Abstract

Preparation of PU powder

Flexible PU waste foams produced by ASR were obtained from a local vehicle waste stream recycling company in Taian, Shandong Province, China. The color of the foams was light brown. Some rubber, wood, fibers, paper, glass, and other contaminants in the PU foams were removed and the foam was washed by water and then acetone (obtained from Sinopharm Group Co. Ltd, Beijing). The foams were then air-dried under a hood and ground into a 120-mesh powder using a grinding machine (Laboratory Mill, Model WQM; Jingcheng Chemical Machinery Co. Ltd, Jiangyin).

Adhesive preparation

The pMDI (Lupranate M20FB) with the isocyanate (NCO) content of 31.5 percent was obtained from the BASF Chemical Company (Wyandotte, Michigan). It was a dark yellow liquid with a viscosity of 278 cP at 25°C. The mixed adhesives were prepared by physically mixing pMDI with PU powder of 0 percent moisture content at various ratios (2.5%, 5%, 7.5%, and 10%, based on the weight of pMDI resin). The adhesive mixtures were labeled as pMDI-2.5, pMDI-5, pMDI-7.5, and pMDI-10, respectively.

Adhesive property evaluation

Panel manufacturing

Three pieces of birch (Betula sp.) veneers with a density of about 638 kg/m³, thickness of 1.6 to 1.7 mm, and a moisture content of 8 to 10 percent for the plywood top, middle, and bottom layers were cut in an end-match manner for plywood manufacturing. This allowed us to minimize the influence of wood quality variation on final panel performance. Plywood was fabricated at 300 by 300 mm using a laboratory hot press (Suzhou Xinxieli Machinery Manufacturer Co. Ltd., Suzhou, China). An adhesive application rate was 220 g/m² (single glue line). The adhesives were applied uniformly using a brush on both sides of the middle veneer. The hot press time, temperature, and pressure were 5 minutes, 135°C, and 1.1 ± 0.1 MPa, respectively. Two panels were made for each adhesive formulation. After pressing, the plywood panels were conditioned at 20°C and 60 percent relative humidity for 1 week.

Measurement of the bonding strength

The bond strength was measured in accordance with the Chinese National Standard GB/T 17657-2013 (SAC 2013). Six specimens with dimensions of 100 by 25 mm were cut from each panel to make a total of 12 specimens for each adhesive formulation. From these samples, six specimens were randomly selected and tested for dry and wet strength. Before the testing of the wet strength, specimens were first immersed in boiling water for 4 hours, then dried in an oven at 63°C for 18 hours and immersed in boiling water again for 4 hours. After the second water-immersion treatment step, the specimens were cooled down in water at 25°C and tested for lap shear strength with a CMT 4104 universal testing machine (MTS System Corporation, Minnesota). The wood failure percentage was identified and recorded.

Statistical analysis

Means and standard deviations of the data obtained were calculated. Duncan's multiple range tests with SPSS 13.0 software (Chicago, Illinois) were used to determine the differences among different treatments.

Characterization of the PU foams

After the PU foams were washed with water and acetone and dried, the color of these foams turned light yellow, and a weight loss of 10 to 20 percent was obtained. The weight loss could be from water-soluble, acetone-soluble substances, and volatile compounds of PU foams collected.

The chemical composition of the ground PU foam powder was characterized using FTIR. The FTIR spectrum (Fig. 1) shows the absorption bands of the PU foam powder. The absorption bands at 1,714.5 and 1,531.8 cm⁻¹ correspond to the urethane carbonyl group (C=O in –NH–CO–O–) stretching and N–H bending vibration, respectively. The absorption bands at 1,643.2 and 3,289.4 cm⁻¹ correspond to the urea carbonyl group (C=O in –NH–CO–NH–) and the stretching vibration of N–H groups, respectively. These two groups were the products from the reaction between isocyanate and water (Wu et al. 2003, Wang and Chen 2006, Kwon et al. 2007, Bene et al. 2012). The absorption band at 1,449 cm⁻¹ corresponds to the aromatic ring and is indicated as a possible aromatic source of the isocyanate as one of the raw materials in the preparation of the PU foams (Lee et al. 2005). The intense stretching vibration of the aliphatic –C–O–C– group at about 1,097.8 cm⁻¹ suggests that the main type of raw polyols used in the preparation of the PU foams might be polyether polyols (Mao et al. 2014).

