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© Smithers Rapra Technology, 2013 199 Cellular Polymers, Vol. 32, No. 4, 2013 Foams and Wood Composite Foams Produced by Rotomolding Alexandre Raymond and Denis Rodrigue* Université Laval, Department of Chemical Engineering, 1065 Avenue de la Médecine, Quebec City, Quebec, Canada, G1V 0A6 SUMMARY In this work, a simple method is presented to produce foams and wood composite foams by rotational molding. As a special case, wood flour (maple) / linear low density polyethylene (LLDPE) was used to study the effect of chemical blowing agent (CBA) concentration (0-0.6%wt.) and wood content (0-20%wt.). From the samples produced, a complete characterization was performed including density, morphology, and mechanical properties in tension and flexion. The results show that tensile and flexural moduli increased with wood content, while tensile strength and elongation at break decreased. The optimum wood content was found to be around 20%wt. for the composites and around 0.4% wt. CBA for the range of conditions studied. Keywords: Polyethylene, Foams, Composites, Rotomolding INTRODUCTION Rotational molding (rotomolding, rotational casting, rotocasting) is a low shear process used to produce almost stress free, one-piece hollow products without weld lines [1]. With this process, a very wide range of part dimensions can be produced; i.e. from very small (few grams) to very large (100,000 L reservoirs) [2]. Rotational molding has significant advantages over other processing methods like injection molding and blow molding: easier to get complex shapes (less design constraints), lower capital investment and greater flexibility with respect to material or color change from part to part. These features are well recognized in all production sectors and this is why rotomolding is still today one of the fastest growing process of the plastic industry over the last few years. *[email protected]

Foams and Wood Composite Foams Produced by Rotomolding

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©Smithers Rapra Technology, 2013

199Cellular Polymers, Vol. 32, No. 4, 2013

Foams and Wood Composite Foams Produced by Rotomolding

Foams and Wood Composite Foams Produced by Rotomolding

Alexandre Raymond and Denis Rodrigue*

Université Laval, Department of Chemical Engineering, 1065 Avenue de la Médecine, Quebec City, Quebec, Canada, G1V 0A6

SUMMARY

In this work, a simple method is presented to produce foams and wood composite foams by rotational molding. As a special case, wood flour (maple) / linear low density polyethylene (LLDPE) was used to study the effect of chemical blowing agent (CBA) concentration (0-0.6%wt.) and wood content (0-20%wt.). From the samples produced, a complete characterization was performed including density, morphology, and mechanical properties in tension and flexion. The results show that tensile and flexural moduli increased with wood content, while tensile strength and elongation at break decreased. The optimum wood content was found to be around 20%wt. for the composites and around 0.4% wt. CBA for the range of conditions studied.

Keywords: Polyethylene, Foams, Composites, Rotomolding

IntRodUctIon

Rotational molding (rotomolding, rotational casting, rotocasting) is a low shear process used to produce almost stress free, one-piece hollow products without weld lines [1]. With this process, a very wide range of part dimensions can be produced; i.e. from very small (few grams) to very large (100,000 L reservoirs) [2]. Rotational molding has significant advantages over other processing methods like injection molding and blow molding: easier to get complex shapes (less design constraints), lower capital investment and greater flexibility with respect to material or color change from part to part. These features are well recognized in all production sectors and this is why rotomolding is still today one of the fastest growing process of the plastic industry over the last few years.

*[email protected]

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Even if different resins can be used for rotomolding, polyethylene has more than 85% of the market today [1-2]. But for each resin, there is always the possibility to improve on the properties (mechanical, thermal) via structural modifications. Two well-known modifications are: adding reinforcement to produce composites (higher mechanical properties) [3-4], and adding a blowing agent to foam the material (lower density and higher insulation) [5-7]. Although some literature can be found on composites and foams as separate materials, almost nothing is available on composite foams (combining both into one).

