Biomacromolecules 2008, 9, 615–623
615
Biodegradable Foam Plastics Based on Castor Oil Hong Juan Wang,† Min Zhi Rong,*,‡ Ming Qiu Zhang,‡ Jing Hu,‡ Hui Wen Chen,§ and Tibor Czigány⊥ Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, OFCM Institute, School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, People’s Republic of China, Materials Science Institute, Zhongshan University, Guangzhou 510275, People’s Republic of China, Guangdong Paper Industrial Research Institute, Guangzhou 510300, People’s Republic of China, and Department of Polymer Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Hungary Received August 16, 2007; Revised Manuscript Received November 4, 2007
In this work, a simple but effective approach was proposed for preparing biodegradable plastic foams with a high content of castor oil. First of all, castor oil reacted with maleic anhydride to produce maleated castor oil (MACO) without the aid of any catalyst. Then plastic foams were synthesized through free radical initiated copolymerization between MACO and diluent monomer styrene. With changes in MACO/St ratio and species of curing initiator, mechanical properties of MACO foams can be easily adjusted. In this way, biofoams with comparable compressive stress at 25% strain as commercial polyurethane (PU) foams were prepared, while the content of castor oil can be as high as 61 wt %. The soil burial tests further proved that the castor oil based foams kept the biodegradability of renewable resources despite the fact that some petrol-based components were introduced.
Introduction In the last few decades, steadily increasing efforts have been devoted to managing the problem of synthetic polymer wastes. With the observation that natural cellulose- and protein-based materials are biodegradable, the importance of natural products for replacing petroleum-derived materials becomes very clear with increasing emphasis on the environmental issues. The best examples of biopolymers based on renewable resources are cellulosic plastics, polylactides (PLA), starch plastics, and soybased plastics.1 Originally, biopolymers were intended to be used in packaging industries, farming and other applications with minor strength requirements. Performance limitations and high cost are the major barriers to widespread acceptance of biopolymers as substitutes for traditional nonbiodegradable polymers. Considering that most foam plastics used in packaging field are not biodegradable and their discards occupy tremendous space, there is a growing urgency to develop novel biofoams. However, the typical biopolymers mentioned above are less employed for making foam plastic, probably due to their high cost and processing problems. A few researchers started to prepare starch-based foams2–13 and natural fiber/starch foam composites,14–16 targeted for usage in fast food packaging and containers. In fact, plant oil, predominantly made up of triglyceride molecules, is an ideal replacement material to manufacture foam plastic as they are renewable, easily processing, offering comparable performance and cost with petroleum-based foams, * Corresponding author: e-mail,
[email protected]; phone & fax +86-20-84114008. † Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, OFCM Institute, School of Chemistry and Chemical Engineering, Zhongshan University. ‡ Materials Science Institute, Zhongshan University. § Guangdong Paper Industrial Research Institute. ⊥ Department of Polymer Engineering, Budapest University of Technology and Economics.
and optionally biodegradable. Many plant oils have been successfully utilized as polyols for the preparation of polyurethane (PU) foam plastics.17–22 Castor oil possessing hydroxyl groups, for example, filled the role of polyols in fabrication of PU elastomers and semirigid foams.23–25 Javni et al.26,27 prepared rigid PU foams using soy polyols that were made from epoxidized soybean oil. Bhattacharya et al.28 yielded flexible PU foams with modified soy-based polyols. Narine et al.29 fabricated polyols and PU foams from seed oils. It is worth noting that, however, the plant-oil-based PU foams contain a large fraction of petroleum-based species because of the 1:1 stoechiometry of the polyol addition reaction with diisocyanate. Biodegradability of these foams is still an open question. It was reported that very little (0-7.5%) of the carbon in vegetableoil-based PU could be converted to CO2 over the biodegradation course.30 Besides polycondensation polymerization, triglyceride can also be polymerized via free radical polymerization. One might attach vinyl functionalities to triglycerides through the reaction between epoxy and hydroxyl groups on triglyceride molecules and acrylic acid or maleic anhydride.31 These vinyl-functionalized plant oils can then be blended with a reactive diluent and cured by free radical polymerization. In this way, the ratio of the plant oil to diluent can be conveniently adjusted. Mechanical and biodegradable properties of the polymerized plant oil are also controlled. Preparation of biofoams containing higher content of acrylated epoxidized soybean oil by means of free radical polymerization and pressurized carbon dioxide has been demonstrated by Wool et al.32 The authors of the present article decided to synthesize plastic foams based on castor oil through a simple approach. Castor oil is a traditional renewable feedstock widely applied in industrial chemical products, like paints, coatings, inks, lubricants, etc.33 It originates from vegetable and consists mainly of esters of ricinoleic acid. The presence of hydroxyl groups and double bonds makes the oil suitable for many reactions and modifications. In this work, castor oil was first converted into
10.1021/bm7009152 CCC: $40.75 2008 American Chemical Society Published on Web 12/29/2007
616 Biomacromolecules, Vol. 9, No. 2, 2008
its maleic acid ester (i.e., maleated castor oil, MACO) by the reaction with maleic anhydride without the aid of any catalyst. Afterward, MACO was free-radically copolymerized with reactive diluents (styrene, St) and cured to form foam plastics using conventional foaming techniques. Meanwhile, cell structure and compressive properties of the foams prepared at different ratios of MACO/St and curing conditions were characterized. In addition, biodegradability of the forms was investigated by soil burial tests. It is believed that the present work would offer a simple and cost-effective route for producing biodegradable plastic foams with high content of castor oil.
