- Email: [email protected]
Synthesis and properties of polyurethane wood adhesives derived from crude glycerol-based polyols
Author’s Accepted Manuscript Synthesis and properties of polyurethane wood adhesives derived from crude glycerol-based polyols Shaoqing Cui, Xiaolan Luo, Yebo Li www.elsevier.com/locate/ijadhadh
PII: DOI: Reference:
S0143-7496(17)30136-7 http://dx.doi.org/10.1016/j.ijadhadh.2017.04.008 JAAD2038
To appear in: International Journal of Adhesion and Adhesives Received date: 17 October 2016 Accepted date: 12 April 2017 Cite this article as: Shaoqing Cui, Xiaolan Luo and Yebo Li, Synthesis and properties of polyurethane wood adhesives derived from crude glycerol-based p o l y o l s , International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2017.04.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and properties of polyurethane wood adhesives derived from crude glycerol-based polyols Shaoqing Cui, Xiaolan Luo, Yebo Li* Department of Food, Agricultural, and Biological Engineering, The Ohio State University /Ohio Agricultural Research and Development Center, 1680 Madison Ave, Wooster, OH 44691-4096, USA * Corresponding Author. Tel.: + 1 330 263 3855; Fax: + 1 330 263 3670. E-mail address: [email protected] (Y. Li); [email protected].
1
Abstract Crude glycerol, a waste stream of the biodiesel production process, is low-cost renewable feedstock for the production of chemicals and polymers. In this study, polyurethane (PU) adhesives were synthesized from crude glycerol-based polyols (CG-based polyols) for wood bonding applications. Effects of different variables, including hydroxyl values of CG-based polyols, chain extenders, and the molar ratio of NCO/OH on the properties of PU adhesives were investigated. The chemical structures of PU adhesives were characterized, and their thermal, mechanical, and chemical resistance properties were evaluated. The experimental results indicated that an increase of the NCO/OH molar ratio (1.3) substantially improved bonding strength by up to 38 MPa. Higher thermal stability and stronger chemical resistance to hot and cold water and to alkali and acid solutions were observed comparing to vegetable oil-based adhesives. However, the effect of the hydroxyl value of polyols on bonding strength was not significant. Additionally, bond strength of crude glycerol-based PU adhesives was comparable to that of some commercial PU wood adhesives. All these properties demonstrated the potential of CG for PU wood adhesive applications, particularly for fast-curing uses. Keywords: Crude glycerol, Bio-polyol, Wood adhesive, Polyurethane
2
1. Introduction Urea-formaldehyde and phenol-formaldehyde based adhesives have been widely used as wood panel adhesives, foundry sand binders, and for the bonding of papers (Pizzi, 2015). However, they are easily hydrolyzed and, under cyclic moisture or warm and humid conditions, can undergo stress scissions (Robert, et al. 1994; River, et al. 1994). Moreover, the release of formaldehyde has posed serious environmental and health concerns. To address these problems, polyurethanes (PUs) have been widely introduced for adhesive production. PUs are versatile polymeric materials that demonstrate excellent flexibility, strong adhesion, high performance at low-temperature, and good stability (De Gray, 1998; John and Joseph, 1998; Malavasic, et al. 1992; Zhang, et al. 2014; Zhang, et al. 2017). In general, PUs are produced by the reaction of polyols and isocyanates, most of which are petroleum-based. Concerns over dwindling petroleum reserves have led to investigations into substitutes that are produced from renewable sources. Because of the limited choice of isocyanates, a majority of the research on renewable substitutes used for PU production has focused on the polyol component. Natural oils (Abraham, et al. 2007; Shah, et al. 2001; Roh, et al. 2008), lignocellulosic biomass (Nadi, 2005; Brioner, 2011), and carbohydrates such as starch (Menezes, et al. 2007) have been widely used for the production of bio-based polyols. Specifically, vegetable oil and starch derived polyols have been successfully used to synthesize PU adhesives. Studies reported that the adhesion strength of PU adhesives derived from castor oil-based polyols was ten times higher than that of commercially available adhesives, when the PU adhesives were applied to wood joints (Keyur, et al. 2003; Desai, et al. 2003). PU adhesives produced from canola 3
oil-based polyols also demonstrated lap shear strengths superior to those of three commercial PU adhesives (Kong, et al. 2011). Natural adhesives derived from wheat and oil palm starch also have been demonstrated to meet industrial standards (Japanese) in terms of mechanical properties (Kushairi, et al. 2015). In recent years, there has been increasing interest in value-added processing of crude glycerol into chemicals and polymers (Luo, et al. 2016; Pagliaro, et al. 2007; Tan, et al. 2013). Crude glycerol is a low-value byproduct which is primarily obtained from the biodiesel production process (Luo, et al. 2016). It has potential as a renewable substitute for petroleum-based feedstocks. Studies have reported the successful production of CG-based polyols with properties suitable for PU applications (Luo, et al. 2014; Luo, et al. 2013). Crude glycerol impurities, such as fatty acids and/or fatty acid methyl esters, were found to be critical for the production of high quality polyols and PU foams and coatings (Hu, et al. 2014). Compared to vegetable oils, crude glycerol is an inexpensive byproduct and does not compete directly with food supplies. Therefore, the use of crude glycerol in the production of bio-based PU adhesives is expected. In this study, the feasibility of crude glycerol-based polyols for the production of PU adhesives for wood bonding was investigated. The effects of parameters, including hydroxyl number of CG-based polyols, chain extenders, and the molar ratio of NCO/OH on PU adhesive properties, were studied and the optimized parameters were determined. In addition, the properties of PU adhesives from CG-based polyols were compared with some commercial wood adhesives to investigate their potential applications for wood bonding.
4
2. Method and materials 2.1 Materials Four CG-based polyols with hydroxyl numbers of 486, 422, 296 and 191 mg KOH/g, and acid numbers below 3 mg KOH/g, were obtained from Bio100 Technologies, LLC (Mansfield, OH). The characteristics of CG-based polyols involving the main composition, averaged molecular weight and functionality were supplemented and are listed in Table 1 and their representative schematics also shown in Fig.1 in the supplementary file. Polyethylene glycerol with an average molecular weight of 400 (PEG 400), 1, 4-butanediol, ethylene glycol, and neopentyl glycol were purchased from Fisher Scientific (Pittsburgh, PA). Polymeric methylene-4-4’-diphenyl isocyanate (pMDI) was obtained from Covestro Bayer Materials Science (Pittsburg, PA). Dibutyltin dilaurate (DBTDL) was obtained from Pfaltz & Bauer (Waterbury, CT). The commercial PU adhesives used for property comparison were Vibrs-tite PU adhesive (ND Industries, Inc., Clawson, MI), BOSTIK’S BEST wood flooring urethane adhesive (Bostik, Inc., Middleton, MA), and waterproof Franklin Titebond PU glue (Franklin International, Columbus, OH). 2.2 Adhesive preparation The preparation of PU adhesives was carried out at room temperature under nitrogen protection by mixing CG-based polyols with pMDI in the presence/absence of chain extenders using DBTDL as a catalyst (0.1 wt% based on the total weight of polyols and pMDI). Tables 1-3 show the formulations for the synthesis of PU adhesives from CG-based polyols with different hydroxyl numbers (Table 1), PU adhesives with different chain extenders from CG-based polyols with a hydroxyl number of 296 mg KOH/g (Table 2), and 5
PU adhesives from CG-based polyols with a hydroxyl number of 296 mg KOH/g with different molar ratios of NCO/OH (Table 3). Table 1 Formulation for the synthesis of PU adhesives from CG-based polyols with different hydroxyl numbers Polyols Adhesives
Hydroxyl number
pMDI
Molar ratio
mass (g)
mass (g)
of NCO/OH
(mg KOH/g) CGPU-191-1.3
191
5
2.99
1.3
CGPU-296-1.3
296
5
4.64
1.3
CGPU-422-1.3
422
5
7.38
1.3
CGPU-486-1.3
486
5
7.63
1.3
Table 2 Formulation for the synthesis of CG-based PU adhesives with different chain extenders Adhesives
Chain extender
Chain extender
Polyola mass
pMDI
mass (g)
(g)
mass (g)
CGPU-BD-1.3
1,4-butanediol
0.5
5
4.65
CGPU-EG-1.3
Ethylene glycerol
0.5
5
4.65
CGPU-NG-1.3
Neopentyl glycerol
0.5
5
4.65
CGPU-1.3
--
--
5
4.65
Note: a Polyol hydroxyl number: 296 mg KOH/g, and molar ratio of (NCO/OH) is 1.3.