One of the most used reinforcement over the last 10 years has been natural (cellulosic) fibres. This material was introduced into different resins to improve mechanical properties, reduce raw material costs and give a “natural” look (aesthetics) [8]. One major drawback of natural fibres, is their hydrophilic nature leading to poor adhesion to hydrophobic matrices and their tendency to absorb moisture (humidity) from the atmosphere. Recently, Ward-Perron and Rodrigue [9] showed that under most rotomolding conditions (biaxially rotating mold heated at 10oC/min), wood fibres can lose most of their humidity (total content less than 1%wt.) in less than 5 min. This time is believed to be enough so the material can be considered dried before the polymer resin starts to melt and no prior wood treatment is needed (drying), thus simplifying greatly processing in comparison with extrusion and injection molding of typical wood-plastics composites (WPC). Furthermore, since all the materials are in a powder/particle form (polymer, wood, chemical blowing agent), simple dry-blending can be used to mix all the components which is again less time and energy consuming than standard melt blending (extrusion + pelletizing/pulverizing).

The main objective of this work was to produce foams and composite foams by rotational molding using the single step process. In particular, the effect of chemical blowing agent (0-0.6%wt.) and wood content (0-20%wt.) was investigated using linear low density polyethylene as the matrix. To produce the rotomolded parts, all the materials were dry-blended before being introduced into the mold (no drying). As a first step, no surface treatment of the wood particles was done to improve adhesion with the polymer matrix.

ExpERIMEntAl

Materials

The polymer used was a grade of linear low density polyethylene (LLDPE): EXXONMOBIL LL 8460.29. This polymer (ExxonMobil Chemical Canada) has

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a melt flow index of 3.3 g/10 min (2.16 kg, 190oC) and a density of 938 kg/m3. As reinforcement, maple wood flour was purchased from PWI Industries (St-Hyacinthe, Quebec, Canada). The material received was only sieved and no surface treatment was performed. Only particles between 125 and 250 microns were used as they correspond to the average LLDPE particle size as similar particle size will limit powder segregation in the dry-blending and initial rotomolding steps. For foaming, the chemical blowing agent (CBA) used was Celogen 754A (Chempoint, USA). Figure 1 presents typical images of the materials used in a powder form.

Figure 1. Scanning electron micrographs of the raw materials: (a) LLDPE, (b) Maple flour, and (c) Celogen 754A

Sample Preparation

First, all the materials were dry-blended using an anchor-type mixer for 3 to 4 minutes. To simplify the process, the wood flour was not dried before processing as most of the volatiles inside the wood particles are believed to be removed in the first minutes of the rotational molding cycle as discussed previously [9]. Figure 2 shows typical drying curves of the wood powder using different amounts of material to simulate different powder bed thicknesses

Figure 2. Drying curves for the maple wood flour used at a temperature ramp of 8.3°C/min and different powder bed thickness

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(4, 9 and 14 mm). The initial humidity was around 5.9%wt. for standard room conditions (23oC, 50% RH). The measurements were performed at a heating ramp of 8.3oC/min (from 50 to 150oC) to simulate typical rotomolding conditions inside the mold. The tests were done on a MX-50 moisture analyzer (A&D Company, Tokyo, Japan). The results indicated that up to 9 mm, less than 1% humidity (material considered dried) is left in the wood particles for times higher than 10 min (typical value before the polymer starts to melt and sticks to the mold walls). Even for a 14 mm powder bed, the humidity is around 2.5% for this time. Nevertheless, these drying results can be considered as worse-case scenario for two reasons. First, wood content in the “composite” bed (blend of wood flour and LLDPE) was kept low (20% maximum here) so the total humidity is directly proportional to wood content as polyethylene humidity can be considered negligible (about 0.2%) [9]. Second, the measurements in the humidity balance were performed under static conditions. In a biaxially rotating mold, it is believed that heat and mass transfer is highly improved due to convection, thus decreasing substantially the time to remove humidity in the wood particles. Based on this analysis, humidity in the wood flour can be assumed to be almost zero under standard rotomolding conditions and no prior drying step is needed before molding.

