The impact of molded pulp product process on the mechanical 1 properties of molded BCTMP 2

9 This study was carried out using bleached softwood Chemi-Thermo-Mechanical Pulp to evaluate the 10 influence of Molded Pulp Products’ manufacturing process parameters on the finished products’ 11 mechanical and hygroscopic properties. A Taguchi table was done to make 8 tests with specific process 12 parameters such as moulds temperature, pulping time, drying time and pressing time. The results of 13 these tests were used to obtain an optimized manufacturing process with improved mechanical 14 properties and a lower water uptake after sorption analysis and water immersion. The optimized process 15 parameters allowed us to improve the Young’ Modulus after 1h immersion of 58% and a water uptake 16 reduction of 78% with the first 8 tests done. 17


Introduction 19
As single use plastic packaging is becoming more controversial in the current environmental context, 20 industrials have been studying other materials and manufacturing processes to propose new products 21 with a lower environmental impact. Some industrials have chosen to turn to wood cellulose fiber 22 historically used for the pulp and paper industry. 23 There are several processing methods known to extract cellulose fibers from wood chips. These 24 processes either use mechanical technique, chemical technique, thermal technique or a combination of 25 these techniques. A widely known and used technique in the paper industry is called kraft process [1,2] 26 and uses a combination of thermal, mechanical and mostly chemical products to obtain cellulose fibers 27 free of hemicellulose, lignin, pectin and other wood molecules. 28 Another fiber extraction method used is called Chemi-Thermo-Mechanical Pulping process or 29 for several applications as it is a less-expensive pulp to produce with a much higher yield of pulp 32 produced. 33 In this method, the wood chips are first pretreated with about 2% of sodium sulfite or hydrogen 34 peroxide, then they are sent to specific refiners with high pressure and steam to free the fibers. The high 35 temperature and presence of steam allow the fibers to soften and reduce their cutting and fines 36 formation. This pulp may be further bleached (BCTMP) to obtain a whiter pulp even if the result cannot 37 be as white as for kraft pulps since lignin is maintained in the CTMP process [2]. 38 Processed pulp (kraft or BCTMP) or recycled paper are used as raw material to make 3D products 39 using a specific manufacturing process called Molded Pulp Products process (MPP process). The first 40 patent of a molded pulp processing was published in 1890 [4], whereas a patent for a manufacturing 41 machine was published in 1903 by Mr. Martin L. Keyes [5] . This process has been developed and 42 modified but its use in industry was first limited to the manufacturing of egg trays. The MPP process can 43 vary depending on the final destination of the manufactured product. According to the International 44 Molded Fiber Association (IMFA) [6], there are 4 categories of MPPs depending on the process used [7], 45 which are described as followed: 46  "Thick-wall": made using one mould resulting in a smooth side (in contact with mould) and a 47 rougher side. The piece is oven dried, has a thickness of 5 to 10 mm and is mainly used to transport 48 and protect heavy products (vehicle parts, motors, ...) 49  "Transfer Molded": made using two moulds, a forming one (same as the "thick-wall" process) and 50 a transferring one. The surface in contact with the forming mould is smoother than the other 51 surface. The resulting piece is also oven dried, 3 to 5 mm thick and is used to make egg trays or 52 other product packaging to protect objects such as cellphones, electrical appliances and drink trays 53  "Thermoformed" or "Thin-wall": made using a number of moulds to press and dry the piece. The 54 resulting product is 2 to 4 mm thick, denser than for previous processes and gives a smooth surface 55 to both sides. Making a "thin-wall" product allows to obtain a piece with a similar finish to 56 thermoformed plastic products and may be used to make drink trays, cellulose tableware such as 57 plates, bowls or food trays that require a good product visual 58 Page 3 / 17 on the finished piece. This process may be used for food applications that require a specific barrier 61 that cannot be done by the cellulose product alone 62 In this study, the "thin-wall" method was tested with 4 moulds used: a forming one, a transferring 63 one and two moulds to press and dry the piece. In order to better understand the influence of the MPP 64 process on the properties of the resulting product, 6 different process parameters were analyzed by 65 changing temperature or specific step time. The result of these experiments will allow us to obtain a 66 manufacturing process with specific parameters that gives higher mechanical properties and limited 67 water uptake following water immersion. Tensile tests in different conditions and water uptake analysis 68 were carried out to compare the influence of the process parameters tested on the structure and 69 properties of the manufactured MPPs. Water uptake tests were made following 1h water immersion and 70 sorption analysis 5 different water activities (aw). Tensile tests included initial analysis as well as after 71 1h water immersion and sorption analysis in the same conditions as water uptake tests. 72 As 6 process parameters were chosen to be tested at 2 levels each, a total of 64 tests should be done 73 to correctly analyze all process possibilities. To lower the number of tests, a design of experiment was 74 done following the Taguchi method, allowing us to lower the number of tests needed to 8. With the 75 experimental results of these 8 specific tests, the Taguchi Table allowed us to obtain theoretical results  76 of all the other tests possibilities. 2 final tests with optimized process parameters from the initial 8 were 77 done to confirm the Taguchi method used for this study.  Samples cutting in the shape of shouldered bars using a punch cutting object 85  Conditioning of the samples depending on the test to be done 86 The first step uses a molded pulp processing machine whose diagram is shown in Figure 1.