Experimental Section Materials. Castor oil was purchased from Tianjin Fucheng Chemical Factory, China. Both maleic anhydride (MA, C.P.) and the diluent monomer styrene (St, C.P.) were supplied by Tianjin Damao Chemical Factory, China. The curing initiators, benzoyl peroxide (BPO, A.R.) and cyclohexanone peroxide (CHP, C.P.), were produced by Guangzhou Chemical Reagent Co., China, and Shanghai Reagent Co., China, respectively. The curing accelerants, N,N-dimethylaniline and cobaltous naphthenate (Co2+ 8%), were supplied by Shantou Guanghua Chemical Co., China, and Shanghai Reagent Co., China, respectively. NaHCO3 (A.R.) produced by Guangzhou Chemical Co., China, was used as the blowing agent. The surfactant, polyoxyethylene nonyl phenyl ether (OP10, C.P.), was purchased from Tianjin Medicine Co., China. Other reagents and solvents were conventional commercial products. All the chemicals were used without further purification. Synthesis and Characterization of Maleated Castor Oil. Maleic anhydride (MA) and castor oil (2.5:1 by mole) were added in a 250 mL four-necked round-bottom flask equipped with a stirrer, a thermometer, and an inlet of dry nitrogen. The reaction proceeded with continuous stirring at various temperatures (85, 100, 120, 130 °C, respectively) for a period of time (0.5-9 h). Meanwhile, the acid numbers at different reaction times were determined to monitor the reaction procedure. The sample (0.5-1 g) was dissolved in 50 mL toluene in a conical flask. Then 5 mL of water was added into the mixture, which was shaken and heated in a 50 °C water bath for 30 min. When the mixture was cooled down to room temperature, 1 mL of phenolphthalein color-indicator was added and the mixture was titrated by 0.1 mol/L KOH ethanol solution until the pink color lasted for 30 s. The acid number was calculated from:
acid number ) 56.1AC ⁄ G where A denotes volume of the consumed KOH solution, C the concentration of KOH solution, and G the sample weight. Molecular weights of castor oil and MACO were measured by using a Walter 208LC gel permeation chromatograph (GPC) at room temperature with THF as the solvent. The molecular structures of castor oil and MACO were analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectrometer, respectively. For 1H NMR inspection, the samples were prepared by dissolving approximately 20 mg of product in 0.5 mL of deuterated chloroform. This solution was then analyzed by a Varian Mercuryplus 300 NMR spectrometer (300 MHz, spectrum width of 3689.22 Hz, pulse width of 4.75 µs, 32 scans at 293 K, 90° pulse width of 9.5 µs). FTIR spectra were collected by a Nicolet/Nexus 670 FTIR spectrometer within the wavenumber range of 400–4000 cm-1 at a 4 cm-1 resolution. All the samples were prepared by coating the reaction product on the surface of a KBr tablet. Preparation and Characterization of MACO Foams. The synthesized MACO was first mixed with the diluent monomer (St), initiator (3 phr BPO or 5 phr CHP), accelerants (0.3 phr N,N-dimethylaniline or 3 phr cobalt naphthenante), surfactant (2 phr OP-10), and blowing agent (different amounts of NaHCO3) in a mold. When the foam was prepared at 65 °C, BPO and N,N-dimethylaniline were used as the initiator system, while CHP and cobalt naphthenante were used at room
Wang et al. temperature. After agitation for several minutes when the mixture became viscous, 4 phr of water was added to the system, and then the mixture was poured into a special open mold to foam and set. A postcuring at 100 °C for 2 h was carried out for all the foams. When the dosage of NaHCO3 changed from 1.5 to 3.75 phr, the foam’s density decreased from 0.35 to 0.12 g/cm3. The apparent density of the foams was measured according to GB6343-1995 (technically equivalent to ASTM D 1622-03):
F)m⁄V where m and V represent the weight (kg) and volume (m3) of the foam, respectively. Each datum was the average of results from five samples. The compressive strength of the foams was evaluated on a CMT7503 universal tester following the procedure specified in GB 8813-88 (technically equivalent to ASTM D 1621-00 or ISO 844). The specimen was a column with diameter of 42.5 mm and thickness of 36 mm, which was compressed between two stainless steel plates at a crosshead speed of 2 mm/min. The compressive modulus was determined from the initial slope of the stress–strain curve, and the strength was calculated from the load at a strain of 10%. Each datum was the average of results from five samples and the standard deviation of the measurements was within 10%. Morphology of the foam-cutting surface was observed by a Hitachi S-520 scanning electron microscope (SEM). The foams were cut into slices with thickness of about 2 mm and then coated using Aurum sputtering. On the basis of the morphologies of foams, the cell diameters were determined according to GB/T12811-91 standard method (technically equivalent to ASTMD 3576-98) as follows. Drawing a reference line on a SEM photo of the foam, counting the number of cell walls that intersect the reference line, and dividing the length of the reference line by the number of the counted cells to obtain the average cell chord length, t. The reference line length should be suitable to the cell size being measured. A minimum cell count of 20 should be adequate. Eventually, the cell size, d, for each direction can be estimated from
