Fabrication of water-soluble loose-fill foam from tamarind ( Tamarindus indica L.) seed polysaccharide by mechanical frothing and freeze-drying process

Abstract

The water-soluble loose-fill foam obtained from tamarind seed polysaccharide (TSP) was successfully prepared by a combination of mechanical frothing and freeze-drying process. The effects of TSP concentration, plasticizer content, and surfactant content on the cellular morphology, physical properties, mechanical properties, and moisture absorption were investigated. The cellular structure of TSP foam exhibited an open cell structure with a non-uniform size of the cell window, and the density varied in a range of ∼0.006–0.106 g/cm³. Foam preparation with high TSP concentration, low plasticizer as well as glycerol content enhanced the mechanical properties of the obtained foam, including tensile strength, compressive strength, and hardness. The high compressive strength of TSP foams up to ∼1.03 MPa can be produced which demonstrates that TSP foam is capable to use as a loose-fill product.

Keywords

Water-soluble loose-fill foam tamarind seed polysaccharide xyloglucan mechanical frothing freeze-drying

Introduction

Loose-fill packaging foam is a material used in protective packing products during transportation in order to provide cushioning and protecting of article packaged during shipment between manufacturers, distributors, retailers, and consumers. Over the last decades, expanded polystyrene-based foam has grown in this application due to its low density, high strength, and low cost. However, a large amount of solid waste is generated and leads to environmental impacts because it is a single-use product and exceedingly difficult to degrade. Therefore, eco-friendly loose-fill foams produced from biodegradable polymer based on thermoplastic starch (neat thermoplastic starch, thermoplastic starch+polyvinyl alcohol, and thermoplastic starch+poly(lactic acid))^1–3 have been developed for packaging distribution system. However, the amount of starch that is available for human consumption is also another matter for concern. This is due to the global demand of starchy food will continue to rise throughout this century⁴ as well as the amount of starch plant production that is available for human consumption depends on global climate. Therefore, an interesting way of creating low cost and ecofriendly loose-fill foam is the use of other polysaccharides that can be generated from agro-waste industries to explore the value-added applications and can be a potential alternative to petroleum-based counterparts.

Tamarind (Tamarindus indica L.) seed polysaccharide (TSP) is a branched polysaccharide extracted from tamarind seed, a discarded waste of the commercial utilization of the fruit in Thailand. The chemical structure of TSP composes of a β-(1,4)-D-glucan backbone substituted with side chains of α-(1,4)-d-xylopyranose and (1,6) linked [β-d-galactopyranosyl-(1,2)-α-d-xylopyranosyl] (shown in Figure 1) with a molecular weight in the range of ∼700–900 kDa.⁵ TSP molecules can dissolve in water and become viscous fluid (hydrogel) after heating. Moreover, it is easily soluble in water, due to the presence of hydroxyl groups in its structure. From these unique properties, TSP has various applications in several industries, such as a sizing powder and fabric detergents for textile, gum based materials for printing processes, as well as gelling agent for food thickeners.^6,7

However, the previous studies have been reported that there was no melting temperature of TPS⁷ because its structure presented only the XRD amorphous component⁸ with a glass transition temperature (T_g) of ∼250–260°C.^9,10 From this reason, the conventional foaming process (hot melt extrusion) was not practical to covert TSP to a solid foam. Therefore, a challenging foaming process has been proposed in this work to overcome this limitation. A combination technique of mechanical frothing and freeze-drying process was used to produce low density TSP foam, without the use of salt, organic solvent and high temperature. Firstly, the air was pressed into the TSP hydrogel to fabricate foam formation, and the foam was further stabilized by a mechanical frothing technique. Then, in order to change the air trapping gel into a solid foam, it had to be dried through lyophilisation using the freeze-drying process. In this system, the surfactants were used to assist foam forming in TSP as generally used in a foam preparation of natural rubber.¹¹ Moreover, glycerol was used as a plasticizer in order to decrease the brittleness of the polysaccharide foam.¹⁰,^12–15

The goals of this research were to develop a novel eco-friendly loose-fill foam by the mechanical frothing and freeze-drying process, as well as to study the effects of TSP concentrations, plasticizer content, and surfactant content on the TSP foam morphology, physical properties (i.e. density, shrinkage and color), and mechanical properties (i.e. tensile strength, elongation at break, and compressive strength). Densities, color parameters, and moisture absorption properties of the obtained TSP foams were characterized and compared to commercial expanded polystyrene (EPS) foam and soluble loose-fill foam. Moreover, an effect of relative humidity on mechanical properties, and moisture absorption of TSP foams were also investigated.