89
In this process 6 parameters, detailed in Table 1, were tested to observe and analyze their influence 90 on the water uptake and the mechanical properties of the finished product. These parameters are easily 91 changed in a production process depending on the desired production properties and objectives (cycle 92 time, finished product water content). 93 Taguchi methods were used to optimize and reduce the number of experiments without neglecting 96 the experimental possibilities. Table 2 shows the experimental card used and the parameters that were 97 tested in the manufacturing process as well as the levels chosen to analyze the manufacturing process. 98 In order to study 6 parameters with 2 levels and 1 interaction, the L8 Taguchi's matrix is used. The 99 pulping time was the most restraining parameter on the production time and was affected to the first 100 column of the table. Other parameters had roughly the same influence on the process time. No 101 preference was attributed and one interaction was analyzed. We have chosen the interaction between 102 Moulds C-D temperature and dehydration time. 103 In order to correctly compare the factors tested and their influence on the analyzed properties, their 104 effect was calculated following the Equation 1 for the level 1 of each factor and Equation 2 for level 2 of each factor. Yt is the average result of all the results for the analyzed factor. The index k is the studied 106 parameter. 1 and 2 correspond to the level of the parameter (in the studied design of experiment, level 1 107 corresponds to the lower value of the parameter and level 2 to the higher value). 108 Once we obtained the theoretical results with the help of Taguchi method, we made MPPs using 115 process parameters that theoretically gave higher mechanical properties and lower water uptake and 116 compared with the results obtained experimentally. For this comparison, tests were done with samples 117 immersed in distilled water for 1 hour at 23 °C (± 2°C). Samples were weighed once prior to their test 118 conditioning and then after being taken out of water and slightly wiped, then tensile testing was done. 119 This comparison allowed us to test the Taguchi table's efficiency and conclude on the theoretical results  120 given. 121