To produce rotomolded samples, a conventional shuttle-type rotomolding machine with two-arms was used at WES Industries (Princeville, Quebec, Canada). The aluminum mold selected had the shape of a two-level rectangular box with total dimensions of 35 x 21 x 12 cm3. One vent on the side of the mold (see Figure 3a), 12.5 mm in diameter, was filled with glass wool to

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prevent powder losses. Typical examples of the molded parts are presented in Figure 3. After preliminary runs, the optimum processing conditions for the foams were: a 4:1 speed ratio, a heating time of 21 minutes with an oven (natural gas heated) temperature of 280oC and a cooling time of 28 minutes with forced air (blowing fans) during summer time (May-August 2012). For the composite foams, the oven temperature was decreased to 270oC with a cooling time of 24 min. Lower oven temperature is probably related to the nucleating effect of the wood particles (less energy needed to decompose the CBA and nucleate bubbles) and lower cooling time can be associated with lower density reduction (less insulation effect leading to faster heat transfer) as described in the Results section. In all cases, a demolding agent was applied to help part removal. Finally, to eliminate shrinkage and warpage, the parts were internally blown with compressed air. Characterization samples were cut directly inside the flat surfaces of the molded parts.

The first series of samples was performed by changing the CBA content (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6%wt.) to produce the foams Then, a second series of samples was produced using 0.1%wt. of CBA and changing wood flour content (0, 5, 10, 15, 20%wt.). For the composite foams, 0.1% CBA was selected as this concentration was found to give a good compromise

Figure 3. Typical images of the rotomolded parts produced. a) Neat LLDPE (side view), b) neat LLDPE (top view), c) foam (0.1%wt. CBA), and d) composite foam (0.1%wt. CBA + 5%wt. maple flour)

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between density reduction and keeping mechanical strength. More results and information can be found in another recent publication on the properties of the wood composites alone [10].

Density

Density of LLDPE powder, maple flour and molded composites/foams was determined using a gas (nitrogen) pycnometer Ultrapyc 1200e (Quantachrome Instruments, USA). The results reported are the average and standard deviation of a minimum of 5 samples.

Morphology

The morphology of the molded parts was examined via scanning electron microscopy (SEM) on a JEOL model JSM-840A. Each sample was cryogenically fractured in liquid nitrogen to expose the internal structure and the surfaces were coated with a thin layer of Au/Pd. Micrographs were taken at different magnifications to get a complete quantitative morphological analysis.

Foam morphology was characterized via two parameters: cell diameter (D) and cell density (Nf). Cell diameter was determined using Image-Pro Plus 4.5 (Media Cybernetics) on SEM images. The software calculates the average of 90 diameters taken every 2 degree from the geometric center of each cell area. Cell density, defined as the number of cell (N) per cubic centimeter of foam is obtained by [11]:

N

f=

N

A

3 2

(1)

where A is the area of the micrograph in cm2.

Mechanical Properties

Flexural modulus was measured on a model 5565 (Instron, USA) universal tester in a three-point bending geometry (ASTM D790) using a 50 N load cell. Five samples having cross-sections of about 14 x 5 mm2 were tested. The span (distance between supports) was set at 100 mm to limit internal shear in the samples and the travel speed was set at 10 mm/min.

Young’s modulus, tensile strength and elongation at break were determined using a model 5565 (Instron, USA) universal tester with a 500 N load cell.

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Type V dog bone samples were cut from the molded parts according to ASTM D638. The tests were performed at room temperature at a deformation rate of 10 mm/min. A minimum of five samples was used to get an average and standard deviation.

RESUltS And dIScUSSIon

Figure 4 present typical morphologies for the foams and composites foams produced. Even for the neat polymer (0, 0), some porosity can be observed

Figure 4. Typical morphologies of the rotomolded parts. The numbers in parenthesis represent (CBA content, wood flour content) in wt.%

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as rotomolded parts are usually slightly undercooked to limit polymer (and wood here) degradation. Based on the density results of Figure 5, porosity is estimated at 1% (0.925 vs. 0.938 g/cm3) which can modify the final mechanical properties of the parts (as discussed later). Then, it is clear that increasing blowing agent content increased the number of bubble (cell density), thus decreasing density (Figure 4) as more gas is generated and trapped inside the molten polymer matrix. In our case, an optimum CBA content was obtained around 0.5% as higher values did not decreased density further. Actually, density was higher at 0.6% CBA as possible gas loss and cell collapse (broken cell walls) can be seen in sample (0.6, 0) in Figure 4. Nevertheless, a wide range of part density (0.449-0.722 g/cm3) was achieved (Figure 4) with good and uniform cell structure, especially for CBA content between 0.3 and 0.6%.