Experimental

Materials

Tamarind seed polysaccharide was obtained from G.M. Ichihara Co., LTD (Thailand). Glycerol (95.0% purity) was purchased from Ajex Finechem (Australia). A mixture of surfactants (20% w/v) was prepared from potassium oleate (Sigma-Aldrich, Germany) and potassium laurate (Lucky Four Co, LTD., Thailand) (1:1 by volume). Distilled water was used for all experiments. Two types of commercial loose-fill packaging foams, which were (polystyrene foam, CF1) and water-soluble loose-fill foam (unknown, CF2) were kind gifts from a chemical supply company in Thailand. The pictures of solid foam of TSP loose-fill foam, polystyrene loose-fill foam (CF1), and water-soluble loose-fill foam (CF2) are shown in Figure 2(a) to (c), respectively.

Preparation of TSP foam

TSP hydrogel was prepared by heating the TSP in hot water (∼80–90°C) for 30 min. The mixture of surfactant (20% of potassium oleate and 20% of potassium laurate, 1:1 by wt.) and the glycerol were added into the gel and then the foams were made by mechanical frothing with a beating hand mixer (Electrolux, model EHM3407, speed #5). The temperature of TPS hydrogel was maintained at 65–70°C during the foaming process. The TPS foam was poured into a silicone mold and kept in the refrigerator for 24 h before freeze-drying. The formulation of TSP foam is shown in Table 1. It was noted that some formulations were duplicated but presented in different names depending on the influencing study factors.

Table 1.

Chemical compositions used to produce the TSP foam.

Formulations	Compositions
	TSP	Glycerol	Surfactant
	(%)	(g/100 g solids)	(g/100 g solids)
TSPF4	4	10	100
TSPF6	6
TSPF8	8
TSPF10	10
GF0	10	0	100
GF5		5
GF10		10
GF15		15
SF50	10	5	50
SF100			100
SF150			150

Foam morphology

The cellular structure of obtained foam was determined using a scanning electron microscope (SEM) (JEOL, model JSM-5800 LV) with an accelerating voltage of 20 kV at 50x magnification. The foam sample was placed on a sample holder containing carbon adhesive tape before sputter-coating with a platinum.

Physical properties

Density

The TSP foam and commercial foams were equilibrated at a relative humidity of 50% and temperature of 30 ± 2°C for 5 days using a temperature and humidity incubator before testing. The foam samples were cut to a specific rectangular shape with a size of 1.0 × 1.0 × 1.0 cm. The density of specimens was calculated from the mass (g) of foam specimen divided by its volume (cm³).¹⁶

Shrinkage

After freeze-drying, the obtained TSP foams were removed from the mold (5.0 × 5.0 × 5.0 cm) and kept in the polypropylene (PP) bag at an ambient temperature. The evaluation of foam deformation was conducted in accordance with ASTM D 2126.¹⁷ The resulting change in volume of mold and a dried specimen was calculated by Eq. (1):

Shrinkage (%) = [\frac{{volume}_{mold} {- volume}_{dried foam}}{{volume}_{mold}}] \times 100

(1)

Color

The color difference of the TSP foam was determined by a colorimeter (Hunter associate laboratory, USA) using L*a*b* coordinate (lightness (L*), redness/greenness (a*), and yellowness/blueness (b*)). The average value was reported with five measurements per sample.

Mechanical properties

Hardness

The test specimens (5.0 × 5.0 × 5.0 cm) were incubated at 30 ± 2°C with the specified relative humidity of 50 for 5 days using the incubator. Indentation hardness was tested using a Shore O durometer (Frank GmbH, Germany) according to ASTM D 2240.¹⁸ All the reported data were obtained from five test results and each measuring point was 1.0 cm apart.