Table 3 Formulation for the synthesis of CG-based and PEG400-based PU adhesives with different NCO/OH molar ratios Adhesives
NCO/OH
Polyol mass (g)
6
pMDI mass (g)
Based on CG polyols / PEG400
CG based / PEG400 based CGPU-296-1.0/ PEG400PU-1.0
1.0
6
4.30 / 4.36
CGPU-296-1.1 / PEG400PU-1.1
1.1
6
4.72 / 4.47
CGPU-296-1.2 / PEG400PU-1.2
1.2
6
5.16 / 5.15
CGPU-296-1.3 / PEG400PU-1.3
1.3
6
5.56 / 5.54
CGPU-296-1.4 / PEG400PU-1.4
1.4
6
6.08 / 5.94
CGPU-296-1.5 / PEG400PU-1.5
1.5
6
6.44 / 6.34
CGPU-296-1.6 / PEG400PU-1.6
1.6
6
6.87 / 6.73
CGPU-296-1.7 / PEG400PU-1.7
1.7
6
7.32/ 7.16
The hydroxyl number of CG-based polyols varied from 191 to 486 mg KOH/g. The NCO/OH molar ratio varied from 1.0 to 1.7 with an interval of 0.1. The effects of chain extenders, including neopentyl glycol, ethylene glycerol and 1,4-butanediol, on PU adhesive properties were investigated. PU adhesives from PEG400 were also synthesized in order to investigate property differences in comparison with CG-based PU adhesives. 2.3 Wood specimen preparation, bonding and testing Wood pieces were cut into 100 mm×25 mm×3 mm strips and polished using sandpaper before use. The synthesized PU adhesive was applied to the surface of two pieces of wood strips at a thickness of 0.1 mm and a lap joint of 25 m×30 m. A load of 1 kg was placed on top of the lap joint and left for 5 min. After that, the joined wood pieces were kept at room temperature and at humidity of 75±5% for 7 days. The lap joint shear strength of joined wood specimens were measured using a universal testing machine with a stretch rate of 50mm/min, Instron 3366 (Instron Corp., Norwood, MA), in accordance with ASTM D 906. The average value of three replicates for each sample was reported. Green strength is one of important indexes to evaluate adhesive properties. It is closely related to the ability of an adhesive to
7
hold the substrate before reaching its ultimate bond strength when completely cured. Regarding green strength measurement, joined wood specimens were cured at room temperature and then directly subjected to lap shear tests at daily intervals up to 7 days. The curing time was determined starting from CG-based polyol and isocyanate being bonded together and ending with the surface of the adhesives being firm. 2.4 Chemical resistance The chemical resistance of CG-based PU adhesives were tested under four conditions: cold water, boiling water, acid solution, and alkali solution. Wood specimens bonded with PU adhesives were immersed in cold water at 4 oC and hot water at 100 oC for 1 day, and in acid ( hydrochloric acid, pH 2) and alkaline (sodium hydroxide, pH 10) solutions at 80 oC for 1 hour. After that, the specimens were dried at room temperature for 7 days and then subjected to lab shear strength tests as described above. 2.5 FT-IR analysis Fourier transform-infrared (FT-IR) spectra of CG-based polyols and the resulting PU adhesives were obtained on a Spectrum Two IR spectrometer (PerkinElmer, Waltham, MA) with 32 scans at a resolution of 2 cm-1. 2.6 Thermogravimetric analysis Thermogravimetric analysis (TGA) was performed using a Q50 thermogravimeter (TA Instruments, New Castle, DE) by heating PU adhesive samples from 50 to 600 oC at a rate of 10 oC/ min under a nitrogen atmosphere.
8
3. Results and discussion 3.1 Effect of the hydroxyl number of polyols Table 4 shows the effects of the hydroxyl number of CG-based polyols on lap shear strength and curing time of PU adhesives. As the hydroxyl number increased from 191 to 486 mg KOH/g, the average lap shear strength of the CG-based PU adhesives gradually increased from 31.6 to 37.1 MPa, while curing time decreased from approximately 5 to 2.5 min. These results were mainly due to the increased crosslinking density of PU adhesives. Under the same NCO/OH molar ratio, as the hydroxyl number of polyols increased, the content of hydroxyl groups increased, causing more isocyanates to participate in the reaction. This resulted in the formation of a more compact crosslinking structure in the PU adhesives. Similar results also have been observed for castor oil-based PU adhesives (Keyur, et al. 2003; Moghadam, et al. 2003). Compared to castor oil-based PU adhesives and other vegetable oil-based PU adhesives (Ang, et al. 2014; Li and Li, 2014), CG-based PU adhesives showed shorter curing times. This might be caused by the low molecular weight of the CG polyols and the residual glycerol, which plays a role as chain extender (Luo, et al. 2013; Hu and Li, 2015). As demonstrated in Table 1 of the supplementary file, the free glycerol content of the employed CG-based glycerol ranges from 5.7% to 20.9% and molecular weight from 932 to 669. Besides, with an increasing hydroxyl number, the hard segment of the obtained adhesives increased from 37.9% to 60.8%, which further confirmed the results of increasing lap shear strength and decreasing curing time. The results in Table 4 also indicate that the hydroxyl number of the CG-based polyols had more obvious effects on curing time than on adhesion strength. Considering a balance between lap shear strength and curing time, the 9
CG-based polyol with a hydroxyl number of 296 mg KOH/g was chosen as a suitable polyol for further studies. Table 4 Effects of the hydroxyl number of CG-based polyols on lap shear strength and curing time of PU adhesives Adhesives
Hydroxyl number of CG
Lap shear strength
Curing time
polyol (mg KOH/g)
(MPa)
CGPU-191-1.3
191
31.6±3.3
4.0~5.0
37.9
CGPU-296-1.3
296
35.8±4.3
3.5~4.5
48.6
CGPU-422-1.3
422
36.3±2.7
3.5~4.0
57.4
CGPU-486-1.3
486
37.1±2.0
2.5~3.5
60.8
(min)
Hard segment (wt %)
3.2 Effect of chain extenders The effect of chain extenders on lap shear strength of CG-based PU adhesives are shown in Table 5. No obvious change was observed with the addition of a chain extender. PU adhesives containing 1, 4-butanediol and neopentyl glycol resulted in a lower lap shear strength than that of an analogue with ethylene glycol as a chain extender. This was most likely because both 1, 4-butandiol and neopentyl glycol had a longer chain length than that of ethylene glycol, thus improving the flexibility of PU structures. In addition, the presence of two side methyl groups of neopentyl glycol had a plasticizing effect on PU performance to some extent, reducing the lap shear strength of its PU adhesives. Compared to PU adhesives with the addition of chain extenders, CG-based PU adhesives without chain extenders exhibited the highest lap shear strength. This was mainly due to the presence of residual glycerol (around 6.5%) in CG-based polyols (Luo, et al. 2013; Hu and Li, 2014; Hu, et al. 2015), resulting in the formation of a crosslinked structure and further increasing the lap shear strength of the PU adhesives.
10
Table 5 Lap shear strength of CG-based PU adhesives with the addition of different chain extenders Samplea
Chain Extender
Lap shear strength (MPa)
CG-BD-1.3
1,4-butanediol
35.7±3.3
CG-EG-1.3
Ethylene glycerol
33.5±2.7
CG-NG-1.3
Neopentyl glycerol
34.5±1.8
CG-1.3
None
35.8±4.3
a
Note: The experiments were carried out with NCO/OH of 1.3 and hard segment around 49.1%.
3.3 Effect of NCO/OH ratio In this study, the effects of the NCO/OH molar ratio on the properties of CG-based PU adhesives that were produced by the reaction between CG-based polyols with a hydroxyl number of 296 mg KOH/g and pMDI, were investigated. In order to examine the property differences in PU adhesives from different structures of polyols, PU adhesives from PEG400, which has an approximate hydroxyl number of 280 mg KOH/g, were also produced with the same NCO/OH molar ratios, from 1.0 to 1.7. Table 6 presents the properties of PU adhesives from both polyols under the varying NCO/OH molar ratios. An obvious increase in bond strength with both CG-based and PEG400-based PU adhesives was observed as the NCO/OH molar ratio incrementally increased from 1.1 to 1.3. However, with a further increase of the NCO/OH molar ratio from 1.4 to 1.7, the bond strength of both PU adhesives gradually decreased. An optimal NCO/OH molar ratio for the highest shear strength of both PU adhesives was 1.3. As the NCO/OH molar ratio increased, PU adhesives with higher crosslink densities were achieved as demonstrated with increasing hard segment contents, resulting in increased rigidity of the adhesive. Moreover, the increased NCO/OH molar ratio provided more free isocyanates, which can react with active groups on the surface of wood samples, and further strengthen the adhesive bond (Keyur, et al. 2003). However, when beyond a
11
critical ratio (1.3), an increase in the NCO/OH molar ratio will likely cause more complex side reactions, such as the reaction of isocyanate groups with urethanes to form allophanates, the reaction of isocyanate groups with water present in wood to form ureas, and the reaction of ureas with isocyanate groups to form biuret, which increased the stiffness of PU adhesives and resulted in decreased adhesion strength (Moghadam, et al. 2016; Kong, et al, 2011). Additionally, increased hard segment content might enhance the rigidity of a bonded joint which, in contrast, may reduce the bond strength. Similar changes in adhesion strength with an increase in the NCO/OH molar ratio have been reported for PU adhesives from ricinoleic acid-based polyols (Moghadam, et al. 2016; Kong, et al. 2011; Saetung, et al. 2015). As the NCO/OH molar ratio increased from 1.0 to 1.7, the curing time of PU adhesives from CG-based polyols and PEG400 was reduced from 11 to 3 min and from 65 to 45 min, respectively. Compared to PEG-based PU adhesives, CG derived adhesives showed much higher lap shear strengths but shorter curing times. This was mainly ascribed to structural differences between the two polyols and the residual glycerol in CG-based polyols. CG-based polyols are mainly composed of monoglycerides, diglycerides, and glycerol, whose hydroxyl groups are similar to PEG400. In contrast, PEG400 is a linear polyether with two terminal hydroxyl groups. It is more flexible than CG-based polyols, therefore, lower adhesion strength was obtained. As mentioned in section 3.2, the residual glycerol could act as a cross-linker to react with isocyanates for the formation of a compact network structure, resulting in high adhesion strength of the PU adhesive. It was concluded that both cross-linking density and network structure affected the adhesive properties to some degree.
