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Effect of Molding Parameters on Young’s Modulus of an Injection Molded Low-Density Polyethylene (LDPE) César Leyva-Porras,†,‡ Miguel A. Esneider-Alcalá,† Alberto Toxqui-Terán,† Alfredo Márquez-Lucero,§ and Josué A. Aguilar-Martínez*,† †

Centro de Investigación en Materiales Avanzados S.C. Unidad Monterrey, Alianza Norte 202 Parque de Investigación e Innovación Tecnológica (PIIT), Apodaca, N. L., México, C.P. 66600 ‡ Centro de Investigación en Materiales DIP-CUCEI, Universidad de Guadalajara, Av. Revolución # 1500, Col. Olímpica, Guadalajara, Jal., México, C.P. 44430 § Centro de Investigación en Materiales Avanzados S.C. (CIMAV), Av. Miguel de Cervantes # 120, Complejo Industrial Chihuahua, Chih., México, C.P. 31350 ABSTRACT: Injection molding is a process employed worldwide to manufacture polymer parts. The final properties of the molded part largely depend on the processing conditions used during the manufacturing process. Therefore, it is necessary to develop empirical approaches that help to understand the relationship between the processing conditions and the final properties of the polymer. In this paper we study the effect of the processing conditions of the injection molding process on the Young’s modulus of a low-density polyethylene (LDPE). The effect of both the barrel temperature and the mold temperature was investigated using analysis of variance (ANOVA) and the effect of the levels of each parameter was examined using the surface response methodology (SRM). The ANOVA results showed that the mold temperature is the parameter that most significantly impacts the Young’s modulus, followed by the barrel temperature, while the combined interaction of both is negligible. SRM showed that the Young’s modulus increases with the mold temperature and decreases with the barrel temperature. Based on the SRM, an empirical equation is proposed which can be used to predict the modulus employing only the barrel and mold temperatures. The changes in the microstructure of the injection molded part are discussed in terms of the crystallinity degree. All this was corroborated with X-ray diffraction (XRD) and differential scanning calorimetry (DSC). properties;13,14 where a high degree of crystallinity results in a harder, stiff, and less ductile behavior. In this way, it is also possible to achieve an improvement in the mechanical properties by simply changing the processing variables during the IM process.9,14−18 When these processing variables are changed, the microstructure of the polymer is affected,19 which in turn can be quantified by means of mechanical testing and moreover by determining the Young’s modulus. The relationships between the microstructure and the mechanical properties of injection molded polymers have been widely studied and the main results indicate an increment in the mechanical response as the degree of crystallinity increases.14,18,20−23 Therefore, it is of practical significance to understand the development of the microstructure during the IM of semicrystalline polymers, which is also important for optimizing processing variables such as injection rate, melt temperature, mold temperature, packing pressure, and holding time.11,14,24−27 It is necessary to develop engineering procedures and relationships that even with empirical and phenomenological approaches can contribute to solve that issue.17,20 The purpose of the present work is to optimize the processing parameters of an injected molded LDPE semicrystalline polymer and relate the effects of these conditions on

1. INTRODUCTION Low-density polyethylene (LDPE) is a semicrystalline thermoplastic widely used due to its properties such as thermal, chemical, and mechanical resistance. Usually it is manufactured in products including films, bottles, and other molded parts.1−4 Due to its good processability and flexibility it is molded by injection, extrusion, and blowing. Of these processes, injection molding (IM) is one of the industrially preferred methods for manufacturing polymeric products due to the high degree of automation, high production rates, and dimensional stability of the molded parts.5−7 During the IM cycle the polymer undergoes both thermal and mechanical effects through the interaction with the geometry of the IM machine (barrel, nozzle, and mold cavities) and the molding conditions (temperature, pressure, and time).7 IM process of thermoplastics basically consists of four stages: filling, packing/holding, cooling, and ejection.8−10 From these stages, cooling is crucial since in this step the solidification of the polymer occurs. During solidification, the microstructure of the polymer is developed producing regions with different arrangement within the molded part.11 Likewise, semicrystalline polymers commonly present two phases well distributed: amorphous and crystalline regions. The crystalline region typically consists of crystal lamellae of folded chains, whereas in the amorphous region, polymer chains are arranged randomly.12 Because for a semicrystalline polymer the final properties greatly depend on the relative amount and distribution of these two phases, it is possible to modify the degree of crystallinity to tailor those © 2013 American Chemical Society

