Transparent packaging material from the Arecanut leaf sheath -

Transparent packaging material from the Arecanut leaf sheath

Synthetic, non-degradable packaging materials have become a severe environmental threat, and therefore the demand is growing for environmentally friendly materials for sustainable packaging. This study presents a feasible method of developing a novel, sustainable packaging material using the Arecanut leaf sheath (ALS). CarboxyMethyl Cellulose was coated on the material for improving its mechanical properties. The novel material has an areal density of 132.5 g per square meter and 1 mm thickness. The material also possesses a tearing strength of approximately 71% of polythene and exhibits approximately 5% elongation at failure. The bursting strength of the material was found to be three times higher (0.3 kg/cm²) than that of polythene (0.1 kg/cm²). The breaking force of the material was approximately 70% and 22% of polythene in lengthwise and widthwise directions, respectively. Novel material shows a good thermal stability where initial degradation occurred in the temperature range of 200–375 °C.

Materials and Methods

The main material used in the present study is the Arecanut leaf sheath (ALS). The sheath along with the husk which primarily protects the betel nut in the tree falls naturally once it is dried. The fallen leaves were collected fresh from a farm. ALS was removed and cleaned with fresh water before processing further.

Preparation of the Film

Samples of ALS (20 cm × 20 cm) were soaked in water for 48 hrs. Soaking in water allowed loosening of fibers in the ALS and peeling off the layers. Subsequently, the layers on the abaxial surface of the sheath were removed manually until a sheet-like layer was formed. Small fibres entangled on the formed thin sheets were wiped away using a brush. The samples were further soaked in water for 8 hrs to facilitate the smooth removal of remaining fibers. The soaking and fiber removal processes were repeated until no further fibers could be removed through that process. The second step, the samples were bleached using Hydrogen Peroxide.  Hydrogen peroxide is stable when pH level is below 7 and it becomes unstable as the level of alkalinity increases. Bleaching was performed at a temperature of 90–95 °C using 600 ml of the bleaching solution. Samples of ALS obtained after the first step were further used in the bleaching process carried out in five different time durations (40 min, 50 min, 60 min, 70 min and 80 min), in order to assess the effect of bleaching time on the level of transparency. During the bleaching process, remaining fibres in the sample were loosened and were easily removable. Therefore, as the third step, the samples were taken out every 10 min, washed with water at room temperature and the loosened layers were removed using a brush. This process was repeated by re-introducing the samples to the bleaching solutions and removing the fibers. For instance, the process was repeated four times for the 40 min samples, five times for the 50 min sample, and seven times for the 70 min sample. After keeping the samples in the bleaching solution in pre-defined time intervals, they were taken out and dried in an air-conditioned room. The samples tend to curl with the time and became nonplanar. In order to eliminate this problem, the dried samples were sent through a pad batch, by applying a pressure of 2 bar. This method was successful in achieving a planar sample.

Coating Using CarboxyMethyl Cellulose (CMC)

Carboxymethyl cellulose (CMC) is a sodium salt derivative of cellulose. Unlike cellulose, it is water soluble and functions as a suspending agent, stabilizer, film former or a thickening agent. CMC glue was prepared by mixing 4 g of CMC powder in 100 ml of water. The mixture was refrigerated overnight in an airtight container resulting in smooth, thick glue. This resulting glue was thinned by mixing water appropriately. Samples (15 cm × 15 cm) were obtained from the bleached ALS sheets. Those samples were mounted on a fat board and the coating was applied evenly on one surface of the sample using a smooth brush. Coated samples were then kept for drying in the room temperature for 5 hrs before turning over. Subsequently, both sides of the samples were coated and kept for drying.

Measuring Mechanical Properties of Samples

Tensile Strength

ASTM D638 standard method was used to determine the behavior of the samples subjected to an axial stretching load. This was achieved by clamping the samples between the jaws of the universal tensile tester and uniformly extending it until failure. It was visually observed that ALS fibers are aligned in one direction. Therefore, the tensile behavior of ALS was anticipated to be different along the aligned fibers and across the fibers. Consequently, tensile tests were carried out both along and across the aligned fibers.

Water Permeability

Permeability of a material can be defined as the ability of liquid water to penetrate from one side of the material to the other. A sufficient resistance to permeability is required from a commercial level packaging material. Permeability of the samples was tested in accordance with the AATCC 127 standard, using a hydrostatic pressure tester. Three specimens per sample were cut with an area of 200 mm×200 mm along the diagonal direction. Distilled water at 21±2 °C was used for the test on a test area of 100 cm². The water pressure was increased at a constant rate, until three water droplets appeared in different places on the specimen.

Tearing Strength

Tearing strength can be defined as the strength required starting or continuing the tear in a material under specific conditions. A sufficient tearing strength is an important property expected from a packaging material. Tearing strength is measured in accordance with the ASTM D412 standard test method, using an Elmendorf tear tester. The Elmendorf tear tester determines the tearing strength by measuring the work done in tearing through a fixed length of the test specimen. Specimens of dimension 100 mm×75 mm were placed on the tester after creating a small slit as specifed by the ASTM D412 standard test method. During the test, the tester tears the ALS specimen from the end of the slit to the opposite edge over a distance of 43 mm.

