Statistical experimental design and modeling of polypropylene–wood fiber composites

Polymer Testing 19 (2000) 419–428

Experiment Design

Statistical experimental design and modeling of polypropylene–wood fiber composites Thais H.S. Costaa, Daisy L. Carvalhoa, Denise C.S. Souzaa, Fernanda M.B. Coutinhoa,b,*, Jose´ Carlos Pintoc, Bohuslav V. Koktad a

Instituto de Macromole´culas Professora Eloisa Mano, UFRJ, P.O. Box 68525, 21945-970, Rio de Janeiro, RJ, Brazil b Departamento de Processos Industriais, IQ/UERJ, Rio de Janeiro, RJ, Brazil c Programa de Engenharia Quı´mica/COPPE, UFRJ, Rio de Janeiro, RJ, Brazil d Centre de Recherche en Paˆtes et Papier, Universite´ du Que´bec a` Trois Rivie`res, Que´bec, Canada Received 23 December 1998; accepted 12 February 1999

Abstract The influence of the amount of maleated polypropylene coating of vinyl-tris (2-methoxy ethoxy) silane treated wood-fibers (2 or 10 wt%), type of matrix (polypropylene or maleated polypropylene) and composition (10, 20 or 30 wt%) on the tensile and flexural performance of polypropylene–wood fiber composites was studied. The effect of these variables on tensile strength, Young’s modulus, elongation at yield and flexural strength was determined through a 22·31 factorial design. The analysis of variance of the experimental and predicted data shows that the constructed models provide a fair approximation of actual experimental measurements. Finally, experimental details regarding the preparation of optimum composites as predicted by empirical models are discussed.  2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Polypropylene–wood fiber composites may be used as substitutes for more expensive and/or less environmentally friendly materials [1]. Polypropylene is a recyclable polymer and wood fibers derive from a renewable source and are biodegradable. The use of wood fibers in a polypropylene * Corresponding author. E-mail: [email protected] 0142-9418/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 1 4 - 8

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matrix includes many benefits, such as improved dimensional stability of composites, lower processing temperatures, increased heat deflection temperature, improved “wood” surface appearance, lighter products, low volumetric cost, up to 30% reduced cycle time for injection molded products and production of good performance materials [2]. However, when the inherent polar and hydrophilic wood fiber is compounded with a non-polar thermoplastic such as polypropylene, if no compatibilization of the two materials is developed, the weak fiber/matrix interfacial adhesion gives rise to composites which present poor mechanical properties [3–7]. In this work the surface of wood fibers was modified through the use of a silane coupling agent and coating with maleated polypropylene. Since processing conditions also affect the properties of the composite, optimum processing conditions were used to reduce the number of variables in the statistical experimental design [8,9]. Besides the analysis of variables responsible for the change in the properties measured, the experimental design allows the determination of the experimental error and the building of empirical models which correlate independent variables with the dependent measured properties. Therefore, the effect of maleated polypropylene coating (2 or 10 wt%), type of matrix (polypropylene or maleated polypropylene) and composition (10, 20 or 30 fiber wt%) was analyzed in tensile and flexural tests of polypropylene–wood fiber composites [10–12].

2. Experimental 2.1. Materials Polypropylene (PP Espheripol H206, melt flow index = 10 g/10 min) and maleated polypropylene (MAPP, molecular weight (Mw) = 106 000) were supplied by OPP Petroquı´mica S/A. Wood fibers were received from Centre de Recherche en Paˆtes et Papiers (Universite´ du Que´bec a` Trois Rivie`res) in the form of chemithermomechanical (CTMP) aspen air dried pulp (impregnation solution used was 8% of Na2SO3 and it was steam-cooked at 128°C for 10 min). Dicumylperoxide (DCUP) was supplied by Atochem–Pennwalt S/A and the silane coupling agent used, A172 (vinyl-tris (2-methoxy ethoxy) silane), was supplied by Union Carbide and donated by Osi Specialties Ltd. The solvents employed were o-dichlorobenzene, hexane and methanol of research grade and were donated respectively by Hoechst do Brasil, CENPES/Petrobra´s and Prosint S/A. 2.2. Wood fiber treatment 2.2.1. Silane coupling agent CTMP aspen pulp oven dried (60°C) for 24 h was refluxed for 3 h in a 4% w/w A172 methanol solution containing 2% w/w dicumylperoxide in a glass reactor. At the end of the reaction, methanol was eliminated by vacuum and the pulp was oven dried (60°C) for 24 h. The treated pulp was ground and screened to 60 mesh size.

