's initial temperature. The initial mixture (about 30 cm3) expands to produce foams between 250 and 350 cm3 in volume (8 to 12 times its initial volume), depending on the water quantity used and temperature conditions. It is not required to use a fume hood during the synthesis if the recommended MDI is used. However, if another polyisocyanate such as toluene diisocyanate (TDI) is used, the synthesis of the foam and handling of TDI must be in a fume hood (see supporting information). After 1 h, the foams can be removed from the beaker. However, the foams should cure for at least 1 week before
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making the density measurements. The polyethylene beaker is the best choice for performing the foam synthesis because it is easy to detach the foam from this type of material. In addition, some pressure must be applied to the foam and the beaker to remove the foam, and the risk of beaker breakage is avoided if it is made from polyethylene or another thermoplastic. An interesting alternative to the described synthesis procedure is to use molds with different shapes (preferably made of some thermoplastic). To do this, simply pour the final liquid mixture into the mold and remove the foam after 1 h. This can be used to pedagogically illustrate the industrial benefits of the polyurethane foam technologies, that is, obtaining foam objects with different shapes. In fact, molding is commonly used in the polyurethane industry to obtain a range of products. For example, in the automotive industry, the polyurethane foam molding is used to produce steering wheels, car seats, and car consoles, among others. The soles of many sport shoes are also produced in an analogous way. Density Measurement During the synthesis of foams, students can evaluate the effect of the quantity of water on the foam rise inside the flasks. The students can easily observe that the foams with more water tend to expand and raise more. Differences among samples can also be observed in the cellular structure of the foams because foams produced with more water quantities have larger cells. This can be explained by the quantity of CO2 generated, which is proportional to the water content in the formulation. Usually the water content is expressed as a percentage of polyol mass, that is, mass of water per mass of polyol. These differences are reflected in the apparent bulk density of the foams and can be quantified by a simple procedure. The apparent bulk density reflects the density of the cellular material including the cells (6). In fact, the cells are small holes in the polymeric material and, thus, when their volume is considered, the apparent density becomes much lower than the effective density of the polymer, which constitutes only the cell walls. The apparent bulk density of the synthesized foams is measured in this experiment by determining the mass and geometric volume of foam samples. From the synthesized foam samples, students cut small cubes of known size (about 1 to 2 cm per side) using a sharp knife. The sizes of the samples are determined using a ruler or a Vernier caliper. In principle, one cube per foam type is sufficient, but more may be made to obtain replicate measurements. If a cork borer is available, it can be used instead to cut foam cylinders of known size. The geometrical shape of the samples should be as perfect as possible since the volume will be estimated using the mathematical definition for the chosen geometrical shape. The foam cubes are weighed using an analytical balance with at least 1 mg precision. The apparent bulk density, dapp, can then be calculated by, m dapp ¼ V where m is the mass and V is the geometrical volume. Alternatively, the volume of the samples can be determined by submerging the foam samples in a half-full measuring cylinder and record the rise in the water level. A small thin wire or spatula can be used to pull down the foam piece inside the water. However, if this approach is followed, the sample weighing should be made before the volume measurement, to avoid errors in the mass determination due to excessive moisture in samples. 214
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Figure 1. Apparent bulk density of foam produced using the formulations listed in Table 1. The bars represent the typical standard deviation of measurements ((2.5 kg m-3).