Bursting Strength

In addition to tensile strength, bursting strength of a packaging material is also important since the material is stressed in all directions at the same time. Bursting strength is measured in accordance with the ISO 2759 standard test method.

Thermal Gravimetric Analysis (TGA)

Thermal stability of CMC coated sample was checked using Thermal Gravimetric Analysis (TGA) (TGA55, TA instruments, USA) and Platinum 100 µl pan. Selected temperature range was − 80 to 450 °C with 10 °C/min ramp, under nitrogen (99% w/w) atmosphere with a gas flow rate of 40 ml/ min.

Transparency Analysis

Transparency levels of the ALS bleached samples before the coating application and the final CMC coated sample were observed both visually and by using a transmittance test. The transmittance of the CMC coated sample was tested by UV–visible spectrophotometer. The test was conducted by using a sample of 1 cm × 5 cm size and by scanning the transmittance in the visible spectrum range (400–700 nm).

Results

The application of coating has a direct impact on improving the tensile strength of the sample. For this study, CMC is used as the coating because it is a sustainable and degradable biomaterial, derived from cellulose. Application of synthetic based coatings would further enhance the tensile strength yet hinder the sustainability aspects of the material. It can be observed that the load bearing capacity in the length direction is considerably lower than its counterpart. Water permeability test was conducted using three specimens, and the average value obtained for the three specimen tested was 1140 mm of water pressure. The results indicate that the ALS sample could withstand in light rain or snow conditions, without penetrating water inside.

Tearing Strength: Average values obtained for the breaking force of tested samples were 12.78 N in lengthwise direction and 40.33 N in widthwise direction. Bursting strength of the CMC coated sample was obtained as follows. Pressure gauge with diaphragm and the sample 2.3 kg/ cm² for 15 cent minutes. Pressure gauge without the sample 2 kg/cm² for 15 cent minutes. Bursting strength of the CMC coated sample 0.3 kg/ cm²

Comparison of the results

The desirable properties of the newly developed packaging material were compared with the commercially available polythene material. CMC coated sample shows better mechanical properties than the uncoated ALS sample. Water permeability of the material was found to be in par with that of polythene where both the materials resisted 114 cm of water pressure. In terms of tearing strength, the novel material was found to be consistent of at least 70% of the strength of polythene width wise, yet 20% in lengthwise. Moreover, the bursting strength of the novel material was observed to be three times higher (0.3 kg/cm²) than that of polythene (0.1 kg/cm²). However, it can also be observed that the tensile strength of the novel material along its width is less than that of its length. Moreover, it is also observed that the width wise tensile strength is considerably lower than that of polythene. However, the tensile strength of the novel material can still be considered adequate for light weight packaging applications. The elongation at break varied from 5 to 5.5% for the novel material. It can be observed that the percentage elongation is higher for the ALS material than previously studied Polyacetic Acid (PLA) based packaging materials. The novel material presented herein needs further improvements in terms of tensile strength as well as elongation at break. TGA plays an imperative role in the determination of the thermal stability of the materials. For CMC coated sample, the highest mass loss (48.528%) can be observed in the temperature range of 200–375 °C. This is corresponded to the second decomposition step and it may be attributed to the initiation step to decompose ALS. Therefore, the TGA spectrum of the packaging film demonstrated a high thermal stability while maintaining the flexibility. The transmittance level of the novel material varied from 54 to 68% in the visible spectrum, whereas for polythene packaging films, this range may vary from 80 to 90%. While the novel material shows nearly 70% of the transparency level of polythene, it is considerably higher than other newly developed bio plastics, such as protein/starch-based materials. Present study used bleaching to achieve the higher transparency of the material, as bleaching process can reduce the lignin content of the ALS sample. Lignin absorbs light and acts as a barrier for transparency, therefore, it is recommended to investigate other effective methods of removing lignin, which could improve the level of transparency in the packaging film.

Conclusion

The present study investigates the feasibility of using ALS as a raw material to develop a sustainable packaging material. The newly developed material shows desirable properties of a packaging material. One of the challenges presents in currently available degradable materials such as degradable polythene is their opacity. The newly developed material achieved a sufficient transparency level to visualize an object contained behind it. Moreover, this packaging material is produced with minimum impact to the environment. The raw material, which is the arecanut leaf sheath, is taken when it is fallen of the tree. The bleaching process and coating were non-toxic resulting in a readily degradable material. Therefore, this is most suitable for single use, disposable packaging applications such as food, small size clothing and other lightweight items. Further experiments to improve the mechanical properties of the material are required before this concept can be commercialized. Therefore, a feasible method to combine multiple sheets has also to be investigated.

Reference:

Dissanayake, D.G.K., Weerasinghe, D., Perera, T.D.R., Bandara, M.M.A.L., Thathsara, S.K.T. and Perera, S., 2021. A sustainable transparent packaging material from the arecanut leaf sheath. Waste and Biomass Valorization, 12(10), pp.5725-5742.