Elsevier

Polymer Testing

Volume 38, September 2014, Pages 100-107
Polymer Testing

Material behaviour
Effects of entrance convergence angle and fiber content on the rheological properties of Pithecellobium clypearia fiber/high-density polyethylene composites

https://doi.org/10.1016/j.polymertesting.2014.06.010Get rights and content

Abstract

The entrance converging flow method was used to investigate the influences of entrance convergence angle and fiber content on the rheological behavior of Pithecellobium clypearia fiber (PCF)/high-density polyethylene (HDPE) composites. Shear and extensional rheological parameters were also measured. It is found that the extensional action exerted on the materials reaches a maximum at a die angle of 120° at certain plunger rates. Wall slippage occurs easily for PCF/HDPE composites with higher fiber content with a die entry angle of 120°, causing a substantial decrease in the entrance pressure drop of the orifice die. Both shear and extensional non-Newtonian indices of HDPE and PCF/HDPE composites were less than 1, consistent with shear- and tension-thinning rheological behavior. Within the fiber content range used in this study, the shear viscosity increased slightly with fiber content, while the extensional viscosity decreased initially and then increased with increasing fiber content.

Introduction

In recent years, wood plastic composites (WPCs) have become widely used in many fields, such as landscape areas, the automotive industry and furniture. Many kinds of plant-based powders have been used in WPCs, including wood powder, bamboo powder, rice husk powder, rice straw powder, peanut shell powder and sawdust. The aspect ratios (ratio of fiber length to diameter) of these powder materials are typically from 1:1 to 4:1. However, the composites obtained by adding plant powders show weak mechanical strength, so plant powders mainly play the role of filler to reduce the cost of composites. In fact, as shown in Fig. 1, many plants have natural fiber cells with large aspect ratios and excellent mechanical properties. If the fiber cells can be separated from plant waste, then the fiber cells can be used as reinforcement for polymer materials.

However, plant fibers with large aspect ratios readily aggregate. Strong forces have to be applied to disperse fiber aggregates. In strong shear flow fields, plant fibers are vulnerable to shear stress and their aspect ratio decreases, which has adverse effects on the mechanical properties of plant fiber-reinforced polymers [1]. Extensional stress in extension flow fields facilitates the orientation and disentanglement of fibers, which helps to maintain their aspect ratio. Therefore, to study the rheological properties of plant fiber-filled polymer composites in extensional flow fields has practical importance for developing high-performance fiber composites.

Many researchers have studied the extensional rheological properties of polymers. Trouton proposed the concept of viscosity drag coefficient (extensional viscosity) in 1906, and Fano studied the spinnability of polymer solutions in 1908 [2], but further studies on extensional rheology developed slowly until the corresponding measuring instruments were developed in the 1960s. The commonly used measuring techniques and instruments for determining the extensional rheological parameters of polymers include the Meissner-type rheometer [3], [4], filament stretching method [5], [6], Rheotens melt strength method [7], [8], Sentmanat extensional rheometer [9], [10], biaxial extensional rheometer [11], [12] and entrance converging flow method [13], [14], [15], [16], [17]. Because plant fiber/polymer composites show weak melt strength, and the composite melt strip easily breaks under uniaxial tension, the entrance converging flow method is more suitable to measure the extensional rheological properties of plant fiber/polymer composites than other methods.

The entrance converging flow method has been developed extensively. Cogswell [13], [14] first developed a model for flow-in converging dies where the extensional and shear components were treated separately, and derived analytical solutions for the entrance pressure drop (EPD) components caused by extensional flow and shear flow in convergent wedged and tapered dies. Based on Cogswell's work, Binding [15] assumed that the relationship between extensional viscosity and extensional rate also satisfies the power law. With his coworkers, he then investigated the rheological behavior of entrance converging flow using a modified capillary device [16], and simulated the pressure distribution in convergence and expansion flow runners using the Oldroyd-B fluid model [17]. Liang [18] analyzed the streamline and EPD of the fluid in the convergent region of a die with a flat entrance, and derived the equations for streamlined entrance convergent flow. Gibson [19] proposed a model to describe the relationship between EPD and extensional stress. We compared the experimental results of our previous trials and found that the Gibson model is more suitable than the other methods discussed above to characterize the extensional rheological properties of plant fiber-filled polymer composites.