Tensile properties

Tensile testing of TSP foam was performed according to the ASTM D 357¹⁹ using a universal testing machine (LLOYD, model LR 30k, USA) with a 25-N load cell, crosshead speed of 2.5 mm/min and initial grip length of 5.0 cm.⁵ The foam specimen was prepared in a rectangular plate (2.5 × 10.0 cm), and incubated at 30 ± 2°C with the desired relative humidity (50 and/or 75% RH) for 5 days using the incubator. The average tensile strength and elongation at break were reported from five replicate samples tested.

Compressive property

The foam specimens used for the compression test were cut to a size of 1.5 × 1.5 × 0.75 cm²⁰ and kept in a specific relative humidity (50 and/or 75% RH) for 5 days before testing. The compressive strength at 25% strain of TSP foam was determined according to the D1621–73²¹ using the universal testing machine with a 25 kN force range and a crosshead speed of 5 mm/min at room temperature.

Moisture absorption

The test specimens (5.0 × 5.0 × 5.0 cm) were dried in a hot air oven at 60°C for 24 h, and kept in the polypropylene bag. The TSP foams were then conditioned in the incubator with a relative humidity of 50% and 75% RH with the temperature of 30 ± 2°C for 15 days. At the specific time, weight gain of the test specimens was measured and the moisture absorption (%) was calculated by Eq. (2):

Moisture absorption (%) = [\frac{{weigth}_{at spc . time} {- weight}_{initial}}{{weight}_{initial}}] \times 100

(2)

Statistical analysis

In this study, a Completely Randomized Design (CRD) was implemented and all experiment were run in triplicates (n = 3). Levels of factor were decided by using the obtained information from preliminary experimental works. SPSS was used to perform statistical data analysis (SPSS Inc., Chicago, IL, USA). All experimental data were analyzed of variance and mean comparisons by using One-Way ANOVA and Duncan’s multiple rang test. Data are presented as mean ± standard deviation and the probability value of P < 0.05 was considered as significant.

Results and discussion

Cellular morphology

Figure 3 shows SEM micrographs of the fracture surfaces of TSP foam at the solid content of 4%, 6%, 8%, and 10%wt, with glycerol and surfactant content at 10 g/100 g solids and 100 g/100 g solids, respectively. It was illustrated that TSP foams showed open-cell structure with thin cell walls. Moreover, the cells were not uniform and the window sizes varied in a broad range (∼20–600 μm), roughly estimated from SEM images. The soft TSP foams were obtained when using TSP concentration of 4% and 6%wt, led to the collapse of the cell structure as shown in Figure 3(a) and (b), respectively. At these low TSP content, the viscosity of the foam gel might be too low to generate sufﬁcient strength to hold the expanding bubble, then limiting the radial expansion of the gel. By contrast, when the TSP content were increased to 8% and 10%wt, the window shape of the foam became more defined, mostly round shape, and rigid texture was observed at the edge of the cell window. The most uniform and the largest window size were observed at the TSP content of 10%wt (Figure 3(d)).

In order to investigate an effect of plasticizer content on the cell morphology of TSP foam, the TSP content was kept at a constant level of 10%wt. The glycerol content had an effect to the cell morphology of TSP foam as shown in Figure 4. It was clearly observed that the larger cell size seemed to develop with glycerol content growth in the foam probably because higher internal plasticization increased the macromolecular chain mobility, resulting the viscosity reduction²² of TSP and enhancing the expansion of air bubble.

Additionally, the surfactant (foaming agent) amounts played an important role to control the cell morphological structure of the TSP foams. In this part, the TSP amount, and glycerol content were fixed at 10%wt and 5 g/100 g solids, respectively, and the amounts of surfactant had been varied from 50 to 150 g/100 g solids. Figure 5 shows that different cell structure was observed at different levels of surfactant added. It was obvious that the cell population significantly increased with increasing the amounts of surfactant. The optimum amount of surfactant (150 g/100 g solid, Figure 5(c)) facilitated the formation of TSP foam with more uniform structure.

Physical properties

The shrinkage in TSP foam was observed when the foam density was below 0.05 g/cm³ (TSP concentration was ≤6%wt and surfactant content up to 150 g/100 g solids, shown in Table 2). This might be due to the low strength of TSP foam structure with a high displacement of air. The density of TSP foam was significantly influenced by the TSP concentration and the surfactant content, while the glycerol content did not affect the foam density.

Table 2.

