Rheology and tribology of starch + κ-carrageenan mixtures

Abstract

In this study, we investigated the rheological and tribological properties of biopolymer mixtures of gelatinized corn starches (0.5 - 10.0 wt%) and κ-carrageenan (κC) (0.05 - 1.0 wt%). Two different starch samples were used. The first starch (CS1), despite extensive heating and shearing contained "ghost" granules, while the second starch (CS2) had no visible ghost granules after the same gelatinization process as CS1. Apparent viscosity measurements demonstrated that κC + CS1 mixtures were shear thinning liquids, with viscosity values being lower than the corresponding weight average of the values of the individual equilibrium phases at shear rates < 50 s^-1 . Tribological results revealed that κC ≥ 0.5 wt% was required to observe any decrease in friction coefficients in the mixed lubrication regime. Starch (CS1) showed an unusual behavior at ≥ 5 wt%, where the friction coefficient decreased not only in the mixed regime but also in the boundary regime, probably due to the presence of the "ghost" granules, as the latter became entrained in the contact region. The CS1 + κC mixtures showed significantly lower friction coefficients than that of pure CS1 and κC in the mixed regime. However, the CS2 + κC mixture (i.e., containing no ghost granules) showed similar behavior to pure κC in the mixed regime, while lower friction coefficients than that of the pure CS2 and κC in the boundary regime. These findings illustrate new opportunities for designing biopolymer mixtures with tunable lubrication performance, via optimizing the concentrations of the individual biopolymers and the gelatinization state of the starch.

Keywords: biopolymer mixtures; corn starch; polysaccharides; rheology; tribology; κ-carrageenan.

FIGURE 1

(i) Friction coefficient (μ) versus entrainment speed (U), (ii) apparent viscosity (η) versus shear rate (γ), and (iii) friction coefficient (μ) as a function of product of entrainment speed and effective viscosity (Uη) of (a) κC and (b) CS1 at various concentrations (κC: 0.05 wt% ( ), 0.1 wt% ( ), 0.5 wt% ( ), and 1.0 wt% ( ); CS1: 0.5 wt% ( ), 1.0 wt% ( ), 2.0 wt% ( ), 3.0 wt% ( ) and 5.0 wt% ( ). Phosphate buffer is used as a control ( ). Values represent means and error bars represent the SDs for at least three measurements on triplicate samples (n = 3 × 3)

FIGURE 2

Optical (a) and confocal (b) micrographs of 1 wt% CS1 after gelatinization. The bright regions in (b) are due to CS1 labeled with Rhodamine Blue. Scale bar is 50 μm

FIGURE 3

Friction force versus applied load for κC (■) and CS1 (□) when sheared between polydimethylsiloxane (PDMS) ball and disc at a constant speed of 0.005 m/s. Values represent means and error bars represent the SDs for at least three measurements on triplicate samples (n = 3 × 3)

FIGURE 4

(i) Apparent viscosity (η) as a function of shear rate (γ) and (ii) friction coefficients (μ) versus entrainment speed (U) of biopolymer mixtures at (a) lower biopolymer concentrations: (1.5 wt% CS1 + 0.15 wt% κC, ) and (b) high biopolymer concentrations (2.5 wt% CS1 + 0.25 wt% κC, ) plus the controls of 0.15 wt% κC ( ), 0.25 wt% κC ( ), 1.5 wt% CS1 ( ) and 2.5 wt% CS1 ( ) alone. The weight average values of the corresponding individual controls for the mixtures are also shown ( ). Values represent means and error bars represent the SDs for at least three measurements on triplicate samples (n = 3 × 3)

FIGURE 5

Friction coefficient (μ) versus entrainment speed (U) of biopolymer mixtures using CS2 without the “ghost” granules, that is, 2.5 wt% CS2 + 0.25 wt% κC, ( ) and controls of 2.5 wt% CS2 ( ) and 0.25 wt% κC ( ). Values represent means and error bars represent the SDs for at least three measurements on triplicate samples (n = 3 × 3). The inset is an optical micrograph of CS2 starch after gelatinization, illustrating the lack of “ghost” granules. The weight average values of the corresponding individual controls for the mixtures are also shown ( ). Error bars represent SDs