Characterization techniques of Molded Pulp products 122
Tensile tests were made using an MTS 10kN machine with a load cell of 250N, at a speed of 1 123 mm/min. At least 5 samples were done for each condition tested. A shouldered testing bar shaped punch 124 cutter was used to obtain the testing samples from the 3D product with the desired shape and size as 125 shown in Figure 2 and with dimensions given in NF EN ISO 527-2 by specimen 5A type [8]. With this Page 6 / 17 cutting method, the samples obtained had exactly the same shape, thus reducing the possible errors in 127 length or width measures. However, the thickness could be different and was measured on all samples 128 prior to their testing. 129 Initial tensile tests were carried out on unmodified samples. For the analysis of sorption behavior, 130 samples were kept in desiccators at a specific water activity (aw) and at 23°C (± 2°C), as shown in Table  131 3. Samples were weighed at their initial condition, then oven dried at 105°C for 48h, weighed again and 132 put in controlled humidity chambers at 23 °C (± 2°C) until equilibrium was reached. Once sorption 133 equilibrium was reached, samples were once again weighed and tensile tests were performed. 134 The samples' water uptake was calculated with the following formula: 138 Where Wf is the sample's weight after sorption test and W0 the dry sample's weight 140 It is interesting to analyze the effect of relative humidity (or aw) on their weight and mechanical 150 properties. τ is the weight gain after test and aw is the water activity in the desiccator. K, C and τm are 151 variables that are specific to each sample tested and may be obtained using solver parameter in excel. τm 152 is the water content adsorbed on the first layer, also known as the monolayer moisture content. C is a 153 constant related to the chemical potential's difference of the sorbate molecule (water for this study) in 154 the upper layer and the monolayer whereas K is a constant related to the multilayers' heat 155 properties [16]. These 2 constants vary with the temperature. C gives an indication on the isotherm type 156 given by IUPAC and if 0 > C > 2, the isotherm is type III whereas if C > 2 it is a type II isotherm [17]. 157 Young Modulus was calculated using the test results following the Equation 5. Smax is the maximum 159 slope of the force -elongation curve (with the force given in N, and the elongation given in mm/mm), l 160 is the initial length of the sample in mm, b is the sample width in mm and t is the sample thickness in 161 mm. 162 The resulting effects of each processing parameter tested for the sorption behavior is described in 166 Table 4, the effects improve the property when it is negative since the objective is to limit the water 167 uptake. We observe that for all aw tested, the effects of each factor are all less than 1 which means, they 168 have a small impact on the weight gain of the samples tested. However, when we observe which level of 169 each parameter has the impact limiting the weight gain, we note that they are different depending on 170 the process parameter observed and the aw tested.  The mould B temperature has a better impact at lower temperature for aw of 0.1 to 0.5, however, the 186 effect is better at higher temperature for aw of 0.75 and 0.98. For the moulds C and D temperature, we 187 observe that the effect is optimized at higher temperature for all aw expect 0.98 where a lower moulds' 188 temperature is more effective. As the temperature of the moulds C and D is applied at the same time as 189 pressure, the drying phenomena of the MPP is different from the mould B where only temperature is 190 applied. The MPP is in contact with mould B for about 10 seconds, the product is not dry when it is 191 transferred to the mould C. However, this short contact time with the mould B at 120°C allows the 192 finished product to have a limited water uptake at high aw. 193 Several studies [21,22] show that the Water retention value of cellulose pulp is lower when a higher 194 temperature is applied to dry the product. A higher process temperature limits the ability of cellulose 195 fibers to absorb water molecules. Chen et Al [21] also observed that a higher drying temperature 196 increases the amount of lactones produced which can be translated by a lower swelling ability of the and observed that a higher temperature increases the product's density. With a higher density, they 199 observed that the tensile strength is also improved. Marta  Hunt et al. [26] studied the influence of pressure on cellulose fibers properties and observed that a 217 drying in a hot press strongly limits the material's shrinkage as well as reduces the product's thickness 218 and water content. Joelsson et al. [27] observed that a higher processing temperature and a higher 219 pressure increase the material's density, thus resulting in a lower pores size. Reducing the pores size 220 limits the water uptake and fiber swelling while increasing the cellulose's degree of crystallinity. 221 The analyzed factors had different effects depending on the aw tested as the resulting effects (level 1 222 or level 2) varied when aw was modified. In order to better understand the influence of the factors on the 223 water uptake after sorption analysis, the GAB equation was used. When modifying the variables in the 224 GAB equation, we can observe that depending on the variable analyzed, the resulting slope will either 225 have a high or a low shift as shown in When C is varied between 5 and 15 the resulting slope is close to the control with a slight modification 230 at lower aw, showing that C is a factor translating the water uptake for aw between 0 and 0.5. The 231 variation of K between 0.8 and 0.9 shows bigger modifications in the slope as we observe a similar slope 232 to the control for lower aw. However, when aw is higher than 0.6, we can observe that the slope with 233 K=0.9 diverges from the control and results in a higher water uptake (39% at 1 compared to 25% for the 234 control slope). This variable translates the behavior of the water multilayer on the cellulose fibers 235 surface. Bedane et al [15] showed that the sorption slope is different when comparing the moisture 236 content on the monolayer and the multilayer. They observed that a lower monolayer content is preferred 237 to maintain the product's optimized properties. 238 With a higher K, the cellulose fiber is better able to adsorb water molecules thus increasing the 239 multilayer and water uptake. A lower K indicates that the product is less capable of adsorbing water 240 molecules thus limiting the water uptake at higher aw. τm is the product's monolayer uptake, above this 241 water content, the bilayer is created. 242 With a lower τm, we observe that the slope is lower for all aw, indicating that with a lower monolayer 243 limit the multilayer is also limited. This may be explained with higher inaccessible sites for water 244 molecules to be adsorbed on thus reducing the product's water uptake for both the monolayer and the 245 multilayers. 246 The variables τm, C and K were analyzed to understand the influence of the process parameters on 247 the global sorption behavior for all water activities as shown in Table 5. Water uptake (%) a w Control C = 5 C = 15 K = 0.8 K = 0.9 τm = 2 τm = 6 When comparing the results of the process effects (Table 5) and the slope modification (Figure 3), 250 we can observe that in order for C to have a higher effect result on the GAB equation modification 251 compared to K and τm, it needs to be a higher number. It means that a small modification of K and/or τm 252 has a much higher impact on the equation than C. Moreover, each constant has a different effect 253 depending on the parameter tested. In order to limit the water uptake, C, K and τm must have the lowest 254