12
Table 6 Lap shear strength and curing time of designed PU adhesives based on different NCO/OH molar ratios CG-based PU a
Lap shear
Curing
Hard
PEG400-based b
Lap shear
Curing
Hard
strength
time (min)
segment
strength
time (min)
Segment (wt %)
PU adhesives
(MPa) CGPU-296-1.0
16.6±2.3
10~11
42.1
PEG400PU-1.0
0.8±1.3
60~65
40.7
CGPU-296-1.1
31.6±3.3
7~7.5
44.4
PEG400PU-1.1
2.3±1.7
60~65
43.1
CGPU-296-1.2
33.2±2.7
5~5.2
46.6
PEG400PU-1.2
12.2±2.1
55~60
45.2
CGPU-296-1.3
36.8±3.5
4~4.5
48.6
PEG400PU-1.3
17.4±2.5
55~60
47.2
CGPU-296-1.4
35.8±3.1
3~3.6
50.4
PEG400PU-1.4
17.1±2.3
50~55
49.0
CGPU-296-1.5
33.9±2.4
3~3.7
52.1
PEG400PU-1.5
16.4±1.5
50~55
50.8
CGPU-296-1.6
31.6±3.5
3~3.5
53.8
PEG400PU-1.6
16.1±1.6
45~50
52.4
CGPU-296-1.7
31.0±3.5
3~3.5
55.3
PEG400PU-1.7
15.5±1.4
45~50
53.9
adhesives
(MPa)
(wt %)
Notes: a 296 is the hydroxyl number of the CG polyol. Numbers of 1.0-1.7 are the NCO/OH molar ratios used in the formulations. b
1.0-1.7 are the NCO/OH molar ratios used in the formulations.
3.4 Properties of adhesives derived from blended polyols Blends of different ratios of CG-based polyols and PEG400 were investigated to produce PU adhesives with improved curing times using an NCO/OH molar ratio of 1.3. Table 7 shows the formulations for the production of PU adhesives from the blended polyols (CG-based polyols and PEG400) and their adhesion properties. The strength of hybrid adhesives were greatly improved compared to PEG400-based adhesives, and their curing times were significantly reduced from 60 min to 6~8 min. This significant change in adhesive performance was probably ascribed to free glycerol present in the CG-based polyols. In order to examine the effect of glycerol on the properties of PEG400-based PU adhesives, mixtures of pure glycerol and PEG400 were employed for the production of PU adhesives. The adhesive properties are shown in Table 8. As the content of pure glycerol in PEG400 increased from 0 to 40%, the curing time significantly decreased from 60 to 15 min. With a further addition of glycerol to PEG 400 (80% glycerol), the curing time decreased to 6 min. 13
Therefore, the removal of glycerol in CG-based polyols was required to improve its curing time. Table 7 Formulations for the production of PU adhesives from blended polyols and their adhesion properties Adhesives
CG polyol content
Mass of CG
Mass of
Mass of
Lap shear
Curing
samples*a
in the blend (w/w)
polyol (g)
PEG400 (g)
pMDI (g)
strength (MPa)
time (min)
PU-0-100
0%
0
4.0
3.41
17.4±2.5
55~60
PU-10-90
10%
0.4
3.6
3.47
25.4±1.7
20~25
PU-20-80
20%
0.8
3.2
3.50
29.0±1.9
18~22
PU-30-70
30%
1.2
2.8
3.53
30.9±1.2
15~17
PU-40-60
40%
1.6
2.4
3.56
31.2±1.5
13~15
PU-50-50
50%
2.0
2.0
3.58
33.2±2.4
8~10
PU-60-40
60%
2.4
1.6
3.61
33.5±2.1
8~10
PU-70-30
70%
2.8
1.2
3.64
33.6±2.1
6~8
PU-80-20
80%
3.2
0.8
3.67
34.3±2.6
6~8
PU-90-10
90%
3.6
0.4
3.70
35.0±1.9
4~5.5
PU-100-0
100%
4.0
0
3.73
35.8±2.7
4~5.5
a
Note: The molar ratio of NCO/OH is 1.3.
Table 8 Adhesive properties based on PEG-400 with different contents of glycerol Pure glycerol content
Lap shear strength
Curing time
(w/w)
(MPa)
(min)
0
17.3
55~60
40%
18.1
15~20
80%
18.4
6~10
* Formulation of this designed experiment: PEG-400 4g, pMDI 3.4g, molar ratio of –NCO/OH 1.3, and different contents of pure glycerol.
3.5 Green strength In this study, the curing performance of PU adhesives from CG-based polyols with NCO/OH molar ratios from 1.2 to 1.4, were investigated by measuring lap shear strength over one week. It can be seen from Fig.1 that the adhesion strength of three CG-based PU adhesives increased markedly during the first 4 days, and then increased slightly until day 6. Almost no significant change in adhesion strength was observed between days 6 and 7 of testing. Approximately 57% and 86% of the maximum strength were obtained after curing for 2 and 14
4 days, respectively. This suggested that the curing reaction occurred quickly during the first 4 days and then slowed down until full curing was achieved after 6 days. As for individual adhesive samples, the adhesion strength increased as the NCO/OH ratio increased from 1.2 to 1.3 and then decreased with a further increase of the NCO/OH ratio to 1.4. This could be explained by the occurrence of more complex side reactions which was discussed in section 3.3. Compared with some reported vegetable oil-based PU adhesives, CG-based PU adhesives showed a shorter curing time when their bond strength reached 85% of the maximum strength. They also exhibited higher bond strength after 4-day curing (Keyur, et al. 2003; Kong, et al. 2011). 3.6 Chemical resistance The chemical resistance of CG-based PU adhesives, with a NCO/OH molar ratio of 1.3, to cold water, hot water, and acid and alkali solutions is shown in Fig. 2. GC-based PU adhesives showed superior resistance to cold water. Hot water, acid solution, and alkali solution had negative effects on the adhesion strength. It was also observed that acid and alkali conditions caused stronger hydrolysis of PU adhesives than hot water. Adhesives fared the worst in alkali conditions, and showed the lowest lap shear strength compared to other conditions. These results are in agreement with previous studies on the chemical resistance of biomass-based PU adhesives (Desai, et al. 2003). The sharp reduction in lap shear strength of CG-based PU adhesives after alkali treatment was probably related to the degradation of the wood surface due to penetration of the alkali solution, thereby deteriorating the contact between the wood and adhesive and loosening the cured bond (Desai, et al. 2003; Zanuttini, et al. 1999; Tamburini, 1970;). 15
3.7 Infrared spectroscopy Fig. 3 shows the FTIR spectra of a CG-based polyol with a hydroxyl number of 296 mg KOH/g and its corresponding PU adhesive with an NCO/OH molar ratio of 1.3. For the CG-based polyol, a strong broad stretching band at 3445 cm
-1
due to hydroxyl groups, a
stretching band at 1745 cm-1 due to carbonyl groups of esters, and a stretching peak at 1100 cm -1 due to C-O bonds were observed in Fig. 3(a). After the reaction of the CG-based polyol with isocyanates, the produced CG-based PU adhesive showed some obvious changes in its FT-IR spectrum (Fig. 3(b)). A strong absorption band at 3340 cm -1 characteristic of the N-H group and an absorption band at around 1736 cm -1 due to carbonyl groups of urethanes were observed, demonstrating the presence of urethane structures in the adhesive. Besides, a clear strong absorption band at 2274 cm-1 was also observed in Fig. 3(b), which indicates the presence of excess –NCO groups in the PU adhesive. The appearance of expected characteristic structures in this PU adhesive confirmed the successful synthesis of PU adhesives from the CG-based polyols. 3.8 Thermal properties Fig. 4 shows thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of a PU adhesive produced from a CG-based polyol with a hydroxyl number of 296 mg KOH/g and an NCO/OH molar ratio of 1.3 under a nitrogen atmosphere. Three stages of weight loss of the CG-based PU adhesive were observed in the DTG curve of CG-based PU adhesive, and the degradation started at about 200 oC and ended at 550 oC, similar to the degradation behavior of vegetable oil-based PUs (Javni, et al. 2000; Ni, et al. 2010.) The first stage of degradation occurred at temperatures from approximately 200 to 300 oC, which resulted from 16
the dissociation of urethane linkages to isocyanates and alcohols and/or to amines and olefins with a loss of CO2 (Javni, et al. 2000). The second and third stages of degradation occurred over the temperature range of 300 to 550 oC, and may have been due to the intensified decomposition of urethane bonds (Lee et. al. 2007) and the scission of fatty acid chains in the polyol structures (Luo et al. 2013; Kong, et al. 2011). Overall, the CG-based PU adhesive showed good thermal stability compared to analogs derived from vegetable oils (Dweck, et al. 2004; Javni, et al. 2000). 3.9 Comparison with commercial wood adhesives A PU adhesive prepared from a CG-based polyol with an NCO/OH molar ratio of 1.3 was compared with three commercial PU wood adhesives. As shown in Table 9, CG-based PU adhesives had lap shear strengths superior to that of the commercial PU wood adhesives, which suggests that CG-based PU adhesives have potential for wood bonding applications. Further investigation is needed to develop methods to prolong the curing time for use in other applications. Table 9 Comparison of CG-based PU adhesives and three commercial PU wood adhesives Sample
Lap shear strength (MPa)
Bostik’s Best adhesive
35.5±2.7
J.E. Moser’s Wood Glue
30.2±3.6
Sikaflex PU adhesive
37.7±2.1
CG-based PU adhesive
36.8±2.5
17
4. Conclusions In this study, PU adhesives were successfully synthesized from CG-based polyol and pMDI. With an increase of the hydroxyl number of CG-based polyols, the resulting PU adhesives showed increased bond strength but faster curing time. The molar ration of NCO/OH of 1.3 was preferred in terms of adhesion strength. CG-based PU adhesives exhibited superior resistance to cold water, moderate resistance to hot water, and relatively weak resistance to acid and alkali solutions. Compared to some commercial PU wood adhesives, CG-based PU adhesives demonstrated higher bond strength, suggesting they have potential for wood bonding applications. Due to the fast curing time, the applications of CG-based PU adhesives may be targeted at fast-curing adhesive applications.