Received: Revised: Accepted: Published: 5666

November 26, 2012 February 23, 2013 March 19, 2013 March 19, 2013 dx.doi.org/10.1021/ie3032422 | Ind. Eng. Chem. Res. 2013, 52, 5666−5671

Industrial & Engineering Chemistry Research

Article

tested at a higher deformation rate (500 mm/min), the specimen was broken and this happened in the period of time required by the norm. Hence, we decided to employ the deformation rate of 500 mm/min for determining the Young’s modulus. This modulus was estimated for every IM condition, testing a set of 10 specimens in each case. The overall changes in the crystallinity from each sample were analyzed by X-ray diffraction (XRD), in an X’Pert diffractometer (Panalytical B.V., Almelo, The Netherlands) equipped with a Cu-kα radiation and θ−2θ geometry in the range of 15−40 2θ degrees, step size of 0.02°, and 10 s per step. We employed this technique to qualitatively identify the variation in the microstructure from a more representative area on the molded specimen. Typically, a sample piece of 2 × 2 cm was used for this purpose. For calculating the crystallinity percentage of the injection molded samples, differential scanning calorimetry (DSC) analyses were performed. Samples of 10 mg were extracted from the same region on the injected specimen and encapsulated in standard aluminum pans. Indium was used as a standard to calibrate the temperature scale and the enthalpy of melting. The thermograms were acquired at a heating rate of 10 °C/min over a range of 25−150 °C. The percentage of crystallinity was calculated using the following equation:

Young’s modulus. To accomplish this, we employed a full factorial 3k experimental design together with the response surface methodology (RSM). With this factorial design, the interpretation of the results is simplified by analyzing the influence of the process variables and their interactions.28 From these data, the RSM is employed for determining the contribution of each of the factors, helping to identify the optimum conditions for the process with the goal of maximizing the Young’s modulus. The quality of the data is estimated using an analysis of variance (ANOVA) procedure, calculating in this way the experimental error.29 The results show that both the mold temperature and the temperature of the barrel have an effect on the microstructure of the molded part of LDPE, with the former being the one that most affects the Young’s modulus.

2. EXPERIMENTAL SECTION 2.1. Materials. A semicrystalline thermoplastic resin based on low-density polyethylene (LDPE) manufactured by Petróleos Mexicanos (product number PX18450 G, PEMEX, México) was used for preparing the injection-molding specimens. This material was selected not only for being a semicrystalline thermoplastic, but also because it is widely used in the manufacturing of IM products due to its high melt flow index (45 g/10 min). The semicrystalline LDPE presents typically a Young’s modulus of 100−500 MPa, melting point between 130 and 140 °C, density of 0.9180 g/cm3, and enthalpy of fusion of 135 J/g. This polymer resin is in the form of pellets, which were dried overnight at 80 °C in order to remove any surface moisture. 2.2. Injection Molding. The preparation of the injection molded specimens was carried out in a 55-Ton Negri-Bossi injection molding machine model V55-200 (Milan, Italy). The mold geometry for these specimens fulfills the requirements according to the norm ASTM D 638. Typical dimensions of the necked specimens are 25.4 × 2.54 × 0.254 cm (L × W × T). The barrel temperature of the injection molding machine has four heating zones, identified as Z1, Z2, Z3, and Z4. Zone 1 is the coldest region of the barrel and from here, the temperature difference between the zones increases 5 °C. Thus, zone 4 is the one with the highest temperature, corresponding to the injection nozzle and from now on we refer to it as the melt temperature. Melt temperatures were set at three different levels: 150, 170, and 190 °C. From these levels, the temperature inside the barrel can be inferred. As an example, when the melt temperature was set at 190 °C, the temperature profile for the remaining zones was fixed as 175, 180, and 185 °C for Z1, Z2, and Z3, respectively. For each IM condition, mold temperature was set at three levels: 10, 40, and 80 °C. Consequently, the experimental design consists of the combination of the three levels of the IM process and the three levels of temperature in the mold. This makes a total of nine possible conditions. 2.3. Testing Methods and Apparatuses. For the evaluation of Young’s modulus, the tensile tests at room temperature were performed using an Instron Universal Machine (Illinois Tool Works Inc.) with a load cell of 500 kg and a deformation rate of 500 mm/min. According to ASTM D638, the deformation rate of the Universal machine must be set in such a way that the specimen fully breaks in a period of time between 0.5 and 5 min. Due to the flexible nature of the LDPE, we found that at low deformation rate (2 mm/min) the specimen was deformed but did not break. Likewise, when