result. 255
A lower drying and pressing time in moulds C and D and higher temperatures (moulds B, C and D), 256 pulping time and dehydration time lower τm result. When τm is reduced, the cellulose-water monolayer 257 is filled at a lower aw. This may be explained by a limited access to cellulose surfaces with the presence 258 of smaller pores or a lower cellulose surface fibrillation. As there is less availability for water molecules 259 to be fixed to cellulose, its' monolayer is more rapidly filled. With a lower τm the resulting MPPs are then 260 less hydrophilic, thus reducing the global water uptake for all aw. 261 When the pulping time, dehydration time, pressing time and mould B temperature are at level 1, and 262 Moulds C-D temperatures and drying time are at level 2, C is reduced. This may be explained by a higher 263 degree of crystallinity as several studies [21,28] showed that a higher crystallinity results in a lower water 264 uptake. The cellulose amorphous phase is more hydrophilic with more available sites when compared 265 to crystalline phase 266 In order for K to be lower, all process parameters were at level 1 except for the drying time of moulds 267 C and D. As K is a factor for multilayers and mostly impacts the equation at higher aw [15,16], this lower 268 result shows that for the given parameters the water vapor adsorption saturation may be reached at a 269 lower water content. One explanation can be the reduction of pores size with the specific process 270 parameters chosen.

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This morphology modification given by the analysis of the process parameters allows us to infer that 272 the mechanical properties of the MPPs may also be changed and optimized with the correct choice of 273 parameters. 274

Mechanical properties depending on the process parameters 275
The initial tensile tests were performed to compare the influence of the MPPs production parameters 276 without adding any other specific condition to the finished products. These results were compared in 277 Table 6 with the tensile tests done after water sorption with aw at 0.98 as it is the state in which MPPs 278 had the lower mechanical property over all the aw spectrum. 279 improve the mechanical properties with higher tensile strength as the average pore size is reduced and 294 the density improved. It was also observed that the crystallinity is improved with high pressure and high 295 temperature, which is translated by a lower water uptake and higher mechanical properties.
These mechanical and hygroscopic results show that with a specific manufacturing process, we may 297 obtain MPPs with a lower water uptake and higher mechanical properties. As only 8 tests were done on 298 the 64 possible, 2 more tests were performed to confirm the theoretical results given by the Taguchi  299   table used.  300 3.3. Process manufacturing optimization to improve MPPs properties 301 The Taguchi table allowed us to obtain theoretical results on the water uptake and mechanical  302 properties tested experimentally. With these results, 2 process parameters gave good results with a 303 limited water uptake and higher mechanical properties. We decided to make MPPs following the specific 304 process parameters given in 305 Table 7. 306 We then tested the samples and compared the theoretical results with the experimental results, 308 shown in Figure 4. We observe that for test n°9, the experimental results are better than the theory, with 309 a lower water uptake and a higher Young's Modulus in both initial state and after 1h water immersion. 310 For the test n°10, the initial Young's Modulus is higher than the theory, however the water uptake and 311 Young's Modulus after immersion are not better than the theory even if the results are close. 312 With experimental results close to the theory for tests 9 and 10, we infer that the Taguchi Table gives  313 conclusive results and allows us to rapidly and efficiently improve the MPPs processing with specific 314 process manufacturing parameters. With the results of these 2 tests, we were able to obtain improved 315 mechanical and hygroscopic properties on the MPPs made with bleached CTMP. When comparing the 316 results of the tests 9 and 10 with the first 8 tests, we observe an optimization in the water uptake and 317 initial mechanical properties. We obtain an average decrease of the water uptake of 15% for test 9 and 318 over 78% for test 10 whereas the overall Young's Modulus after 1h of water immersion is improved by 319 19% for test 9 and 58% for test 10. These specific process parameters used together efficiently improve 320 the hygroscopic and mechanical properties of the MPPs made.  This study allowed us to optimize the hygroscopic and mechanical properties of BCTMP without 326 using additives and by optimizing the manufacturing process. 327 We observed that pulping lowers the cellulose crystallinity thus increasing the swelling and WTR of 328 cellulose fibers. With higher moulds' temperature, we showed that Swelling and WTR decreased as 329 crystallinity increased and we also obtained lower pore size. When pressure is applied, the product's 330 density increased and pore size reduced. 331 As a result, these modifications in the samples morphology either improved or decreased the 332 mechanical properties. We were able to further understand the MPPs sorption behavior and improved 333 the water uptake with the help of Taguchi table and GAB  It was interesting to analyze the process parameters by making optimized tests. These tests showed 336 that with specific parameters, we were able to further improve the hygroscopic and mechanical 337 properties of the MPPs. With test n°10 the water uptake was on average 78% lower than all 8 tests made 338 with the Taguchi table as  The datasets used and/or analyzed during the current study are available from the corresponding 345 author on reasonable request 346