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2.2.2. Polypropylene coating 10 (2% w/v coating) or 50 g (10% w/v coating) of MAPP was mixed with 500 ml of odichlorobenzene in a 1 liter glass reactor and, after 24 h of swelling, the mixture was heated to 180°C until complete dissolution. When the temperature of the viscous solution reached 130°C, the A172 treated wood fibers were added. The mixture was then cooled to room temperature, thoroughly washed with toluene and hexane and finally dried at 60°C for 24 h. The resulting solid mixture was then ground. 2.3. Composites preparation The mixtures of polypropylene (or maleated polypropylene) and coated A172 treated wood fibers were compounded in a Haake Rheomix 600 equipped with roller blades rotor at 180°C for 10 min at 60 rpm. After the addition of the matrix, the filler was added as soon as the registered torque indicated that the polymer melt had reached a steady state (2 min). Tensile and flexural specimens were obtained by compression molding in a Carver press at 188°C under a pressure of 4 MPa followed by cooling in another press equipped with refrigeration facilities. Rectangular specimens were cut from the pressed sheets to size (100 × 8 × 0.9 mm). They were measured with the aid of a micrometer for tensile (ASTM D882 83) and flexural tests (ASTM D790M-84 three-point bending, adapted to the available support span). The mechanical property measurements were carried out in an Instron Tester (model 4204) with a load cell of 1 kN, crosshead speed of 5 mm/min in tensile tests and 6.5 mm/min in flexural tests. A minimum of four samples were tested.

3. Results and discussion Twelve good performance [8] polypropylene–wood fiber composites were prepared according to a 22·31 factorial design where the variables chosen were: fiber content (F), percentage of MAPP coating on the wood fiber (C) and type of matrix (M). Table 1 presents these variables with their normalized levels while Table 2 shows the corresponding composites of the experimental matrix. The mechanical properties (responses) selected from tensile data were stress at maximum load (TS), tensile modulus (TM) and elongation at yield (E). The selected responses of flexural data were stress at maximum load (FS) and flexural modulus (FM). Using standard least-squares parameter estimation procedures and statistical analysis, empirical models were built and parameter values and covariances were computed. The models may be generically described by Eq. (1):

Table 1 Experimental variables Variable

F

Level Normalized level

10 ⫺1

C 20 0

30 +1

2 ⫺1

M 10 +1

PP ⫺1

PPMA +1

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Table 2 Experimental matrix Composite 1 2 3 4 5 6 7 8 9 10 11 12

F

C

M

⫺1 ⫺1 ⫺1 ⫺1 0 0 0 0 1 1 1 1

⫺1 ⫺1 1 1 ⫺1 ⫺1 1 1 ⫺1 ⫺1 1 1

⫺1 1 ⫺1 1 ⫺1 1 ⫺1 1 ⫺1 1 ⫺1 ⫺1

P = (A1 ± 2sA1) + (A2 ± 2sA2)F + (A3 ± 2sA3)R + (A4 ±

(1)

2sA4)M + (A5 ± 2sA5)F*R + (A6 ± 2sA6)F*M + (A7 ± 2sA7)R*M where P = measured property, A = parameter that correlates a property with a variable or interaction between two variables and sAn = standard deviation of parameter An. Tables 3–7 present tensile and flexural data of the twelve composites, the experimental error and the predicted values given by the model. All parameters were calculated based on a 95% confidence level. Empirical models were developed sequentially starting with the main sources of variations, including the interaction when necessary and only retaining the significant effects. Table 3 Tensile stress at maximum load Composite Experimental value (MPa)

1 2 3 4 5 6 7 8 9 10 11 12

35.4 41.0 36.8 41.7 35.9 44.3 38.2 42.4 40.6 48.6 40.0 47.2

34.8 41.2 39.0 39.9 37.5 46.7 42.3 44.0 35.9 45.0 41.3 44.8

34.0 40.9 37.7 41.3 35.2 44.8 38.9 39.3 39.6 48.7 — 45.0

Average (MPa) 33.7 43.0 37.0 39.4 34.2 46.2 40.6 40.8 38.6 46.0 — 45.5

34.7 40.6 — 41.0 33.5 44.0 40.6 — 39.3 48.7 — 43.8

— — — — — — — — 36.6 48.4 — 46.1

35.0 40.8 36.9 41.3 34.7 44.2 39.4 41.6 38.6 48.5 40.6 46.6

SD (MPa) Predicted value (MPa) 0.7 0.9 1.0 1.0 1.6 1.2 1.6 2.0 1.8 1.7 0.9 1.2

34.0 41.7 38.0 40.2 36.1 44.7 39.6 42.6 38.2 47.7 41.1 45.1

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Table 4 Tensile modulus Composite Experimental value (MPa)

1 2 3 4 5 6 7 8 9 10 11 12

1042 1288 1034 966 1291 1472 1194 1086 1451 1374 1432 1548

1284 927 1110 1100 1256 1371 1282 1138 1135 1656 1542 1654

1323 1100 1030 1177 1182 1566 1240 1171 1538 1349 — 1228

1215 1196 1192 1041 1169 1514 1208 1086 1278 1295 — 1462

1127 1202 — 1015 1240 1412 1301 — 1345 1624 — 1446

— — — — — — — — 1492 1507 — 1373

Average (MPa)