18
Acknowledgements This project is supported by funding from USDA-NIFA Critical Agricultural Materials Program (No. 2012-38202-19288). The authors would like to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for reading through the manuscript and providing useful suggestions. The authors would also like to thank Josh Borgemenke (Department of Food, Agricultural and Biological Engineering, OSU) for giving critical suggestions.
19
References Abraham, T.W., Carter, J.A., Malsam, J., Zlatanic, A.B., 2007. Pittsburg State University, Oligomeric Polyols from Palm-based Oils and Polyurethane Compositions Made Therefrom, WO2007123637. Ang, K.P., Lee, C.S., Cheng, S.F. Chuah, C.H., 2014. Polyurethane wood adhesive from palm oil-based polyester polyol. J. Adhes. Sci. Technol. 28, 1020-1033. Briones, R., Serrano, L., Younes, R.B., Mondragon, I., Labidi, J., 2011. Polyol production by chemical modification of date seeds. Ind. Crops. Prod. 34, 1035-1040. De Gray, D., 1998. PUR adhesives offer solutions for assembly challenges. Adhes. Age. 41, 23-24. De Menezes, A.J., Pasquini, D., Curvelo, A.A., Gandini, S., 2007. Novel thermoplastic materials based on the outer-shell oxypropylation of corn starch ganules. Biomacromolecules. 8, 2047-2050. Desai, S.D., Anurag, L.E., Kumar, S.V., 2003. Biomaterial Based Polyurethane Adhesive for Bonding Rubber and Wood Joints. J. Polym. Res. 10, 275-281. Desai, S.D., Patel, J.V., Sinha, V.K., 2003. Polyurethane adhesive system from biomaterial-based polyol for bonding wood. Int. J. Adhes. Adhes. 23. 393-399. Dweck, J., Sampaio, C.M.S., 2004. Analysis of the thermal decomposition of commercial vegetable oils in air by simultaneous TG/DTA. J. Therm. Anal. Calorim. 75: 385-391. Ebewele, R.O., River, B.H., Myers, G.E., 1994. Behavior of amine-modified urea-formaldehyde-bonded wood joints at low formaldehyde/urea molar ratios. J. Appl. Polym. Sci. 52, 689-700. Hu, S., Luo, X., Li, Y. 2015. Production of polyols and waterborne polyurethane dispersions from biodiesel-derived crude glycerol. J. Appl. Polym. Sci. 132, 41425-41433. Hu, S., Li, Y., 2014. Polyols and polyurethane foams from acid-catalyzed biomass liquefaction by crude glycerol: effects of crude glycerol impurities. J. Appl. Polym. Sci. 131, 9054-9062. Javni, I., Petrovic, Z.D., Guo, A., 2000. Rachel Fuller. Thermal stability of polyurethane based vegetables oils. J. Appl. Polym. Sci. 77, 1723-1734. John, N., Joseph, R., 1998. Rubber solution adhesives for wood to wood bonding. J. Appl. Polym. Sci. 68, 1185-1189. Keyur, P.S., Sujata, S.K., Natvar, K.P., Animesh, K.R., 2003. Castor Oil based polyurethane adhesives for wood-to-wood bonding. Int. J. Adhes. Adhes. 23, 269-275. Kong, X.H., Liu, G.G., Curtis. J.M., 2011. Characterization of canola oil based polyurethane wood adhesives. Int. J. Adhes. Adhes. 31,559-564. Lee, C.S., Ooi, T.L., Chuan, C.H., 2007. Synthesis of Palm Oil-based Diethanolamides. J. Am. Oil. Chem Soc. 84, 945-952. 20
Li, A., Li, K., 2014. Pressure-sensitive adhesives based on epoxidized soybean oil and Dicarboxylic acids. ACS Sustainable Chem. Eng. 2, 2090-2096. Luo, X., Ge, X., Cui, S., Li, Y., 2016, Value-added processing of crude glycerol into chemicals and polymers. Bioresource Technol. 215, 144-154. Luo, X., Hu, S., Zhang, X., Li, Y., 2013. Thermochemical conversion of crude glycerol to biopolyols for the production of polyurethane foams. Bioresource Technol. 139, 323-329. Luo, X., Li, Y., 2014. Synthesis and characterization of polyols and polyurethane foams from PET waste and crude glycerol. J. Polym. Environ. 22, 318-328. Malavasic, T., Cernilec, N., Mirceva, A., Osrekar, U., 1992. Synthesis and adhesive properties of some polyurethane dispersion. Int. J. Adhes. Adhes. 112,38-42. Moghadam, P.N., Yarmohamadi, M., Hasanzadeh, R., Nuri. S., 2016. Preparation of Polyurethane Wood adhesives by polyols formulated with polyester polyols based on castor oil. Int. J. Adhes. Adhes. 68, 273-282. Munchow, E.A., Ferreira, A.C., Machado, R.M., Ramos, T.S., Rodrigues-Junior, S.A., Zanchi, C.H., 2014. Effect of Acidic solutions on the surface degradation of a micro-hybrid composites Resin. Braz. Den. J. 25, 321-326. Nadji, H., Bruzzese, C., Belgacem, M.N., Benaboura, A., Gandini, A., 2005. Oxypropylation of lignins and preparation of rigid polyurethane foams from the ensuing polyols. Macromol. Mater. Eng. 290, 1009-1016. Ni, B., Yang, L., Wang, C., 2010. Synthesis and thermal properties of soybean oil-based waterborne polyurethane coatings. J. Therm. Anal. Calorim. 100: 239-246. Pagliaro, M., Ciriminna, R., Kimura, H., Rossi, M., Pina, C.D., 2007. From Glycerol to Value- Added Products. Angew. Chem. Int. Ed. 46, 4434-4440. Pizzi, A., 2015. Synthetic Adhesives for wood panels: Chemistry and Technology, in: Mittal K.L. (Eds.), Progress in adhesion and adhesives. Scrivener Publishing, Beverly, pp. 66-74. River, B.H., Ebewele, R.O., Myers, G.E., 1994. Failure mechanisms in wood joints bonded with urea-formaldehyde adhesives. Eur. J. Wood Prod. 52, 179-184. Roh, Y., Kumar, R., Zhao, C. L., Kaczan, D., Smiecinski, T.M., 2008. BASF, Polyol formed from an epoxidized Oil, WO2009138411. Saetung, A., Tsupphayakorn-ake, P., Tulyapituk, T., Seatung, N., Phinyocheep, P., Pilard, J. F., 2015. The chain extender content and NCO/OH ratio flexibly tune the properties of natural rubber-based waterborne polyurethanes. J. Appl. Polym. Sci. 132, 42505 -42517. Salleh, K.M., Hashim, R., Sulaiman, O., Hiziroglu. S., Nadhari. W.N.A.W., Karim. N.A., Jumhuri. N., Ang, L. Z. P., 2015. Evaluation of properties of starch-based adhesives and particleboard manufactured from them. J. Adhes. Sci. Technol., 29, 319-336. Shah, A.M., Shah, T.M., 2001. Process for Production Polyols and Polyols for Polyurethane, US6258869. Tamburini, U., 1970. Alkaline degradation of wood: Effects on Young’s Modulus. Wood Sci. 21
Technol. 4, 284-291. Tan, H.W., Abdul Aziz, A.R., Aroua, M.K., 2013. Glycerol production and its applications as a raw material: A review. Renew. Sust. Energ. Rev. 27, 118-127. Zhang, L. H., Brostowitz, N.R., Cavicchi, K.A., Weiss, R.A., 2014. Perspective: Ionomer research and applications. Macromol. React. Eng. 8, 81-99. Zhang, L.H., Chen, L.F., Rowan, S.J., 2017. Trapping dynamic disulfide bonds in the hard segment of thermoplastic polyurethane elastomers. Macromol. Chem. Phys. 218, 1600320 (1-10).