d ) 1.623t Biodegradability Study of MACO Foams. The degree and rate of aerobic biodegradation of the MACO foams were determined in terms of soil burial under laboratory conditions. The foams were cut into slices with thickness of about 2 mm, and then buried in soil. The samples (1.0 g) were mixed with 72 g of soil, 24 g of topsoil, 8 g of mature compost, and 50 g of water in 250 mL glass beaker. The beaker was kept in an oven controlled at the desired temperature (30 °C). The soil composition and compost conditions resembled those described in ASTM D 5988-03. After a given time, the samples were taken out of the container, washed thoroughly, and dried in vacuum. Variations in weight and mechanical properties of the foams (columnar samples with diameter of 42.5 mm and thickness of 36 mm) before and after the biodegradation were evaluated. The change in cellular structure of the foams was observed using SEM.
Results and Discussion Synthesis and Characterization of Maleated Castor Oil (MACO). Castor oil contains hydroxyl groups and double bonds, but the double bonds are not active enough to take part in free radical polymerization. To overcome this shortcoming, castor oil must be modified by something that is able to provide reactive double bonds. In this context, MACO was synthesized through the reaction of maleic anhydride with the hydroxyl groups in triglycerides of castor oil (Figure 1). To monitor the esterification reaction course, acid number was used, from which the esterification yielding can be estimated. Figure 2 indicates that the acid number drastically decreases at the beginning of the reaction and then levels off due to the reduction of the component’s concentration. It is clear that the higher reaction temperature favors the reaction as a result
Plastic Foams Based on Castor Oil
Biomacromolecules, Vol. 9, No. 2, 2008 617
Figure 1. Schematic illustration of the reaction of castor oil with maleic anhydride to produce (1) a single MACO or (2) a dimer.
Figure 2. Change of acid number with reaction time during MACO synthesis at different temperatures.
Figure 3. FTIR spectra of castor oil (CO) and MACO.
of higher reaction speed. The majority of the reaction is nearly completed within 6 h at 85 °C, while 2 h is sufficient for the reaction at 120 °C. At the end of the reaction, the esterification yielding is about 80%. FTIR spectra of castor oil and MACO are compared in Figure 3. For castor oil, the peak at 3450 cm-1 belongs to -OH absorption, while that at 1635 cm-1 can be assigned to -CdCdouble bond. When castor oil reacts with maleic anhydride, the -OH peak is evidently decreased, while the double bond peak is enhanced. No peaks corresponding to cyclic anhydride at 1779 and 1849 cm-1 can be perceived in the resultant MACO,34 indicating that almost all of maleic anhydride has been consumed to react with castor oil. Generally, castor oil reacts with maleic anhydride following the first route shown in Figure 1. Under higher reaction temperature, however, complete esterification of maleic anhy-
Figure 4. (a) GPC chromatographs of castor oil (CO) and MACO. (b) Deconvolution of GPC chromatograph of MACO into four distinct peaks.
dride may occur to form an oligomer as shown by the second route in Figure 1. This is confirmed by the higher weight average molecular weight of MACO than that of castor oil (Figure 4 and Table 1). For castor oil, only one narrow peak is observed on its gel permeation chromatograph (Figure 4a), which corresponds to the individual triglyceride molecules. In contrast, MACO exhibits multipeaks due to the appearance of single MACO and a certain amount of dimers, trimers, and even oligomers in the product. Accordingly, the gel permeation chromatography (GPC) curve of MACO can be deconvoluted into four distinct peaks as shown in Figure 4b, giving molecular weights of these components. Furthermore, the ratios of the peak areas can be used to determine the molar fraction of dimers, trimers, and oligomerized triglycerides (Table 1). The percentages of both dimers (Mw ) 3135) and oligomerized triglycerides
618 Biomacromolecules, Vol. 9, No. 2, 2008
Wang et al.