SD (MPa) Predicted value (MPa)

1085 1245 1113 990 1266 1442 1248 1086 1472 1441 1487 1461

115 138 76 81 51 78 46 42 151 151 78 146

Average (MPa)

SD (MPa) Predicted value (MPa)

1165 1211 1096 1007 1267 1357 1264 1219 1368 1502 6050 1430

Table 5 Strain at yield Composite Experimental value (MPa)

1 2 3 4 5 6 7 8 9 10 11 12

8.3 12.7 12.3 14.3 5.9 7.9 8.2 10.1 6.4 10.8 9.1 10.5

8.4 13.4 12.1 12.5 6.3 9.0 9.6 12.4 5.3 7.1 9.2 10.8

8.6 12.8 11.1 14.0 5.8 8.8 8.2 8.0 5.8 11.3 — 10.3

8.7 13.8 — 12.8 6.2 8.8 8.7 10.1 5.2 8.2 — 10.7

8.3 11.8 — 13.3 5.7 8.3 8.9 — 5.7 8.5 — 10.8

— — — — — — — — 4.9 8.4 — 10.7

8.3 12.3 11.7 13.8 5.8 8.1 8.5 10.1 5.7 9.6 9.1 10.6

0.2 0.7 0.6 0.8 0.3 0.4 0.6 1.8 0.5 1.6 0.0 0.2

8.3 11.9 10.9 13.0 6.7 10.2 9.6 11.5 5.1 8.5 8.3 10.1

Experimental data and predicted data were compared through the F test [12] and by analysis of both values through an error bar as shown in Figs. 1–5. According to the results obtained, deviations between model predictions and experimental values and the experimental errors are statistically equivalent, so that the empirical models presented in this paper provide an adequate description of the main sources of variation of the measured mechanical properties. Table 8 presents the tensile and flexural models and the variance analysis.

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Table 6 Flexural stress at maximum load Composite

1 2 3 4 5 6 7 8 9 10 11 12

Experimental value (MPa)

45.1 45.7 44.4 36.2 57.3 57.2 43.3 44.8 60.9 57.9 46.9 46.8

42.0 42.8 49.4 37.4 51.3 58.2 45.6 46.6 57.3 59.8 55.2 49.9

41.2 43.9 46.9 36.9 45.4 53.9 46.9 49.8 64.5 59.4 52.3 52.5

— 47.3 41.0 41.7 49.7 55.2 47.9 43.3 60.9 56.7 50.8 46.7

— 48.7 — — 49.5 55.2 46.7 43.3 58.0 57.0 45.6 50.3

Average (MPa)

SD (MPa)

Predicted value (MPa)

43.2 47.2 42.7 38.9 50.6 56.2 45.0 44.0 59.5 57.4 46.3 48.5

2.8 2.1 2.4 3.9 5.5 1.4 2.5 1.1 2.0 0.6 0.9 2.4

Average (MPa)

SD (MPa)

Predicted value (MPa)

5184 4333 4319 4231 5759 5156 4791 3962 6356 5158 4916 5269

2412 563 732 426 39 109 94 366 115 227 418 25

4978 3916 4505 4003 5582 4586 4972 4535 6187 5256 5438 5068

44.2 45.8 43.2 40.3 51.4 53.3 47.1 44.4 58.7 60.8 50.9 48.5

Table 7 Flexural modulus Composite 1 2 3 4 5 6 7 8 9 10 11 12

Experimental value (MPa) 6889 3935 4836 4532 5761 5233 4724 4220 6274 5318 4620 5287

4683 3955 4664 5113 6767 5811 4303 4296 5121 4721 6398 5329

3478 4609 4478 4240 6232 5221 5420 4671 7226 4942 6119 5151

— 4564 3801 3930 4328 5413 4926 3294 4879 4947 5715 5012

— 4731 — — 5706 5079 4857 3703 6437 4997 5211 5251

3.1. Tensile stress at maximum load The results obtained and shown in Table 8 indicate that all the variables analyzed affect the stress at maximum load. The effect of the fiber content is similar in both polymer matrices and does not depend on the coating. This is probably due to the good compatibility between the fiber and the matrix and because the fiber has a good reinforcing influence on the composites. The effect of the polymer matrix, however, depends on the amount of maleated coating, decreasing from 4.3 to 1.5 as the amount of coating changes from 2% to 10%. As the MAPP coating increases, the true amount of fiber in the composite decreases, reducing the reinforcement level of the fiber. This means that as the amount of coating increases, differences observed between

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Fig. 1. Tensile stress at maximum load of composites.