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Adhesive viscosity

The viscosity of the adhesives is presented in Table 1. The statistical analysis demonstrates that the viscosity of the pMDI–PU mixing increased with PU powder content (increased from 278 cP for pMDI to 3,280 cP for PMDI-10). In other words, the viscosity of the adhesives was closely related to the amount of PU powder. It was estimated that as the amount of PU powder in the adhesives increased, the foaming structure of the PU powder could absorb more pMDI resin, especially the smaller molecules in the pMDI resin, and this affected the flow during mixing. So, the viscosity of the adhesive would increase accordingly, which helped to maintain pMDI on the veneer surface and prevent the resin from penetrating into the wood cell walls. This could help solve the problem of a starved bond-line on the veneer surface when using pMDI as a plywood binder. However, as the amount of PU powder added was increased, the viscosity eventually became too high and made dispersion difficult in a plant setting. According to viscosity results (Table 1) and lab practice, a PU powder content higher than 10 percent might result in viscosities that are too high for application onto plywood or LVL.

Table 1.— Viscosities of the adhesives.

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FTIR spectrum of adhesives

Figure 2 shows the FTIR spectrum of the adhesive liquid mixtures at various percentages of PU powder substitution (2.5%, 5%, 7.5%, and 10%). The absorption band at 2,242 cm⁻¹ corresponds to the isocyanate group (–NCO). The spectrum of pMDI resin was similar to those of pMDI-2.5, pMDI-5, pMDI-7.5, and pMDI-10 adhesives, indicating that the reaction might not happen immediately after the PU powder was mixed with the pMDI. During plywood and LVL manufacturing, the addition of PU powder into pMDI resin might not affect the pot life of pMDI resin greatly and this will provide enough time for adhesive application.

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Figure 3 shows the FTIR spectrum of the cured adhesives obtained by mixing PU powder into pMDI at various weight percentages (2.5%, 5%, 7.5%, and 10%). The subtle change in absorption peaks with the addition of PU powder suggests that the pMDI structure was not altered substantially. This was probably because the PU powder addition amount was still very small (no more than 10%, based on the weight of pMDI resin), or that the FTIR used was not sensitive enough to tell the difference. The absorption band at 1,662 cm⁻¹ should correspond to the carbonyl group (–C=O) in substituted urea, which was the reaction product of –NCO groups and water (Mao et al. 2014). The cured mixed adhesives still showed obvious absorption peaks of the –NCO group (absorption bands at 2,252 cm⁻¹) in the spectrum. This result indicates that even though the pMDI and mixed adhesives were cured at 120°C for 3 hours, some residual –NCO groups were still left in various proportions (Li et al. 2017). As the amount of PU powder in the adhesives was increased, the strength of absorption bands of –NCO groups decreased, indicating that some reactions might happen between –NCO groups and other functional groups or materials in PU powder. The potential reactions could be that (1) –NCO groups in pMDI resin reacted with trace water in PU powder or (2) –NCO groups in pMDI resin reacted with –NH– groups in PU powder (Fig. 1). Either reaction might result in the increase of the cross-link density of cured adhesives, which would contribute to the cohesion strength of cured mixed adhesives; or it may be simply that too much PU powder added in might reduce the density of –NCO groups in the mixtures.

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TG analysis of the adhesives

The TG curve of the PU powder sample is shown in Figure 4 and the weight loss of PU is shown in Table 2. The PU powder had two different weight-loss regions. The first weight-loss region occurred between 200°C and 300°C and exhibited a weight loss of 28 percent. The thermal degradation of PU materials started at a temperature of 200°C. At this temperature, the C–O bond in the urethane group started to break and generate isocyanate and polyols (Yuan et al. 2005). This process lasted until the temperature reached 300°C. At 300°C, the degradation of the urethane group was nearly completed and the curve became stable at higher temperatures. The second weight-loss region was prevalent at 350°C to 400°C, which had a corresponding weight loss of 55 percent. As the temperature increased to 350°C, the curve started to decrease quickly. The reason could be the degradation of the products from the first degradation reaction: isocyanate and polyol. At temperatures higher than 350°C, the isocyanate and polyol started to degrade to generate amines, olefins, and CO₂ (Duquesne et al. 2001). This could result in an obvious weight loss of PU materials.

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Table 2.— Thermogravimetric (TG) parameter of polyurethane (PU) and the adhesives with different PU additions.