Table 1. Number Average Molecular Weight, Mn, Weight Average Molecular Weight, Mw, and Peak Molecular Weight, Mp, of Castor Oil and MACO materials castor oil MACO deconvoluted deconvoluted deconvoluted deconvoluted a
peak peak peak peak
1a 2a 3a 4a
Mn
Mw
Mp
polydispersity
area ratio of deconvoluted peak/entire peaka (%)
1304 1679 10156 4621 2987 1638
1356 1772 11693 5307 3135 1721
1330 1725 10898 4952 3060 1679
1.04 1.06 1.15 1.15 1.05 1.05
5 15 16 64
Refer to Figure 4b for the details of deconvolution of the gel peremation chromatograph of MACO.
Figure 6. Typical specific compressive stress–strain curves of MACO foams prepared at different MACO/St weight ratios. Initiator: BPO. To eliminate the influence of foam density on the compressive properties, the compressive stress is normalized by the densities of the samples so that specific compressive stress (stress/density) is used hereinafter.
Figure 5. 1H NMR spectrum with peak assignments of (a) castor oil and (b) MACO.
(Mw > 5307) in the maleated castor oil are about 16 and 20 mol %, respectively, while that of single MACO (Mw ) 1721) is 64 mol %. Figure 5a illustrates the 1H NMR spectrum of castor oil used in this work with peak assignments. The fatty acid protons ((-CH2)CO-) are in the range of 2.2–2.4 ppm (peak 3), while the methyl protons are at 0.9 ppm (peak 12) and the methylene protons of glycerol are in the range of 4.1–4.3 ppm (peak 1). These peaks can be used as internal standards to quantitatively characterize the content of certain groups because they remain unchanged during the reaction of castor oil with maleic anhydride. Accordingly, it is known that there are 2.44 mol of hydroxyls per triglyceride as determined by comparing the peak integral of methylene attached to hydroxyl (CH-OH) at 3.61 ppm (peak 9) with that of the methylene protons of glycerol (peak 1).
Figure 7. Typical specific compressive stress–strain curves of MACO foams of different densities prepared with CHP or BPO as curing initiator at MACO/St 80/20. The unit of the density is g/cm3.
When castor oil is converted to MACO, the peak related with the hydroxyl (CH-OH) at 3.61 ppm nearly disappears (Figure 5b), while a new peak at 5.03 ppm appears, which represents the methylene protons connected to maleate groups (CH-O-CO). The result proves that the reaction between castor oil and maleic anhydride has taken place as expected. Similarly, with the peaks at 4.1–4.3 ppm as reference, it is calculated from the integral of the peak at 5.03 ppm that 2.08 mol of hydroxyl groups is consumed. That is, about 85% of the hydroxyl groups of castor oil have reacted with maleic anhydride. It is known that isomerization of maleate to fumarate (cis to trans) may occur due to prolonged heating.35 Such isomerization
Plastic Foams Based on Castor Oil
Biomacromolecules, Vol. 9, No. 2, 2008 619
Figure 8. Surface morphologies of MACO foams of different densities prepared with BPO or CHP as curing initiator at MACO/St ) 80/20: (a) Initiator, BPO; density, 0.161 g/cm3. (b) Initiator, BPO; density, 0.256 g/cm3. (c) Initiator, BPO; density, 0.346 g/cm3. (d) Initiator, CHP; density, 0.253 g/cm3.
Table 2. Results of Power-Law Regression of the Data in Figure 9 Using Equations 1 and 2 weight ratio of MACO/St
n
p
A (MPa)
B (MPa)
90:10 80:20 70:30 60:40
1.26 1.65 3.55 3.83
1.24 1.85 3.43 2.99
1.57 6.63 514.0 2350.2
0.11 0.69 26.6 30.0
was also observed in the product of MACO when the reaction time was extended. In Figure 5b, the peaks at 6.7–6.9 ppm represent vinyl protons of fumarate. Actually, fumarate has higher reactivity than maleate when participating in free radical
Figure 9. Density dependence of (a) compressive modulus and (b) compressive strength of MACO foams prepared with BPO initiator at different MACO/St weight ratios.
Figure 10. Time dependence of soil burial biodegradation induced weight loss of MACO foams prepared with BPO initiator at different MACO/St weight ratios (density, 0.200 g/cm3).
620 Biomacromolecules, Vol. 9, No. 2, 2008
Wang et al.
Table 3. Contents of Natural Component and Biodegradability of Plant-Oil-Based Polymers plant-oil-based polymer
species of content of natural natural products component (wt %)
PU
soybean oil
66
PU
castor oil
54
MACO foam castor oil (MACO/St ) 90/10)
61
biodegradability
ref