22
Figures and Tables Fig. 1 Lap shear strength of CG-based PU adhesives prepared at different NCO/OH molar ratios as a function of time Fig. 2 Lap shear strength of CG-based PU adhesives after chemical treatment (cold water, hot water, acid solution, and alkali solution) Fig. 3 FTIR spectra of a CG-based polyol and its derived PU adhesive Fig. 4 Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of a CG-based PU adhesive
23
Figures
Fig.1 Lap shear strength of CG-based PU adhesives prepared at different NCO/OH molar ratios as a function of time
24
Fig. 2 Lap shear strength of CG-based PU adhesive after chemical treatment (cold water, hot water, acid solution, and alkali solution)
25
Fig.3 FTIR spectra of a CG-based polyol and its derived PU adhesive
26
Fig. 4 Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of a CG-based PU adhesive
27
PII: DOI: Reference:
S0143-7496(17)30136-7 http://dx.doi.org/10.1016/j.ijadhadh.2017.04.008 JAAD2038
To appear in: International Journal of Adhesion and Adhesives Received date: 17 October 2016 Accepted date: 12 April 2017 Cite this article as: Shaoqing Cui, Xiaolan Luo and Yebo Li, Synthesis and properties of polyurethane wood adhesives derived from crude glycerol-based p o l y o l s , International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2017.04.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and properties of polyurethane wood adhesives derived from crude glycerol-based polyols Shaoqing Cui, Xiaolan Luo, Yebo Li* Department of Food, Agricultural, and Biological Engineering, The Ohio State University /Ohio Agricultural Research and Development Center, 1680 Madison Ave, Wooster, OH 44691-4096, USA * Corresponding Author. Tel.: + 1 330 263 3855; Fax: + 1 330 263 3670. E-mail address: [email protected] (Y. Li); [email protected].
1
Abstract Crude glycerol, a waste stream of the biodiesel production process, is low-cost renewable feedstock for the production of chemicals and polymers. In this study, polyurethane (PU) adhesives were synthesized from crude glycerol-based polyols (CG-based polyols) for wood bonding applications. Effects of different variables, including hydroxyl values of CG-based polyols, chain extenders, and the molar ratio of NCO/OH on the properties of PU adhesives were investigated. The chemical structures of PU adhesives were characterized, and their thermal, mechanical, and chemical resistance properties were evaluated. The experimental results indicated that an increase of the NCO/OH molar ratio (1.3) substantially improved bonding strength by up to 38 MPa. Higher thermal stability and stronger chemical resistance to hot and cold water and to alkali and acid solutions were observed comparing to vegetable oil-based adhesives. However, the effect of the hydroxyl value of polyols on bonding strength was not significant. Additionally, bond strength of crude glycerol-based PU adhesives was comparable to that of some commercial PU wood adhesives. All these properties demonstrated the potential of CG for PU wood adhesive applications, particularly for fast-curing uses. Keywords: Crude glycerol, Bio-polyol, Wood adhesive, Polyurethane
2
1. Introduction Urea-formaldehyde and phenol-formaldehyde based adhesives have been widely used as wood panel adhesives, foundry sand binders, and for the bonding of papers (Pizzi, 2015). However, they are easily hydrolyzed and, under cyclic moisture or warm and humid conditions, can undergo stress scissions (Robert, et al. 1994; River, et al. 1994). Moreover, the release of formaldehyde has posed serious environmental and health concerns. To address these problems, polyurethanes (PUs) have been widely introduced for adhesive production. PUs are versatile polymeric materials that demonstrate excellent flexibility, strong adhesion, high performance at low-temperature, and good stability (De Gray, 1998; John and Joseph, 1998; Malavasic, et al. 1992; Zhang, et al. 2014; Zhang, et al. 2017). In general, PUs are produced by the reaction of polyols and isocyanates, most of which are petroleum-based. Concerns over dwindling petroleum reserves have led to investigations into substitutes that are produced from renewable sources. Because of the limited choice of isocyanates, a majority of the research on renewable substitutes used for PU production has focused on the polyol component. Natural oils (Abraham, et al. 2007; Shah, et al. 2001; Roh, et al. 2008), lignocellulosic biomass (Nadi, 2005; Brioner, 2011), and carbohydrates such as starch (Menezes, et al. 2007) have been widely used for the production of bio-based polyols. Specifically, vegetable oil and starch derived polyols have been successfully used to synthesize PU adhesives. Studies reported that the adhesion strength of PU adhesives derived from castor oil-based polyols was ten times higher than that of commercially available adhesives, when the PU adhesives were applied to wood joints (Keyur, et al. 2003; Desai, et al. 2003). PU adhesives produced from canola 3
oil-based polyols also demonstrated lap shear strengths superior to those of three commercial PU adhesives (Kong, et al. 2011). Natural adhesives derived from wheat and oil palm starch also have been demonstrated to meet industrial standards (Japanese) in terms of mechanical properties (Kushairi, et al. 2015). In recent years, there has been increasing interest in value-added processing of crude glycerol into chemicals and polymers (Luo, et al. 2016; Pagliaro, et al. 2007; Tan, et al. 2013). Crude glycerol is a low-value byproduct which is primarily obtained from the biodiesel production process (Luo, et al. 2016). It has potential as a renewable substitute for petroleum-based feedstocks. Studies have reported the successful production of CG-based polyols with properties suitable for PU applications (Luo, et al. 2014; Luo, et al. 2013). Crude glycerol impurities, such as fatty acids and/or fatty acid methyl esters, were found to be critical for the production of high quality polyols and PU foams and coatings (Hu, et al. 2014). Compared to vegetable oils, crude glycerol is an inexpensive byproduct and does not compete directly with food supplies. Therefore, the use of crude glycerol in the production of bio-based PU adhesives is expected. In this study, the feasibility of crude glycerol-based polyols for the production of PU adhesives for wood bonding was investigated. The effects of parameters, including hydroxyl number of CG-based polyols, chain extenders, and the molar ratio of NCO/OH on PU adhesive properties, were studied and the optimized parameters were determined. In addition, the properties of PU adhesives from CG-based polyols were compared with some commercial wood adhesives to investigate their potential applications for wood bonding.
4
2. Method and materials 2.1 Materials Four CG-based polyols with hydroxyl numbers of 486, 422, 296 and 191 mg KOH/g, and acid numbers below 3 mg KOH/g, were obtained from Bio100 Technologies, LLC (Mansfield, OH). The characteristics of CG-based polyols involving the main composition, averaged molecular weight and functionality were supplemented and are listed in Table 1 and their representative schematics also shown in Fig.1 in the supplementary file. Polyethylene glycerol with an average molecular weight of 400 (PEG 400), 1, 4-butanediol, ethylene glycol, and neopentyl glycol were purchased from Fisher Scientific (Pittsburgh, PA). Polymeric methylene-4-4’-diphenyl isocyanate (pMDI) was obtained from Covestro Bayer Materials Science (Pittsburg, PA). Dibutyltin dilaurate (DBTDL) was obtained from Pfaltz & Bauer (Waterbury, CT). The commercial PU adhesives used for property comparison were Vibrs-tite PU adhesive (ND Industries, Inc., Clawson, MI), BOSTIK’S BEST wood flooring urethane adhesive (Bostik, Inc., Middleton, MA), and waterproof Franklin Titebond PU glue (Franklin International, Columbus, OH). 2.2 Adhesive preparation The preparation of PU adhesives was carried out at room temperature under nitrogen protection by mixing CG-based polyols with pMDI in the presence/absence of chain extenders using DBTDL as a catalyst (0.1 wt% based on the total weight of polyols and pMDI). Tables 1-3 show the formulations for the synthesis of PU adhesives from CG-based polyols with different hydroxyl numbers (Table 1), PU adhesives with different chain extenders from CG-based polyols with a hydroxyl number of 296 mg KOH/g (Table 2), and 5
PU adhesives from CG-based polyols with a hydroxyl number of 296 mg KOH/g with different molar ratios of NCO/OH (Table 3). Table 1 Formulation for the synthesis of PU adhesives from CG-based polyols with different hydroxyl numbers Polyols Adhesives
Hydroxyl number
pMDI
Molar ratio
mass (g)
mass (g)
of NCO/OH
(mg KOH/g) CGPU-191-1.3
191
5
2.99
1.3
CGPU-296-1.3
296
5
4.64
1.3
CGPU-422-1.3
422
5
7.38
1.3
CGPU-486-1.3
486
5
7.63
1.3
Table 2 Formulation for the synthesis of CG-based PU adhesives with different chain extenders Adhesives
Chain extender
Chain extender
Polyola mass
pMDI
mass (g)
(g)
mass (g)
CGPU-BD-1.3
1,4-butanediol
0.5
5
4.65
CGPU-EG-1.3
Ethylene glycerol
0.5
5
4.65
CGPU-NG-1.3
Neopentyl glycerol
0.5
5
4.65
CGPU-1.3
--
--
5
4.65
Note: a Polyol hydroxyl number: 296 mg KOH/g, and molar ratio of (NCO/OH) is 1.3.