Effect of TSP concentration, glycerol content and surfactant content on shrinkage, density, and color of TSP foam.

	Shrinkage	Density	Color parameters
Formulations	(%)	(g/cm³)	L*	a*	b*
CF1	–	0.006^a	94.4 ± 0.1^e	–3.1 ± 0.0^a	10.1 ± 0.0^a
CF2	–	0.014^b	90.5 ± 0.5^d	–3.0 ± 0.0^b	11.1 ± 0.1^a
TSPF4	50.2 ± 2.4^c	0.039^cd	97.6 ± 3.8^f	–1.5 ± 0.3^bc	18.8 ± 2.4^b
TSPF6	4.0 ± 0.4^a	0.044^d	91.0 ± 1.0^d	–0.4 ± 1.2^d	20.1 ± 0.5^bc
TSPF8	nd.	0.067^f	88.9 ± 1.0^cd	–1.8 ± 0.1^b	22.9 ± 0.4^de
TSPF10	nd.	0.077^g	82.1 ± 1.0^b	–1.1 ± 0.1^bc	23.5 ± 0.9^de
GF0	nd.	0.082^h	80.8 ± 1.7^ab	–0.5 ± 0.1^c	22.9 ± 0.2^de
GF5	nd.	0.086^hi	81.2 ± 0.5^b	–0.6 ± 0.0^cd	22.4 ± 0.6^cd
GF10	nd.	0.078^gh	82.1 ± 1.0^b	–1.1 ± 0.1^bc	23.5 ± 0.9^de
GF15	nd.	0.081^h	80.8 ± 0.6^ab	–0.6 ± 0.2^cd	22.8 ± 1.5^de
SF50	nd.	0.106^j	78.6 ± 1.3^a	0.5 ± 0.2^e	24.3 ± 0.9^de
SF100	nd.	0.086^hi	81.2 ± 0.5^b	–0.6 ± 0.0^d	22.4 ± 0.6^cd
SF150	23.9 ± 1.2^b	0.051^e	87.8 ± 0.2^c	–0.2 ± 1.3^de	25.3 ± 4.0^e

The standard deviation of foam densities was in the range of ∼0.000–0.003 g/cm³. CF (1 and 2) = Commercial foams (#1 and #2), TSPF (4, 6, 8, and 10) = Tamarind seed powder foam with different concentrations (%), GF (0, 5, 10, and 15) = Tamarind seed powder foam with different glycerol content (g/100 g solids), and SF = Tamarind seed powder foam with different surfactant content (g/100 g solids). L* = lightness/brightness, a* = redness/greenness, and b* = yellowness/blueness.The values with different superscript letters in the columns are significantly different (p < 0.05).

The foam densities tended to increase from ∼0.006 to 0.086 g/cm³ when the TSP content increased from 4% to 10%wt, while decrease from ∼0.106 to 0.051 g/cm³ when the surfactant content increased from 50 to 150 g/100 g solids. This result was corroborating the morphological observation in Figure 4 that the proportion of bubble population in TSP media was increased with increasing the surfactant content. As compared to the commercial loose-fill foams (CF1 and CF2), the density of prepared TSP foam was higher than both commercial foams (0.006–0.014 g/cm³) (Table 2).

Most TSP foams showed lower luminosity (L*) than that of the commercial loose-fill foams, except the foam prepared from TSP at the amount of 4% and 6% wt. The decrease of L* and the increase of b* (yellowness/blueness) were presented when increasing the TSP amount. This might be due to the increase of protein content which is an ingredient of TSP (∼15–20%),^23,24 however there was no conclusion for a* (redness/greenness). Plasticizer content had no significant effect on the color of the foams. However, the addition of glycerol caused the increase in L* values.