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The TG curves of the adhesives are shown in Figure 4 and the calculation of weight loss in the adhesive is shown in Table 2. From room temperature to 600°C, there were two different weight-loss regions. The first weight-loss region occurred between 150°C and 300°C, and had a weight loss of about 59, 55, 53, 52, and 51 percent for pMDI, pMDI-2.5, pMDI-5, pMDI-7.5, and pMDI-10 adhesives, respectively. The weight loss decreased with increased PU powder substitution. The reactions of pMDI with water and –NH– groups in PU powder appeared to increase the thermal stability of the cured mixed adhesives (pMDI-2.5, pMDI-5, pMDI-7.5, and pMDI-10) between 150°C and 300°C, resulting in less weight loss than that of pMDI resin alone. The second weight-loss range started at around 400°C and lasted to the end of the TG tests (600°C). During exposure to this temperature range, the weight losses were 19, 24, 26, 26, and 27 percent for pMDI, pMDI-2.5, pMDI-5, pMDI-7.5, and pMDI-10 adhesives, respectively. After the PU powder was added into the pMDI resin, the weight loss increased (from 19% to 27%). This result was opposite of that in the first weight-loss range (from 150°C to 300°C) discussed just above. These weight losses might be from thermal degradation of the PU powder. However, from the TG test results of PU powder, after 400°C, the degradation line already became stable and the PU powder had no obvious weight loss. The probable reason might be that the addition of PU powder into pMDI resin also increased the thermal stability of the PU powder so that the second weight loss of PU powder was postponed to a higher temperature range. The final percentage of degradation remains for all of the adhesives was similar but clearly higher than that of PU powder.

Panel bonding performance

The bond strength and wood failure percentage of the adhesives were determined (Table 3). Both the dry and wet strengths of the mixed adhesives met the requirements of the standard GB/T 17657-2013 (SAC 2013) for outdoor applications (≥1.0 MPa for dry strength and ≥0.7 MPa for wet strength).

Table 3.— Bonding performance of the plywood.

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Direct use of the pMDI resin for bonding plywood in this study was met with some difficulties: when pMDI resin was applied to the veneer surface, it penetrated into the veneer quickly (within 2 to 3 min), resulting in a deficiency of adhesive spread onto the veneer surface. To address this issue, the plywood had to be hot-pressed as soon as the pMDI resin was applied to the veneer. This could be done under lab conditions but was not acceptable in the factory. In a mill, usually, after the adhesive is applied, at least 30 minutes can expire before plywood hot-pressing.

Statistical analysis revealed that the addition of PU powder affected the dry strength of the plywood specimens significantly. The dry bond strength of adhesives increased with PU powder substitution (from 2.5% to 7.5%). At a PU content of 7.5 percent, the strength value reached the highest point (2.68 MPa). As the PU content increased from 7.5 to 10 percent, the dry bond strength value decreased to 1.87 MPa. The dry bond strength is affected by the homogeneity and fluidity of the adhesives (Jin et al. 2010).The addition of PU foam powder into pMDI resin increased adhesive viscosity. Even with a PU foam powder content of 7.5 percent, the pMDI-7.5 adhesive still had an appropriate viscosity and could be spread well on the veneer surface. (In a mill, the adhesive should have viscosity around 2,000 cP.) This could contribute to good dry bond strength. However, when the PU powder content increased from 7.5 to 10 percent, the adhesive viscosity increased quickly from 2,231 to 3,280 cP (Table 1). High viscosity would affect the wetting ability of the adhesive, the spread of the adhesive to the wood surface, and also the depth of penetration of adhesive into wood, in a way that is counterproductive. These are all important factors for the formation of a good dry bond. Therefore, when PU content increased to 10 percent, both the dry bond strength and wood failure percentage decreased obviously, indicating an inappropriate content of PU foam powder used at 10 percent.

Statistical analysis also shows that the addition of PU powder affected the wet strength of plywood specimens. The wet strength of plywood bonded by pMDI resin was 1.07 MPa. When the PU content increased from 2.5 to 7.5 percent, the wet strength value of plywood increased from 1.72 to 2.26 MPa. After a boiling-water treatment, the plywood samples only had a minor decrease in bond strength value when compared with those of dry samples. This indicated that the addition of PU powder into pMDI resin increased both bonding and water resistance of the plywood samples. This might be attributed to the reaction of PU foam powder with pMDI resin, as indicated by FTIR results (Fig. 3). However, when the PU powder content increased to 10 percent, the wet strength value decreased to 1.67 MPa, indicating that too much powder could overwhelm the pMDI system and minimize the cross-link density. This could result in a worse wetting ability of adhesive to veneer surface, lacking in fluency of adhesive spreading, and insufficient penetration of adhesive into wood because of high adhesive viscosity. Moreover, the uniformity of the adhesive layer would also be disturbed by excessive PU powder. Based on bond strength testing, the best addition amount of recycled PU powder was 7.5 percent considering both the bond strength and production practice of plywood.