Table 3 Formulation for the synthesis of CG-based and PEG400-based PU adhesives with different NCO/OH molar ratios Adhesives
NCO/OH
Polyol mass (g)
6
pMDI mass (g)
Based on CG polyols / PEG400
CG based / PEG400 based CGPU-296-1.0/ PEG400PU-1.0
1.0
6
4.30 / 4.36
CGPU-296-1.1 / PEG400PU-1.1
1.1
6
4.72 / 4.47
CGPU-296-1.2 / PEG400PU-1.2
1.2
6
5.16 / 5.15
CGPU-296-1.3 / PEG400PU-1.3
1.3
6
5.56 / 5.54
CGPU-296-1.4 / PEG400PU-1.4
1.4
6
6.08 / 5.94
CGPU-296-1.5 / PEG400PU-1.5
1.5
6
6.44 / 6.34
CGPU-296-1.6 / PEG400PU-1.6
1.6
6
6.87 / 6.73
CGPU-296-1.7 / PEG400PU-1.7
1.7
6
7.32/ 7.16
The hydroxyl number of CG-based polyols varied from 191 to 486 mg KOH/g. The NCO/OH molar ratio varied from 1.0 to 1.7 with an interval of 0.1. The effects of chain extenders, including neopentyl glycol, ethylene glycerol and 1,4-butanediol, on PU adhesive properties were investigated. PU adhesives from PEG400 were also synthesized in order to investigate property differences in comparison with CG-based PU adhesives. 2.3 Wood specimen preparation, bonding and testing Wood pieces were cut into 100 mm×25 mm×3 mm strips and polished using sandpaper before use. The synthesized PU adhesive was applied to the surface of two pieces of wood strips at a thickness of 0.1 mm and a lap joint of 25 m×30 m. A load of 1 kg was placed on top of the lap joint and left for 5 min. After that, the joined wood pieces were kept at room temperature and at humidity of 75±5% for 7 days. The lap joint shear strength of joined wood specimens were measured using a universal testing machine with a stretch rate of 50mm/min, Instron 3366 (Instron Corp., Norwood, MA), in accordance with ASTM D 906. The average value of three replicates for each sample was reported. Green strength is one of important indexes to evaluate adhesive properties. It is closely related to the ability of an adhesive to
7
hold the substrate before reaching its ultimate bond strength when completely cured. Regarding green strength measurement, joined wood specimens were cured at room temperature and then directly subjected to lap shear tests at daily intervals up to 7 days. The curing time was determined starting from CG-based polyol and isocyanate being bonded together and ending with the surface of the adhesives being firm. 2.4 Chemical resistance The chemical resistance of CG-based PU adhesives were tested under four conditions: cold water, boiling water, acid solution, and alkali solution. Wood specimens bonded with PU adhesives were immersed in cold water at 4 oC and hot water at 100 oC for 1 day, and in acid ( hydrochloric acid, pH 2) and alkaline (sodium hydroxide, pH 10) solutions at 80 oC for 1 hour. After that, the specimens were dried at room temperature for 7 days and then subjected to lab shear strength tests as described above. 2.5 FT-IR analysis Fourier transform-infrared (FT-IR) spectra of CG-based polyols and the resulting PU adhesives were obtained on a Spectrum Two IR spectrometer (PerkinElmer, Waltham, MA) with 32 scans at a resolution of 2 cm-1. 2.6 Thermogravimetric analysis Thermogravimetric analysis (TGA) was performed using a Q50 thermogravimeter (TA Instruments, New Castle, DE) by heating PU adhesive samples from 50 to 600 oC at a rate of 10 oC/ min under a nitrogen atmosphere.
8
3. Results and discussion 3.1 Effect of the hydroxyl number of polyols Table 4 shows the effects of the hydroxyl number of CG-based polyols on lap shear strength and curing time of PU adhesives. As the hydroxyl number increased from 191 to 486 mg KOH/g, the average lap shear strength of the CG-based PU adhesives gradually increased from 31.6 to 37.1 MPa, while curing time decreased from approximately 5 to 2.5 min. These results were mainly due to the increased crosslinking density of PU adhesives. Under the same NCO/OH molar ratio, as the hydroxyl number of polyols increased, the content of hydroxyl groups increased, causing more isocyanates to participate in the reaction. This resulted in the formation of a more compact crosslinking structure in the PU adhesives. Similar results also have been observed for castor oil-based PU adhesives (Keyur, et al. 2003; Moghadam, et al. 2003). Compared to castor oil-based PU adhesives and other vegetable oil-based PU adhesives (Ang, et al. 2014; Li and Li, 2014), CG-based PU adhesives showed shorter curing times. This might be caused by the low molecular weight of the CG polyols and the residual glycerol, which plays a role as chain extender (Luo, et al. 2013; Hu and Li, 2015). As demonstrated in Table 1 of the supplementary file, the free glycerol content of the employed CG-based glycerol ranges from 5.7% to 20.9% and molecular weight from 932 to 669. Besides, with an increasing hydroxyl number, the hard segment of the obtained adhesives increased from 37.9% to 60.8%, which further confirmed the results of increasing lap shear strength and decreasing curing time. The results in Table 4 also indicate that the hydroxyl number of the CG-based polyols had more obvious effects on curing time than on adhesion strength. Considering a balance between lap shear strength and curing time, the 9
CG-based polyol with a hydroxyl number of 296 mg KOH/g was chosen as a suitable polyol for further studies. Table 4 Effects of the hydroxyl number of CG-based polyols on lap shear strength and curing time of PU adhesives Adhesives
Hydroxyl number of CG
Lap shear strength
Curing time
polyol (mg KOH/g)
(MPa)
CGPU-191-1.3
191
31.6±3.3
4.0~5.0
37.9
CGPU-296-1.3
296
35.8±4.3
3.5~4.5
48.6
CGPU-422-1.3
422
36.3±2.7
3.5~4.0
57.4
CGPU-486-1.3
486
37.1±2.0
2.5~3.5
60.8
(min)
Hard segment (wt %)
3.2 Effect of chain extenders The effect of chain extenders on lap shear strength of CG-based PU adhesives are shown in Table 5. No obvious change was observed with the addition of a chain extender. PU adhesives containing 1, 4-butanediol and neopentyl glycol resulted in a lower lap shear strength than that of an analogue with ethylene glycol as a chain extender. This was most likely because both 1, 4-butandiol and neopentyl glycol had a longer chain length than that of ethylene glycol, thus improving the flexibility of PU structures. In addition, the presence of two side methyl groups of neopentyl glycol had a plasticizing effect on PU performance to some extent, reducing the lap shear strength of its PU adhesives. Compared to PU adhesives with the addition of chain extenders, CG-based PU adhesives without chain extenders exhibited the highest lap shear strength. This was mainly due to the presence of residual glycerol (around 6.5%) in CG-based polyols (Luo, et al. 2013; Hu and Li, 2014; Hu, et al. 2015), resulting in the formation of a crosslinked structure and further increasing the lap shear strength of the PU adhesives.
10
Table 5 Lap shear strength of CG-based PU adhesives with the addition of different chain extenders Samplea
Chain Extender
Lap shear strength (MPa)
CG-BD-1.3
1,4-butanediol
35.7±3.3
CG-EG-1.3
Ethylene glycerol
33.5±2.7
CG-NG-1.3
Neopentyl glycerol
34.5±1.8
CG-1.3
None
35.8±4.3
a
Note: The experiments were carried out with NCO/OH of 1.3 and hard segment around 49.1%.
3.3 Effect of NCO/OH ratio In this study, the effects of the NCO/OH molar ratio on the properties of CG-based PU adhesives that were produced by the reaction between CG-based polyols with a hydroxyl number of 296 mg KOH/g and pMDI, were investigated. In order to examine the property differences in PU adhesives from different structures of polyols, PU adhesives from PEG400, which has an approximate hydroxyl number of 280 mg KOH/g, were also produced with the same NCO/OH molar ratios, from 1.0 to 1.7. Table 6 presents the properties of PU adhesives from both polyols under the varying NCO/OH molar ratios. An obvious increase in bond strength with both CG-based and PEG400-based PU adhesives was observed as the NCO/OH molar ratio incrementally increased from 1.1 to 1.3. However, with a further increase of the NCO/OH molar ratio from 1.4 to 1.7, the bond strength of both PU adhesives gradually decreased. An optimal NCO/OH molar ratio for the highest shear strength of both PU adhesives was 1.3. As the NCO/OH molar ratio increased, PU adhesives with higher crosslink densities were achieved as demonstrated with increasing hard segment contents, resulting in increased rigidity of the adhesive. Moreover, the increased NCO/OH molar ratio provided more free isocyanates, which can react with active groups on the surface of wood samples, and further strengthen the adhesive bond (Keyur, et al. 2003). However, when beyond a
11
critical ratio (1.3), an increase in the NCO/OH molar ratio will likely cause more complex side reactions, such as the reaction of isocyanate groups with urethanes to form allophanates, the reaction of isocyanate groups with water present in wood to form ureas, and the reaction of ureas with isocyanate groups to form biuret, which increased the stiffness of PU adhesives and resulted in decreased adhesion strength (Moghadam, et al. 2016; Kong, et al, 2011). Additionally, increased hard segment content might enhance the rigidity of a bonded joint which, in contrast, may reduce the bond strength. Similar changes in adhesion strength with an increase in the NCO/OH molar ratio have been reported for PU adhesives from ricinoleic acid-based polyols (Moghadam, et al. 2016; Kong, et al. 2011; Saetung, et al. 2015). As the NCO/OH molar ratio increased from 1.0 to 1.7, the curing time of PU adhesives from CG-based polyols and PEG400 was reduced from 11 to 3 min and from 65 to 45 min, respectively. Compared to PEG-based PU adhesives, CG derived adhesives showed much higher lap shear strengths but shorter curing times. This was mainly ascribed to structural differences between the two polyols and the residual glycerol in CG-based polyols. CG-based polyols are mainly composed of monoglycerides, diglycerides, and glycerol, whose hydroxyl groups are similar to PEG400. In contrast, PEG400 is a linear polyether with two terminal hydroxyl groups. It is more flexible than CG-based polyols, therefore, lower adhesion strength was obtained. As mentioned in section 3.2, the residual glycerol could act as a cross-linker to react with isocyanates for the formation of a compact network structure, resulting in high adhesion strength of the PU adhesive. It was concluded that both cross-linking density and network structure affected the adhesive properties to some degree.