χ = (ΔHexp/ΔH °) × 100

(1)

where ΔHexp is the heat of fusion of the samples obtained from the DSC results after integrating the area under the curve (endothermic peak), and ΔH° is the heat of fusion with 100% crystallinity LDPE, where 290 J/g was used in this study.30 2.4. Performing of ANOVA and Surface Response Analysis. A full factorial experimental design of the type 32 with 10 replicates was used. Factorial design allows determining the effect for a given factor at various levels on one or more response variables.31−33 In this sense, we have two factors, the melt temperature and mold temperature at three levels each. As mentioned before, melt temperature was varied as 150, 170, and 190 °C, while mold temperature was changed as 10, 40, and 80 °C. The contribution of each of the factors and their interactions on the variability of the Young’s modulus was examined using analysis of variance (ANOVA) and the effect of each of the levels was determined using surface response methodology (SRM). Response functions describing variations of dependent variables (melt temperature and mold temperature) with two independent variables (Xi and Xj) can be written as follows: Y = b0 + biXi + bjXj + biiXi2 + bjjXj2 + bijXiXj

(2)

where Y is the predicted response (Young’s modulus), Xi and Xj are the input variables affecting the response. Xi2 and Xj2 are the square effects, XiXj is the interaction effect, b0 is the offset term, bi and bj are the linear effects, bii and bjj are the squared effects, and bij is the interaction effect. The response function coefficients were determined by regression using the experimental data and the Statistica 7.0 software.

3. RESULTS AND DISCUSSION 3.1. ANOVA and Surface Response Analysis. After determining the Young’s modulus by the mechanical tests, the average values of the modulus at different IM conditions were used for calculating the ANOVA; this was performed to quantitatively estimate the relative contribution of each of the 5667

dx.doi.org/10.1021/ie3032422 | Ind. Eng. Chem. Res. 2013, 52, 5666−5671


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of response is the one related with the color fringes. The color of these fringes corresponds to a certain value in the Young’s modulus: a low modulus value is represented by the green color, while the brown color represents the highest value. Similarly, these fringes are not linear but curved. The curved shape allows us to understand that it is conceivable to obtain a similar value of the modulus under different IM conditions. For example, a modulus in the range of 542.82 MPa can be obtained at a melt temperature of 150 °C and a mold temperature of 10 °C or by increasing these temperatures to 170 and 40 °C, respectively. Therefore, it is possible to optimize the process for energy saving purposes while maximizing the Young’s modulus. From the observation explained above, it has been demonstrated that the Young’s modulus of an injected molding LDPE can be modified by simply manipulating the processing parameters: the mold and melt (barrel) temperatures. Another important result extrapolated from the SRM is an equation for predicting the behavior of the modulus at the different processing conditions. The equation calculated for predicting the Young’s modulus, valid only within the range of the measured values and conditions tested in this work, is as follows:

control factors on the measured response. Table 1 shows the results from analysis of variance. Kalay et al.14 provided a Table 1. ANOVA Results for Young’s Modulus parameter

sum squares

df

mean square

F0

(A) melt temperature (B) mold temperature A*B error total

63743.7 710662.1 926.6 7540.6 782873

2 2 1 81 89

31871.9 355331.1 926.6 93.1

342.36 3816.90 9.95

detailed description on how to calculate each of these parameters together with their interpretation. Although the reported error value seems to be large, these values were calculated based on a significance of 1%; hence these results are very reliable. From the values of F0, the mold temperature labeled as B in the table, presents the largest value. On the other hand, the contribution of A, the melt temperature, is an order of magnitude less than the value of B. The contribution of the combined interaction of both parameters A and B is almost negligible due its very low value when compared to the individual values of A and B. These observations suggest that the mold temperature is the one with the more significant effect on the modulus, while the melt temperature has a much smaller effect and the combined effect of both parameters is insignificant. Likewise, a surface response plot was constructed for determining the individual effect of the levels from each factor on the Young’s modulus. Figure 1 shows the contribution of