To analyze further the foam morphology, Table 1 presents the results obtained for average cell diameter and cell density. It is clear again that an optimum is

Figure 5. Density of the rotomolded foams (top) and composite foams (bottom)

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obtained here, but closer to 0.3% CBA where minimum cell size and maximum cell density are obtained simultaneously.

table 1. Morphological parameters of the foams produced

CBA content(%wt.)

D(mm)

Nf(103 cells/cm3)

0.1 471 ± 155 2.66

0.2 445 ± 126 9.54

0.3 401 ± 134 16.4

0.4 458 ± 143 12.6

0.5 482 ± 150 11.0

0.6 513 ± 127 10.0

For the composite foams, the morphology does not seem to be as homogeneous as the neat foams. In this case, cell size distribution is very large due to the presence of wood particles which are known to act as nucleating agents [12] and also limit gas diffusion [13], thus creating possible gas pockets in the polymer melts. Also, the morphology of composite foams is always difficult to quantify as most of the big voids seen in the micrographs are coming from fibre pull-out. Nevertheless, based on the results of Figure 5, density did not changed substantially with wood content up to 15% since the blowing agent content was constant, and increased slightly at 20% wood as wood density (1.480 g/cm3) is higher than LLDPE (0.938 g/cm3). Again, this will affect mechanical properties as discussed next.

For the flexural modulus, Figure 6 shows that decreasing foam density led to substantial decrease: up to 83% lower at 0.6% CBA. This is expected as less material is available to sustain the stresses when density decreases [14-15]. On the other hand, keeping the density almost constant (constant CBA content), adding wood particles led to substantial increase in flexural modulus. In our case, an increase of 62% (from 408 to 655 MPa) was obtained by adding 20%wt. of maple flour. This results clearly shows that composites foam can be made by rotomolding and can have similar flexural modulus as the neat polymer (655 vs. 680 MPa), but at lower density (0.884 vs. 0.925 g/cm3).

Finally, Figures 7-9 present the tensile properties of the foams and composite foams in terms of modulus (Figure 7), strength (Figure 8) and elongation at break (Figure 9). As for flexural modulus results, tensile modulus is decreasing with increasing CBA content. In this case, the decrease is 86% (from 184 to 26 MPa) which is higher than the 62% in flexion. On the other hand, adding wood particles led again to a slight modulus increase (24%) compared to the

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Figure 6. Flexural modulus of the rotomolded foams (top) and composite foams (bottom)

Figure 7. Tensile modulus of the rotomolded foams (top) and composite foams (bottom)

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foam without wood flour. This can be related to possible orthotropic particle orientation inside the sample due to the biaxial rotation of the molding in processing. Small variation in modulus trend can be related to inhomogeneities in the composite foam morphology as presented in Figure 4. This would need more work to improve processing and get optimized results.

For tensile strength, Figure 8 shows that the value decreases with increasing CBA content. Nevertheless, the values for the composite foams are lower than for the neat foam, but almost constant indicating that good contact occurs at the wood-polymer interface. Actually, there was no clear evidence that bad contact (voids, cracks, gas cells) occurred in our samples as presented in Figure 10, even if rotomolding is a low pressure process. Nevertheless, good contact (physical) does not mean good adhesion (chemical). This aspect will also need more work as surface treatment of the wood particles can be made before dry-blending and rotomolding. This aspect is currently being studied.

Finally, Figure 9 presents the results for the elongation at break. Again, the values are decreasing with increasing CBA content as most polymer foams become

Figure 8. Tensile strength of the rotomolded foams (top) and composite foams (bottom)

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more brittle than their polymer matrix counterpart [16-17]. For the composite foams, elongation at break is decreasing even more with increasing wood content as the particles have lower elasticity compared to the polymer matrix.

conclUSIonS

In this work, foams and composites foams were produced via rotomolding. In particular, the chemical blowing agent concentration range studied was 0-0.6%wt., while the maple flour content was varied between 0 and 20%wt. From the results obtained, several conclusions can be obtained:

Rotomolded parts can be produced by the simple dry-blending technique since all the raw materials are in a powder form. But careful control of the particle size distribution must be made to limit possible segregation related to material handling and biaxial rotation of the mold.