12
Table 6 Lap shear strength and curing time of designed PU adhesives based on different NCO/OH molar ratios CG-based PU a
Lap shear
Curing
Hard
PEG400-based b
Lap shear
Curing
Hard
strength
time (min)
segment
strength
time (min)
Segment (wt %)
PU adhesives
(MPa) CGPU-296-1.0
16.6±2.3
10~11
42.1
PEG400PU-1.0
0.8±1.3
60~65
40.7
CGPU-296-1.1
31.6±3.3
7~7.5
44.4
PEG400PU-1.1
2.3±1.7
60~65
43.1
CGPU-296-1.2
33.2±2.7
5~5.2
46.6
PEG400PU-1.2
12.2±2.1
55~60
45.2
CGPU-296-1.3
36.8±3.5
4~4.5
48.6
PEG400PU-1.3
17.4±2.5
55~60
47.2
CGPU-296-1.4
35.8±3.1
3~3.6
50.4
PEG400PU-1.4
17.1±2.3
50~55
49.0
CGPU-296-1.5
33.9±2.4
3~3.7
52.1
PEG400PU-1.5
16.4±1.5
50~55
50.8
CGPU-296-1.6
31.6±3.5
3~3.5
53.8
PEG400PU-1.6
16.1±1.6
45~50
52.4
CGPU-296-1.7
31.0±3.5
3~3.5
55.3
PEG400PU-1.7
15.5±1.4
45~50
53.9
adhesives
(MPa)
(wt %)
Notes: a 296 is the hydroxyl number of the CG polyol. Numbers of 1.0-1.7 are the NCO/OH molar ratios used in the formulations. b
1.0-1.7 are the NCO/OH molar ratios used in the formulations.
3.4 Properties of adhesives derived from blended polyols Blends of different ratios of CG-based polyols and PEG400 were investigated to produce PU adhesives with improved curing times using an NCO/OH molar ratio of 1.3. Table 7 shows the formulations for the production of PU adhesives from the blended polyols (CG-based polyols and PEG400) and their adhesion properties. The strength of hybrid adhesives were greatly improved compared to PEG400-based adhesives, and their curing times were significantly reduced from 60 min to 6~8 min. This significant change in adhesive performance was probably ascribed to free glycerol present in the CG-based polyols. In order to examine the effect of glycerol on the properties of PEG400-based PU adhesives, mixtures of pure glycerol and PEG400 were employed for the production of PU adhesives. The adhesive properties are shown in Table 8. As the content of pure glycerol in PEG400 increased from 0 to 40%, the curing time significantly decreased from 60 to 15 min. With a further addition of glycerol to PEG 400 (80% glycerol), the curing time decreased to 6 min. 13
Therefore, the removal of glycerol in CG-based polyols was required to improve its curing time. Table 7 Formulations for the production of PU adhesives from blended polyols and their adhesion properties Adhesives
CG polyol content
Mass of CG
Mass of
Mass of
Lap shear
Curing
samples*a
in the blend (w/w)
polyol (g)
PEG400 (g)
pMDI (g)
strength (MPa)
time (min)
PU-0-100
0%
0
4.0
3.41
17.4±2.5
55~60
PU-10-90
10%
0.4
3.6
3.47
25.4±1.7
20~25
PU-20-80
20%
0.8
3.2
3.50
29.0±1.9
18~22
PU-30-70
30%
1.2
2.8
3.53
30.9±1.2
15~17
PU-40-60
40%
1.6
2.4
3.56
31.2±1.5
13~15
PU-50-50
50%
2.0
2.0
3.58
33.2±2.4
8~10
PU-60-40
60%
2.4
1.6
3.61
33.5±2.1
8~10
PU-70-30
70%
2.8
1.2
3.64
33.6±2.1
6~8
PU-80-20
80%
3.2
0.8
3.67
34.3±2.6
6~8
PU-90-10
90%
3.6
0.4
3.70
35.0±1.9
4~5.5
PU-100-0
100%
4.0
0
3.73
35.8±2.7
4~5.5
a
Note: The molar ratio of NCO/OH is 1.3.
Table 8 Adhesive properties based on PEG-400 with different contents of glycerol Pure glycerol content
Lap shear strength
Curing time
(w/w)
(MPa)
(min)
0
17.3
55~60
40%
18.1
15~20
80%
18.4
6~10
* Formulation of this designed experiment: PEG-400 4g, pMDI 3.4g, molar ratio of –NCO/OH 1.3, and different contents of pure glycerol.
3.5 Green strength In this study, the curing performance of PU adhesives from CG-based polyols with NCO/OH molar ratios from 1.2 to 1.4, were investigated by measuring lap shear strength over one week. It can be seen from Fig.1 that the adhesion strength of three CG-based PU adhesives increased markedly during the first 4 days, and then increased slightly until day 6. Almost no significant change in adhesion strength was observed between days 6 and 7 of testing. Approximately 57% and 86% of the maximum strength were obtained after curing for 2 and 14
4 days, respectively. This suggested that the curing reaction occurred quickly during the first 4 days and then slowed down until full curing was achieved after 6 days. As for individual adhesive samples, the adhesion strength increased as the NCO/OH ratio increased from 1.2 to 1.3 and then decreased with a further increase of the NCO/OH ratio to 1.4. This could be explained by the occurrence of more complex side reactions which was discussed in section 3.3. Compared with some reported vegetable oil-based PU adhesives, CG-based PU adhesives showed a shorter curing time when their bond strength reached 85% of the maximum strength. They also exhibited higher bond strength after 4-day curing (Keyur, et al. 2003; Kong, et al. 2011). 3.6 Chemical resistance The chemical resistance of CG-based PU adhesives, with a NCO/OH molar ratio of 1.3, to cold water, hot water, and acid and alkali solutions is shown in Fig. 2. GC-based PU adhesives showed superior resistance to cold water. Hot water, acid solution, and alkali solution had negative effects on the adhesion strength. It was also observed that acid and alkali conditions caused stronger hydrolysis of PU adhesives than hot water. Adhesives fared the worst in alkali conditions, and showed the lowest lap shear strength compared to other conditions. These results are in agreement with previous studies on the chemical resistance of biomass-based PU adhesives (Desai, et al. 2003). The sharp reduction in lap shear strength of CG-based PU adhesives after alkali treatment was probably related to the degradation of the wood surface due to penetration of the alkali solution, thereby deteriorating the contact between the wood and adhesive and loosening the cured bond (Desai, et al. 2003; Zanuttini, et al. 1999; Tamburini, 1970;). 15
3.7 Infrared spectroscopy Fig. 3 shows the FTIR spectra of a CG-based polyol with a hydroxyl number of 296 mg KOH/g and its corresponding PU adhesive with an NCO/OH molar ratio of 1.3. For the CG-based polyol, a strong broad stretching band at 3445 cm
-1
due to hydroxyl groups, a
stretching band at 1745 cm-1 due to carbonyl groups of esters, and a stretching peak at 1100 cm -1 due to C-O bonds were observed in Fig. 3(a). After the reaction of the CG-based polyol with isocyanates, the produced CG-based PU adhesive showed some obvious changes in its FT-IR spectrum (Fig. 3(b)). A strong absorption band at 3340 cm -1 characteristic of the N-H group and an absorption band at around 1736 cm -1 due to carbonyl groups of urethanes were observed, demonstrating the presence of urethane structures in the adhesive. Besides, a clear strong absorption band at 2274 cm-1 was also observed in Fig. 3(b), which indicates the presence of excess –NCO groups in the PU adhesive. The appearance of expected characteristic structures in this PU adhesive confirmed the successful synthesis of PU adhesives from the CG-based polyols. 3.8 Thermal properties Fig. 4 shows thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of a PU adhesive produced from a CG-based polyol with a hydroxyl number of 296 mg KOH/g and an NCO/OH molar ratio of 1.3 under a nitrogen atmosphere. Three stages of weight loss of the CG-based PU adhesive were observed in the DTG curve of CG-based PU adhesive, and the degradation started at about 200 oC and ended at 550 oC, similar to the degradation behavior of vegetable oil-based PUs (Javni, et al. 2000; Ni, et al. 2010.) The first stage of degradation occurred at temperatures from approximately 200 to 300 oC, which resulted from 16
the dissociation of urethane linkages to isocyanates and alcohols and/or to amines and olefins with a loss of CO2 (Javni, et al. 2000). The second and third stages of degradation occurred over the temperature range of 300 to 550 oC, and may have been due to the intensified decomposition of urethane bonds (Lee et. al. 2007) and the scission of fatty acid chains in the polyol structures (Luo et al. 2013; Kong, et al. 2011). Overall, the CG-based PU adhesive showed good thermal stability compared to analogs derived from vegetable oils (Dweck, et al. 2004; Javni, et al. 2000). 3.9 Comparison with commercial wood adhesives A PU adhesive prepared from a CG-based polyol with an NCO/OH molar ratio of 1.3 was compared with three commercial PU wood adhesives. As shown in Table 9, CG-based PU adhesives had lap shear strengths superior to that of the commercial PU wood adhesives, which suggests that CG-based PU adhesives have potential for wood bonding applications. Further investigation is needed to develop methods to prolong the curing time for use in other applications. Table 9 Comparison of CG-based PU adhesives and three commercial PU wood adhesives Sample
Lap shear strength (MPa)
Bostik’s Best adhesive
35.5±2.7
J.E. Moser’s Wood Glue
30.2±3.6
Sikaflex PU adhesive
37.7±2.1
CG-based PU adhesive
36.8±2.5
17
4. Conclusions In this study, PU adhesives were successfully synthesized from CG-based polyol and pMDI. With an increase of the hydroxyl number of CG-based polyols, the resulting PU adhesives showed increased bond strength but faster curing time. The molar ration of NCO/OH of 1.3 was preferred in terms of adhesion strength. CG-based PU adhesives exhibited superior resistance to cold water, moderate resistance to hot water, and relatively weak resistance to acid and alkali solutions. Compared to some commercial PU wood adhesives, CG-based PU adhesives demonstrated higher bond strength, suggesting they have potential for wood bonding applications. Due to the fast curing time, the applications of CG-based PU adhesives may be targeted at fast-curing adhesive applications.