Mechanical properties

The basic mechanical properties of obtained foams were determined, i.e., tensile strength, elongation at break, compressive strength, and hardness as shown in Figures 6 and 7. Figure 6(a), (c), and (e) show the tensile strength and elongation at break values of TSP foam at different TSP, glycerol and surfactant content, respectively. The results showed that the tensile strength tended to increase when increasing TSP concentration and varied in a range of ∼0.001–0.017 MPa (Figure 6(a)). This might be due to the higher density of the xyloglucan polysaccharide after cooling. In contrast, by adding a higher amount of glycerol, the tensile strength significantly decreased, while the elongation at break was increased (Figure 6(c) and (e)). It was plausibly due to the reduction in intermolecular forces between the polymer molecules and the increase in the specific volume,²⁵ resulting in the slippage of the branched TSP molecules that enhanced the flexibility and resiliency of the TSP foams.¹⁰,^12–15 Moreover, its brittleness also should be inversely proportional decrease as a report of Brostow and Hagg Lobland.²⁶ In the same way, higher surfactant content also caused the decrease in the tensile strength and increase in the elongation at break. This could be explained that the strength and extension ability of the foam was generally proportional to the bubble population and foam densities.²⁷ The bubble population in TSP matrix was increased with increasing the surfactant content, resulting in lower polymer matrix which acted as mainly load distribution area and it caused lower strength lamella and higher wall bending. The compressive strength of TSP foams (Figure 6(b), (d), and (f)) showed the same trend with tensile strength and varied in a range of ∼0.01–1.03 MPa, while starch-based and virgin EPS loose-fill foam that has been reported in previous works varied in the ranges of ∼0.05–0.10 MPa.^28,29 The maximum tensile strength and compressive strength were achieved when using TSP at 10%wt, glycerol content of 5 g/100 g solids, and surfactant content of 50 g/100 g solids. Moreover, the foam produced from this formula was the only one that the hardness could be tested; see the hardness value in Figure 7(d).

In order to assess the impact of moisture on the mechanical properties of TSP foam, the tensile properties, compressive strength, and hardness of TSP foam with TSP of 10%, glycerol of 5 g/100 g solids, and surfactant of 50 g/100 g solids also investigated at different level of relative humidity (50% and 75%) under temperature of 30°C. As shown in Figure 7(a), (c) and (d), tensile strength, compressive strength, and hardness of TSP foam decreased (∼10–25%) at high relative humidity (75% RH) and constant temperature due to the plasticization of hydrophilic carbohydrate polymer.^30,31 However, the elongation at break of TSP foam increased significantly from ∼1.7 to 10.4% as shown in Figure 7(b).

Moisture absorption

Figure 8 illustrates the moisture absorption results of TSP foam produced from TSP concentration of 10%wt, glycerol content of 5 g/100 g solids, and surfactant content of 50 g/100 g solids with different relative humidity compared to the commercial soluble loose-fill foam (CF2). The results showed that moisture absorption of TSP foam increase rapidly than that of the commercial loose-fill foam, indicating that the composition of TSP foam may have higher hydroxyl group which can interact with water molecules through hydrogen bonding.³² From 0 to 12 h, the moisture absorption of TSP foam increased from ∼3.2–8.9% as the relative humidity increase from 50 to 75% at constant temperature (30°C), while the moisture absorption of commercial foam increased from ∼0.7 to 2.1%. The equilibrium adsorption of TSP foam occurred within 2 days which was faster than the soluble commercial foam for 2 days. However, the moisture absorption of TSP foam can be future improved by structure modification and surface treatment techniques.

Conclusions

Water-soluble loose-filled produced from tamarind seed polysaccharide with open cell structure was successfully prepared by a combination of mechanical frothing and freeze-drying process and might be used as loose-fill application. The properties of TSP foam were influenced by TSP concentration, glycerol content and surfactant content. The shrinkage of obtained foam no longer presented since the foam density was higher than 0.06 g/cm³. The color of TSP foam was significantly affected by the amount of protein containing in tamarind seed polysaccharide. The maximum mechanical properties (i.e., tensile strength, compressive strength, and hardness) were achieved when using TSP concentration of 10%, glycerol content of 5 g/100 g solids, and surfactant content of 50 g/100 g solids. From this formulation, the TSP foam had a higher density 10 times than commercial loose-fill foam (CF1 and CF2) and also more sensitive to moisture than the commercial water-soluble loose-fill foam (CF2). Therefore, this observation points out that its polarity should be future improved in order to increase the product performance.

Footnotes

Acknowledgements

The authors would also like to acknowledge G.M. Ichihara Co., LTD (Thailand) for providing tamarind seed polysaccharide.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by grant no. AGR6102012S from the Research and Development Office, Learning Resource Center, Prince of Songkla University, Thailand.

ORCID iD

Ladawan Songtipya

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