Figure 9. Tensile elongation at break of the rotomolded foams (top) and composite foams (bottom)

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Wood-polymer composites can be produced without the necessity of wood drying before processing. Based on dynamic drying curves and SEM micrographs of the wood-polymer interface in the final parts, no clear voids or gas cells were observed indicating good physical contact between both phases.

The optimum blowing agent content for the processing conditions selected should be around 0.4%. This value represents a compromise between 0.5% giving the lowest density (0.449 g/cm3) and 0.3% giving the most uniform cell structure (lowest cell size of 401 microns and highest cell density of 16.4x103 cells/cm3). For the composite foams, 20%wt. wood flour gave the highest mechanical properties (modulus) in tension and flexion.

Although the wood composites have good aesthetic properties (natural look), more work needs to be done in order to improve the homogeneity of the molded parts, to increase the stress transfer at the wood-polymer interface and to increase even more wood content for enhanced material properties. These investigations are currently under way and the results will be reported in a future communication.

AcKnowlEdgEMEntS

Financial support from the National Science and Engineering Research Council of Canada (NSERC) was appreciated. Technical support from the Research Centre on Advanced Materials (CERMA) was also highly useful. LLDPE samples from ExxonMobil Chemical Canada (Ron Cooke) are highly appreciated. Finally, WES industries inc. is highly thanked for machine time allowing to perform the experimental work related to sample preparation (dry blending) and rotomolding samples. Thanks also to Q. Hatte for the humidity measurements and Y. Giroux for the density measurements.

REfEREncES

1. Crawford R.J. and Throne J.L., Rotational Molding Technology, William Andrew Publishing and Plastics Design Library, USA (2002).

2. Beall G.L., Rotational Molding. Design, Materials, Tooling, and Processing, Hanser Publishers, USA (1998).

3. López-Bañuelos R.H., Moscoso F.J., Ortega-Gudiño P., Mendizabal E., Rodrigue D. and González-Núñez R., Polymer Engineering and Science, 52 (2012) 2489-2497.

4. Ward-Perron N. and Rodrigue D., Proc. of SPE ANTEC, paper 00467 (2012).

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5. Liu G.B., Park C.B. and Lefas J.A., Polymer Engineering and Science, 38 (1998) 1997-2009.

6. Chu R.K.M., Naguib H.E. and Atalla N., Polymer Engineering and Science, 49 (2009) 1744-1754.

7. Emami M., Takacs E. and Vlachopoulos J., Journal of Cellular Plastics, 46 (2010) 333-351.

8. Klyosov A.A., Wood-Plastic Composites, Wiley-Interscience, USA (2007).

9. Ward-Perron N. and Rodrigue D., Proc. of SPE ANTEC, paper 00443 (2012).

10. Raymond A. and Rodrigue D., Proc. of SPE ANTEC, paper 1589808 (2013).

11. Kumar V. and Weller J.E., Polymer Engineering and Science, 34 (1994) 169-176.

12. Rodrigue D., Souici S. and Twite-Kabamba E., Journal of Vinyl Additives and Technology, 12 (2006) 19-24.

13. Mechraoui A., Riedl B. and Rodrigue D., Journal of Cellular Plastics, 47 (2011) 115-132.

14. Barzegari M.R. and Rodrigue D., Journal of Applied Polymer Science, 113 (2009) 3103-3112.

15. Barzegari M.R., Twite-Kabamba E. and Rodrigue D., Journal of Porous Materials, 18 (2011) 715-721.

16. Zhang Y., Rodrigue D. and Aït-Kadi A., Journal of Applied Polymer Science, 90 (2003) 2130-2138.

17. Zhang Y., Rodrigue D. and Aït-Kadi A., Journal of Applied Polymer Science, 90 (2003) 2139-2149.

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