Z = 1615.52 − 11.90X + 0.03X2 − 0.25Y + .04Y 2

(3)

where X is the melt temperature, Y is the mold temperature, and Z is the measured response (Young’s modulus). As observed from the equation, the variable that most affects the modulus is the melt temperature and this effect is negative. Thus, by increasing the melt temperature the modulus will decrease. Similarly, the mold temperature also has a negative impact on the modulus and this effect is 2 orders of magnitude less than that from the melt temperature. However, both the sign and the coefficient corresponding to Y2 have a positive influence on the modulus, which predominates over the impact of the linear Y coefficient. Hence, in order to obtain a large Young’s modulus from this equation, it is recommendable to set the processing conditions on the injection machine at a low melt temperature and at a high mold temperature. 3.2. XRD. As mentioned before, XRD measurements were performed with the aim of qualitatively examining the overall microstructure of the molded specimens. Figure 2 shows typical diffractograms of the LDPE.3,30 The series are plotted as a function of the various combinations of mold and melt temperatures. It can be observed that there is a variation in the intensity for each sample, especially in the main diffraction

Figure 1. Surface response plot for the interaction between melt temperature and mold temperature.

the melt temperature, the mold temperature, and their combined interaction on the modulus. The effect of the melt temperature is low, showing a variation of only 63 MPa when the temperature varied from 150 to 190 °C. On the other hand, the effect of the mold temperature is much more pronounced, since the modulus increases 189 MPa when the temperatures was changed from 10 to 80 °C. The planar shape of the graph indicates that the combined effect from the interaction of the two factors, the melt and mold temperatures, is negligible. These observations confirm those results obtained from the analysis of variance. Another important feature from the surface

Figure 2. Diffractograms of LDPE for different conditions; the labels for each diffractogram correspond to the melt temperature and mold temperature, respectively. 5668



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dependent on viscosity, which depends on temperature. For the polymer to flow into the mold cavities it must be first softened or melted into the injection barrel at a given melt temperature. Then, by increasing the melt temperature, the polymer is more easily deformed by effect of the shear rate, producing a large number of nucleation sites. This is observed as the left shift in the melting temperature. The second important feature in the thermograms is the enthalpy of fusion (ΔHm) which was extracted from the area under the curve of the endothermic peak and further employed to calculate the degree of crystallinity (χ); the details of these results are shown in Table 2. From these data it is evident that

peak (about 2θ = 20°). For a given melt temperature there is a decrease in the intensity of the peak as the mold temperature is decreased, which suggests that the crystallinity decreases as the mold temperature decreases. On the other hand, nonsignificant changes in the peak intensities were observed when increasing the melt temperature for a given mold temperature. Therefore, the melt temperature is not a significant factor affecting the crystallinity of the material. This is consistent with the results of the ANOVA and the SRM. 3.3. DSC and Degree of Crystallinity. After observing the overall changes in the microstructure, we decided to quantify the variations in the degree of crystallinity at the different processing conditions. Figure 3 shows the DSC melting curves

Table 2. Data Obtained from the DSC Experiments melt temperature (°C)

mold temperature (°C)

Tm (°C)

ΔH (J/g)

χ (%)