Hazards MDI is irritating to eyes, respiratory system, and skin. Polymeric MDI is the least hazardous of the commonly available polyisocyanates as it has low vapor pressure (7). This reduces its hazards during handling compared to the other common polyisocyanates (TDI, HDI), but, similar to the other polyisocyanates, is an allergen and sensitizer (7, 8). MDI reacts with hydrogen donors, in some cases violently, and the reaction with water produces carbon dioxide, which can burst containers if water is allowed to enter. Dibutyltin dilaurate is toxic if swallowed and is irritating to eyes and skin. It is also toxic to aquatic organisms, causing long-term adverse effects in the aquatic environment. Glycerol propoxylate and silicone oil are slightly hazardous in case of skin contact (irritant), of eye contact (irritant), and of ingestion. Care should be taken when handling the mechanical stirrer. The sharp instrument used to cut the specimens must be handled with care. Results It is expected that the apparent bulk density decreases with increasing water content in the formulation. Quite often, the masses of the formulation components are expressed as mass percentage of the polyol to indicate their content in the formulation. The results obtained for some foam samples are plotted in Figure 1. The error bars represent the typical standard deviation of replicate measurements ((2.5 kg m-3). The apparent bulk density decreases with the water content and varies from 54 to 33 kg m-3. The foam at the highest water content (4%, foam 5) shows an increase relative to previous foam (at 3.75%, foam 4) and deviates from the main trend of the data. This is caused by a foam collapse during the foam rise, allowing some CO2 to escape without expanding the polymer. This happens when too much gas is produced and the surfactant used (silicone oil) cannot sufficiently stabilize the emulsion. The water content at which this behavior will start to occur depends on the type and quantity of surfactant used. The students use the data collected from their experiments to plot a graph similar to that shown in Figure 1. Students should realize that the results obtained are apparent density values, and for this, it may be helpful to compare the obtained values with the
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In the Laboratory
densities of common substances (liquids or solids), for example, water as a density of about 1000 kg m-3 (1 g cm-3), which is about 20 times more than the highest value in Figure 1. However, the polyurethane polymer density of the sample series is on average 1700 kg m-3 (determined with a gas pycnometer), which means that if nonfoamed polyurethane were produced it would not float in water, contrary to the foam samples. A nonfoamed polyurethane sample may be produced simply by not using water in the formulation (and adjusting the MDI quantity to 2 g), but some difficulties may occur to remove the produced sample and also to obtain a true nonfoamed material, since small quantities of water are often present in the polyols. Each student group (3 to 4 students per group) writes a lab report due the session after the experimental work is complete (usually two sessions are needed for the experimental work). Student groups are allowed to share data with other groups, especially the data obtained with the unique formulation developed by each group. The student groups must answer questions, which directs them to conclusions about polyurethane foams and formulations. (The questions are available in the supporting information.) Before starting the experiment, students are instructed to read the questions so they can plan additional experiments that allows them to obtain results to support their answers. Additional Activities The polymer samples produced using this experimental procedure can be tested in the laboratory to determine other important properties besides the apparent bulk density. The given formulation produces polyurethane foams with elastomeric properties; thus, the tensile and mechanical properties can be determined in samples using relatively simple procedures (9, 10). Some small changes can be made in the formulation by adding a short chain polyol, such as glycerol, to increase the foam stiffness (see the supporting information).
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The rubber (elastic) properties of the polyurethanes may also be useful in the introduction to more theoretical aspects of chemistry. In fact, the rubber-like properties have been proposed by several authors to introduce basic concepts in physical chemistry and thermodynamics to students (11-14). Literature Cited 1. Stenzel, M. H.; Barner-Kowollik, C. J. Chem. Educ. 2006, 83, 1521–1530. 2. Seymour, R. B.; Kauffman, G. B. J. Chem. Educ. 1992, 69, 909–910. 3. Bailey, M. E. J. Chem. Educ. 1971, 48, 809–813. 4. Woods, G. The ICI Polyurethanes Book, 1st ed.; John Wiley and Sons: New York, 1987. 5. Hansen, L. D.; McCarlie, V. W. J. Chem. Educ. 2004, 81, 1581– 1584. 6. D1622-98, Standard Test Method for Apparent Density of Rigid Cellular Plastics; American Society for Testing and Materials: West Conshohocken, PA, 1998. 7. NIOSH Safety and Health Topic: Isocyanates. http://www.cdc. gov/niosh/topics/isocyanates/ (accessed Nov 2009). 8. Hocking, M. B.; Canham, G. W. R. J. Chem. Educ. 1974, 51, A580–A581. 9. Gilmer, T. C.; Williams, M. J. Chem. Educ. 1996, 73, 1062–1065. 10. Stevens, E. S.; Baumstein, K.; Leahy, J. M.; Doetschman, D. C. J. Chem. Educ. 2006, 83, 1531–1533. 11. Mark, J. E. J. Chem. Educ. 2002, 79, 1437–1443. 12. Pellicer, J.; Manzanares, J. A.; Zuniga, J.; Utrillas, P.; Fernandez, J. J. Chem. Educ. 2001, 78, 263–267. 13. Smith, B. J. Chem. Educ. 2002, 79, 1444–1452. 14. Smith, B. J. Chem. Educ. 2002, 79, 1453–1461.
Supporting Information Available Supplemental material and instructor notes on calculations, polyols, isocyanates, CAS numbers, and alternative formulations. This material is available via the Internet at http://pubs.acs.org.
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