18
Acknowledgements This project is supported by funding from USDA-NIFA Critical Agricultural Materials Program (No. 2012-38202-19288). The authors would like to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for reading through the manuscript and providing useful suggestions. The authors would also like to thank Josh Borgemenke (Department of Food, Agricultural and Biological Engineering, OSU) for giving critical suggestions.
19
References Abraham, T.W., Carter, J.A., Malsam, J., Zlatanic, A.B., 2007. Pittsburg State University, Oligomeric Polyols from Palm-based Oils and Polyurethane Compositions Made Therefrom, WO2007123637. Ang, K.P., Lee, C.S., Cheng, S.F. Chuah, C.H., 2014. Polyurethane wood adhesive from palm oil-based polyester polyol. J. Adhes. Sci. Technol. 28, 1020-1033. Briones, R., Serrano, L., Younes, R.B., Mondragon, I., Labidi, J., 2011. Polyol production by chemical modification of date seeds. Ind. Crops. Prod. 34, 1035-1040. De Gray, D., 1998. PUR adhesives offer solutions for assembly challenges. Adhes. Age. 41, 23-24. De Menezes, A.J., Pasquini, D., Curvelo, A.A., Gandini, S., 2007. Novel thermoplastic materials based on the outer-shell oxypropylation of corn starch ganules. Biomacromolecules. 8, 2047-2050. Desai, S.D., Anurag, L.E., Kumar, S.V., 2003. Biomaterial Based Polyurethane Adhesive for Bonding Rubber and Wood Joints. J. Polym. Res. 10, 275-281. Desai, S.D., Patel, J.V., Sinha, V.K., 2003. Polyurethane adhesive system from biomaterial-based polyol for bonding wood. Int. J. Adhes. Adhes. 23. 393-399. Dweck, J., Sampaio, C.M.S., 2004. Analysis of the thermal decomposition of commercial vegetable oils in air by simultaneous TG/DTA. J. Therm. Anal. Calorim. 75: 385-391. Ebewele, R.O., River, B.H., Myers, G.E., 1994. Behavior of amine-modified urea-formaldehyde-bonded wood joints at low formaldehyde/urea molar ratios. J. Appl. Polym. Sci. 52, 689-700. Hu, S., Luo, X., Li, Y. 2015. Production of polyols and waterborne polyurethane dispersions from biodiesel-derived crude glycerol. J. Appl. Polym. Sci. 132, 41425-41433. Hu, S., Li, Y., 2014. Polyols and polyurethane foams from acid-catalyzed biomass liquefaction by crude glycerol: effects of crude glycerol impurities. J. Appl. Polym. Sci. 131, 9054-9062. Javni, I., Petrovic, Z.D., Guo, A., 2000. Rachel Fuller. Thermal stability of polyurethane based vegetables oils. J. Appl. Polym. Sci. 77, 1723-1734. John, N., Joseph, R., 1998. Rubber solution adhesives for wood to wood bonding. J. Appl. Polym. Sci. 68, 1185-1189. Keyur, P.S., Sujata, S.K., Natvar, K.P., Animesh, K.R., 2003. Castor Oil based polyurethane adhesives for wood-to-wood bonding. Int. J. Adhes. Adhes. 23, 269-275. Kong, X.H., Liu, G.G., Curtis. J.M., 2011. Characterization of canola oil based polyurethane wood adhesives. Int. J. Adhes. Adhes. 31,559-564. Lee, C.S., Ooi, T.L., Chuan, C.H., 2007. Synthesis of Palm Oil-based Diethanolamides. J. Am. Oil. Chem Soc. 84, 945-952. 20
Li, A., Li, K., 2014. Pressure-sensitive adhesives based on epoxidized soybean oil and Dicarboxylic acids. ACS Sustainable Chem. Eng. 2, 2090-2096. Luo, X., Ge, X., Cui, S., Li, Y., 2016, Value-added processing of crude glycerol into chemicals and polymers. Bioresource Technol. 215, 144-154. Luo, X., Hu, S., Zhang, X., Li, Y., 2013. Thermochemical conversion of crude glycerol to biopolyols for the production of polyurethane foams. Bioresource Technol. 139, 323-329. Luo, X., Li, Y., 2014. Synthesis and characterization of polyols and polyurethane foams from PET waste and crude glycerol. J. Polym. Environ. 22, 318-328. Malavasic, T., Cernilec, N., Mirceva, A., Osrekar, U., 1992. Synthesis and adhesive properties of some polyurethane dispersion. Int. J. Adhes. Adhes. 112,38-42. Moghadam, P.N., Yarmohamadi, M., Hasanzadeh, R., Nuri. S., 2016. Preparation of Polyurethane Wood adhesives by polyols formulated with polyester polyols based on castor oil. Int. J. Adhes. Adhes. 68, 273-282. Munchow, E.A., Ferreira, A.C., Machado, R.M., Ramos, T.S., Rodrigues-Junior, S.A., Zanchi, C.H., 2014. Effect of Acidic solutions on the surface degradation of a micro-hybrid composites Resin. Braz. Den. J. 25, 321-326. Nadji, H., Bruzzese, C., Belgacem, M.N., Benaboura, A., Gandini, A., 2005. Oxypropylation of lignins and preparation of rigid polyurethane foams from the ensuing polyols. Macromol. Mater. Eng. 290, 1009-1016. Ni, B., Yang, L., Wang, C., 2010. Synthesis and thermal properties of soybean oil-based waterborne polyurethane coatings. J. Therm. Anal. Calorim. 100: 239-246. Pagliaro, M., Ciriminna, R., Kimura, H., Rossi, M., Pina, C.D., 2007. From Glycerol to Value- Added Products. Angew. Chem. Int. Ed. 46, 4434-4440. Pizzi, A., 2015. Synthetic Adhesives for wood panels: Chemistry and Technology, in: Mittal K.L. (Eds.), Progress in adhesion and adhesives. Scrivener Publishing, Beverly, pp. 66-74. River, B.H., Ebewele, R.O., Myers, G.E., 1994. Failure mechanisms in wood joints bonded with urea-formaldehyde adhesives. Eur. J. Wood Prod. 52, 179-184. Roh, Y., Kumar, R., Zhao, C. L., Kaczan, D., Smiecinski, T.M., 2008. BASF, Polyol formed from an epoxidized Oil, WO2009138411. Saetung, A., Tsupphayakorn-ake, P., Tulyapituk, T., Seatung, N., Phinyocheep, P., Pilard, J. F., 2015. The chain extender content and NCO/OH ratio flexibly tune the properties of natural rubber-based waterborne polyurethanes. J. Appl. Polym. Sci. 132, 42505 -42517. Salleh, K.M., Hashim, R., Sulaiman, O., Hiziroglu. S., Nadhari. W.N.A.W., Karim. N.A., Jumhuri. N., Ang, L. Z. P., 2015. Evaluation of properties of starch-based adhesives and particleboard manufactured from them. J. Adhes. Sci. Technol., 29, 319-336. Shah, A.M., Shah, T.M., 2001. Process for Production Polyols and Polyols for Polyurethane, US6258869. Tamburini, U., 1970. Alkaline degradation of wood: Effects on Young’s Modulus. Wood Sci. 21
Technol. 4, 284-291. Tan, H.W., Abdul Aziz, A.R., Aroua, M.K., 2013. Glycerol production and its applications as a raw material: A review. Renew. Sust. Energ. Rev. 27, 118-127. Zhang, L. H., Brostowitz, N.R., Cavicchi, K.A., Weiss, R.A., 2014. Perspective: Ionomer research and applications. Macromol. React. Eng. 8, 81-99. Zhang, L.H., Chen, L.F., Rowan, S.J., 2017. Trapping dynamic disulfide bonds in the hard segment of thermoplastic polyurethane elastomers. Macromol. Chem. Phys. 218, 1600320 (1-10).
22
Figures and Tables Fig. 1 Lap shear strength of CG-based PU adhesives prepared at different NCO/OH molar ratios as a function of time Fig. 2 Lap shear strength of CG-based PU adhesives after chemical treatment (cold water, hot water, acid solution, and alkali solution) Fig. 3 FTIR spectra of a CG-based polyol and its derived PU adhesive Fig. 4 Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of a CG-based PU adhesive
23
Figures
Fig.1 Lap shear strength of CG-based PU adhesives prepared at different NCO/OH molar ratios as a function of time
24
Fig. 2 Lap shear strength of CG-based PU adhesive after chemical treatment (cold water, hot water, acid solution, and alkali solution)
25
Fig.3 FTIR spectra of a CG-based polyol and its derived PU adhesive
26
Fig. 4 Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of a CG-based PU adhesive
27