150 170 190 150 170 190 150 170 190

10 10 10 40 40 40 80 80 80

134.77 134.85 132.14 132.78 132.36 132.25 133.21 133.33 132.51

194.33 192.70 165.10 195.70 197.90 170.70 223.50 204.60 205.80

67.01 66.44 56.93 67.48 68.24 58.86 77.06 70.55 70.96

the degree of crystallinity increases as the mold temperature increases, while it decreases as the melt temperature increases. In terms of microstructure the previous is a normal behavior, since the crystallization inside the injection mold is conducted mainly by dynamics conditions of temperature, where both mold and melt temperature strongly influence the crystallization process, modifying the degree of crystallinity in the final injected part. Kunze et al.40 observed that by increasing the mold temperature, the enthalpy of fusion increases and this behavior is reasonable since the crystals can form better. As known, during injection molding, semicrystalline polymers solidify with different structures along the thickness of the molded part.11,23,27,41 Near the center of the piece where the shear stresses are relatively low, the polymer crystallizes with a spherical morphology called spherulite. These spherulites consist of two parts, an amorphous nucleus and lamellar oriented polymer chains radiating from the core.42 On the other hand, near the wall of the mold where shear stresses are high, the polymer crystallizes in a highly oriented structure called skin layer. Guo et al.43 found that for isotactic polypropylene, the skin layer thickness decreases with increasing the melt temperature, while the spherulite size directly increases with the mold temperature. Based on our results from DSC, we suggest that the size of the spherulites is not increasing but the amount is, i.e. there are more and smaller spherulites. This is observed as the displacement in the melting temperature and as the increment in the degree of crystallinity, both effects related with the mold temperature. These results are in agreement with those found by Torres et al.22 who explained that the increasing in the melting temperature is related with the distribution of the crystalline entities. Additionally, Na et al.34 concluded that for injection molded blends of high-density polyethylene and isotactic polypropylene without orientation, the mechanical properties are positively affected by the phase morphology, crystallinity, and interfacial adhesion among the two immiscible polymers. In this work, the

Figure 3. DSC thermograms for LDPE processed at different molding parameters; the labels for each thermogram correspond to the melt temperature and mold temperature respectively.

of LDPE for the different processing conditions. The most obvious feature observed in this figure is the melting point of LDPE, which is slightly shifted toward the left. This variation in the melting temperature is only about 2 °C and may be caused by a retarded crystallization;34 this retarded crystallization refers to the generation of more nucleating sites for the solidification of the crystalline entities.10,35,36 During the crystallization process of semicrystalline polymers, different types of microstructures are developed which greatly depend on the cooling rate.37 In turn, the cooling rate depends on the temperature difference between the molten polymer, the mold, and the temperature at which the molded specimen is ejected from the mold.25,38 Therefore, as the mold temperature is increased while leaving constant the other temperatures, the cooling time is larger and at the microstructure level the polymer chains are arranged in a more ordered fashion. Fellini et al.27 found that in the case of PBT copolymers, the melting temperature displacement is related with the perfection of the primary crystals formed during the solidification at a constant temperature. Hence, we can relate the changes in the melting temperature with the quality of the crystals, where smaller crystals melt at a lower temperature than those of greater size. Recently, Mykhaylyk et al.39 found that for LDPE, which usually exhibits a wide and continuous molecular weight distribution, the crystallization can be induced by effect of the shear rate. The shorter polymer chains are more readily deformed by the effect of shear and act as nucleation sites. As a consequence, by increasing the time the polymer is kept under stress, the number crystalline cores increases. This effect is 5669



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increase in the Young’s modulus is caused by the increase in crystallinity of the LDPE. Obviously, the crystallinity is being affected mainly by the temperature in both the melt and mold. From these temperatures, the one with the largest impact in the microstructure is the mold temperature. Thus, at the microstructure level the amount of nucleating sites is increasing as the mold temperature is increased, which macroscopically is observed as an increment in the modulus. Among other processing parameters which can affect the microstructure of injection-molded semicrystalline polymers are the injection speed and pressure,43 holding pressure,14,26 packing pressure,19,44 and mold surface energy.10,37 All of these parameters modify in different extent the thickness of the several layers in the microstructure of the polymer and hence the final properties of the molded specimen.

4. CONCLUSIONS The effects of both melt and mold temperatures on Young’s modulus of an injection molded low-density polyethylene were investigated. Results from ANOVA are in agreement with those obtained by surface response analysis. ANOVA results showed that the mold temperature is the parameter that most significantly impacts the Young’s modulus, followed by the melt temperature of the polymer. Surface response analysis showed that when the mold temperature increases the modulus increases, while the melt temperature presented the opposite effect. Therefore, the largest Young’s modulus was obtained when setting the injection molding machine at high mold temperatures and low melt temperatures. The variations in the processing conditions induced changes in the microstructure of the LDPE and these changes were explained in terms of crystallinity. Finally, considering only the parameters tested in this work, an empirical relationship for predicting the Young’s modulus starting from the processing temperatures was proposed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or jaguilar_mtz@hotmail. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Arturo Hernández and Daniel Lardizábal for the technical support provided. REFERENCES

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