Fig. 2. Tensile modulus of composites.

the different polymer matrices decrease. This is reasonable since there is an improvement of fiber/matrix adhesion as the amount of coating in PP composites increases, causing the stress to get closer to that of MAPP composites. According to the model, differences will be negligible when the percentage of coating reaches 14.3%. Besides, the maximum values of stress will be obtained with maximum amount of fibers, minimum amount of coating and matrices of PPMA. This result is confirmed by experiment 10 of Table 3. 3.2. Tensile modulus The second model presented in Table 8 reveals that the amount of fiber has a strong positive effect on the tensile modulus. When there is a good dispersion of wood fibers in the polypropylene matrix the tensile is expected to increase with the addition of fiber. The effect of amount of maleated coating depends on the used matrix being almost 70 to PP matrix and practically nil if

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Fig. 3. Tensile strain at yield of composites.

Fig. 4.

Flexural stress at maximum load composites.

Fig. 5. Flexural modulus of composites.

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Table 8 Models relating the responses to the experimental variables Model

Model variance

TS = (40.8 ± 0.4) + (2.3 ± 0.4)F + (2.9 ± 0.4)M ⫺ (1.4 ± 0.4)C*M 2.03 TM = (1276 ± 32) + (156 ± 40)F ⫺ (35 ± 32)C ⫺ (34 ± 32)C*M 14 316 E = (9.5 ± 0.4) ⫺ (1.5 ± 0.4)F + (1.1 ± 0.4)C + (1.4 ± 0.4)M ⫺ 1.49 (0.4 ± 0.4)C*M FS = (49.0 ± 0.8) + (5.7 ± 1.0)F ⫺ (3.3 ± 0.8)C ⫺ (1.7 ± 1.0)F*C 10 ⫺ (1.1 ± 0.8)C*M FM = (5002 ± 186) + (444 ± 232)F ⫺ (249 ± 186)C ⫺ (281 ± 475 630 186)M a

Experimental F0a variance

Fcb

1.91 12 214 0.73

1.06 1.07 2.05

1.63 1.63 1.63

7

1.48

1.65

1.26

1.65

378 100

Calculated by the ratio between model and experimental variances. From F distribution table with 95% of significance level [12].

b

MAPP is used. However, when compared with the effect of the amount of fiber, the use of different matrices or coating have negligible effects. 3.3. Strain at yield The model constructed for the strain at yield property shows that as the amount of fiber increases the response decreases. The addition of fiber always causes a decrease in strain at yield, however, when there is an adhesion in the fiber/matrix interface strain at yield also increases. On the other hand, the strain increases as amount of coating increases, probably due to the better adhesion of fiber to the matrix and the reduction of the amount of actual wood reinforcement in the composite. The amount of coating also reduces the effect of the matrix on the response; MAPP provides composites with higher strain at yield but this is diminished with a large amount of coating and fiber. Better results are obtained in composites with MAPP matrix, minimum amount of fiber and maximum amount of coating, as shown by result 4 of Table 5. 3.4. Flexural stress at maximum load The model that describes the flexural stress at maximum load data (Table 8) indicates that the fiber amount has a positive effect on the response, but it depends on the amount of maleated coating. The effect varies from 7.4 to 4.0 if the coating amount changes from 10% to 2%. The presence of fiber causes a resistance to flexure in the composite. Again, the presence of coating may influence the weighting procedure, reducing the amount of fiber on the composite. The coating also has a primary negative effect on the flexural strain that depends on the kind of matrix: in PP matrix the effect is smaller, showing that the maleated coating is necessary to improve the adhesion fiber/matrix. Thus, better results of flexural stress may be obtained in composites with maximum amount of fiber and coating and PP matrix (see result 10, Table 6).

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3.5. Flexural modulus The empirical model built to flexural modulus is completely linear. It means that the effect of variables interaction on the response may be neglected. The increase of amount of fiber causes an increase on the modulus; this is reasonable as the fiber plays a reinforcing role demanding a higher stress to suffer some deformation. According to the model, the amount of maleated coating and the matrix have a strong negative effect, decreasing the modulus as their levels are elevated. The effect of amount of coating can be understood as its capacity to reduce the actual amount of fiber added to the composite, as observed in the tensile modulus. The effect of matrix is not so clear, especially if we compare with its effect on tensile modulus. 4. Conclusion The MAPP composites presented the best performance. Tensile and flexural models showed that the presence of fiber in the composites improves the measured properties. The effect of amount of maleated coating depends on the used matrix: it is practically nil for MAPP composites and improves the performance of PP composites. PP composites may have their properties improved by the addition of wood fibers if a good fiber/matrix adhesion is attained. An extrapolation of the models comparing the two matrices can provide the amounts of fiber and MAPP coating to produce a PP composite with a performance comparable to MAPP composites. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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