Gibson [19] divided the entrance zone of a convergent die into two parts, as shown schematically in Fig. 2; region A, between the spherical boundaries at r = r0 and r = r1, and region B, between the boundary r = r1 and entrance of the capillary. The pressure drop in region A contains components related to losses in extensional and shear flow. The extensional flow component increases initially with increasing entrance convergence angle (ECA) in an almost linear fashion. The shear component, which is usually only important at low die angles, approaches infinity when ECA equals zero, but decreases rapidly with increasing ECA. Usually, the shear component in region B is not a substantial part of the overall pressure, so it can be ignored. Then, the average extensional strain rate of a material crossing any spherical surface in region A can be calculated as follows [20], [21]:ε˙¯=QπR3sinα(1+cosα),where ε˙¯ is average extensional strain rate, α is the die semi-angle of the capillary and R is the radius of region A with a value from r1 to r0 (as shown in Fig. 2, between the boundary r = r1 and r = r0). On passing from the entry to the exit boundary in region A, the mean value of extensional strain rate increases substantially, reaching a maximum at the spherical boundary between regions A and B. The maximum value is given by [19]:ε˙¯1=γ˙1sinα(1+cosα)4,where γ˙1 is the apparent wall shear rate of the long capillary.

The maximum extensional stress can be expressed as shown in Eq. (3):σ¯1=kEε˙¯1m=PE23m[1(R1R0)3m]+ϕ(m,α)[sinα(1+cosα)]m.

The extensional non-Newtonian index m and extensional consistency kE can be calculated from the linear fitting results of a plot of lnσ¯1 against lnε˙¯1. Sometimes, the linear fitting results are not stable, so a further iterative procedure is needed to find m and kE, starting with trial value m = n (where n is the shear non-Newtonian index).

Extensional stress σ and shear stress τ are described using independent power law models:σ=kEε˙mandτ=kSγ˙nwhere kS is the shear consistency.

The corresponding extensional viscosity ηE and shear viscosity ηS are given by:ηE=kEε˙m1andηS=kSγ˙n1.

In this study, a new set of tandem capillary dies (TCD) was designed that can be used to evaluate the shear and extensional rheological properties of fiber composites at the same time. The Gibson model is used to evaluate the relationship between EPD and extensional stress in a convergent runner. The influence of different ECA on the extensional rheological behavior of plant fiber composites is investigated, and the extensional and shear rheological properties of composites with different fiber contents are compared. The aim of this research is to provide a method to measure the extensional and shear rheological properties of fiber-reinforced polymer composites, and provide suggestions for mold design and process parameter setting.

Section snippets

Materials

The HDPE 7000F used in this study was obtained from Iran's National Petrochemical Company. The melt flow index of HDPE is 0.04 g/10 min (170 °C, 2.16 kg).

Pithecellobium clypearia (PC) was broken by a crusher (WSG-Y250, Wensui Plastic Machinery Co., Ltd., Guangzhou) and screened through a sieve with 4 mm-diameter mesh. Water was added to the broken PC to give a moisture content of 50 wt%. . Then, the PC fiber bundles were separated into Pithecellobium clypearia fiber (PCF) by steam explosion

Effect of ECA on extensional and shear rate components

Fig. 5 shows the relationship between the ratio of extensional strain rate to shear strain rate (RES) and ECA under constant wall shear rate of the long capillary, which also means under constant plunger descent speed according to Eq. (2). The extensional strain rate increases initially but then decreases with increasing ECA, reaching a maximum at 120°, which indicates that materials will reach maximum extension when using an ECA of 120°.

Effect of ECA on EPD for pure HDPE

Fig. 6 shows the relationship between EPD and ECA for

Conclusions

The entrance converging flow method was used to investigate the influences of ECA and fiber content on the rheological behavior of PCF/HDPE composites, and shear and extensional rheological properties were assessed. The results show that: (1) The extensional rate increases initially and then decreases with increasing ECA at the same plunger speed; the maximum extensional rate is achieved at an ECA of 120°. Therefore, the extensional effect can be adjusted by changing ECA. (2) Wall slippage

Acknowledgments

The authors acknowledge the National Natural Science Foundation of China (Nos. 50903033, 51373058, and 11272093), the Program for New Century Excellent Talents in University (No. NCET-11-0152), the Pearl River Talent Fund for Young Sci-Tech Researchers of Guangzhou City (No. 2011J2200058), and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120172130004) for financial support.

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