Feasibility of producing biodegradable disposable paper cup from pineapple peels, orange peels and Mauritian hemp leaves with beeswax coating

16 Mar.,2023


The company has a group of cooperation teams engaged in the Bagasse Coffee Cups industry for many years, with dedication, innovation spirit and service awareness, and has established a sound quality control and management system to ensure product quality.


Physicochemical characterization of raw biomass

The raw materials used for paper cup forming were characterized in terms of dry matter (total dry solids %), moisture, ash, lignin, cellulose and hemicellulose contents as given in Table 1.

Table 1 Physicochemical properties of raw biomass used

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The Mauritian hemp had the highest moisture content in comparison with pineapple and orange peels. This might be because the hemp leaves also contain a sticky, irritating saponins-containing juice present in the sap (which causes foaming) besides moisture in the plant cell structure [21].

Hemp leaves were also observed to have the highest cellulose content, followed by the pineapple peels. Thus, pulp yield from hemp leaves would be greater since more cellulosic fibers can be recuperated. Higher cellulose content also contributes to the fibers having higher tensile strength [22], which are desirable properties for paper cups. In addition, hemp leaves fiber is longer than that of pineapple peels and orange peels, thereby contributing further to its tensile properties. On the other hand, pineapple peels, with a higher cellulose content than orange peels, have a higher pulp yield and better tensile properties than the latter.

Pineapple peels were found to contain the highest amount of hemicellulose as compared to orange peels and hemp leaves. Despite a higher hemicellulose content indicates that the fiber is highly amorphous with reduced fiber strength [22], it was observed that orange peels contained more hemicellulose than cellulose, while pineapple peels and hemp leaves contained more cellulose than hemicellulose. In this respect, orange peels fiber was inferred to have the lowest fiber strength and mechanical properties, while hemp leaves fibers were considered to have the highest one. Since hemicellulose are easily degraded at high alkali charges, they are easily dissolved away during soda pulping, as a result of which the pulp yield is lower for biomasses having high hemicellulose content.

To produce high-grade paper cups, the lignin content, which is an undesirable polymer in paper making, should be as low as possible [23]. Its removal during pulping requires high amounts of chemicals and energy [24]. In addition, a higher lignin content implies that the cellulosic fibers are bonded more tightly together [25]. This indicates, therefore, that it is more difficult, energy intensive and costly to remove lignin from hemp leaves than from the fruit peels.


Appearance and structure test

Visual inspection of the formed cups is an important step to check for conformity according to GB 18006 (2008) [16]. It gives quality assurance of the product in terms of service performance. In this respect, the color, edge, stability of cup base, texture and cracks were analyzed. The molded paper cups in different ratios are shown in Figs. 2 and 3.

Fig. 2

Molded paper cup-making process

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Fig. 3

Beeswax coating of composite paper cup

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The color, edge and stability of the cup sample and the presence of grease/dirt/dust on its surface were qualitatively analyzed and reported as shown in Table 2.

Table 2 Word codes assigned to each property of a molded cup to check for conformance

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For the inspection of texture and cracks of the paper cups, scores were assigned as given in Table 3 in order to quantitatively compare the different cup compositions.

Table 3 Scores allocated to different grades of texture and number of cracks

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The results for the appearance and structure tests are given in Table 4.

Table 4 Appearance and structure test results

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As per the GB 18006 (2008) standard [16], the color of the cup should be normal, with no discoloration or stains except for decoration purposes. The control cup had colored patterns for decoration purposes, and hence, its color was considered normal. All molded composite cups had a normal, uniform color with no stains. The color of paper cups from both pineapple and orange peels composite was also found to be significantly paler as compared to the original color of the peels and hemp leaves. This can be explained by the degradation of pigments found in the peels at the high temperature as well as the removal of lignin during the pulping process. Similar phenomenon was observed by Park et al. [26], who reported the color of composite boards from cranberry pomace to change due to degradation of red pigments at high molding temperature.

No grease, dirt or dust was present on any cup, and all cups had a stable base. The edges of the cups were trimmed so that they were smooth, tidy and clean. Hence, there was no problem with regard to the initial visual appearance of the paper cups. In terms of texture, however, it was observed that the general trend was a smoother cup with increasing percentage of pineapple and orange peels from 20 to 80%. This might be because the fruits’ pulps were smaller in size, which upon blending, became finer, thereby resulting in a smoother cup. The Mauritian hemp pulp, however, was coarser. Hence, as its percentage in the mix ratio increased, the cup became less smooth.

This observation was in line with that of Gouw et al. [10], who reported coarser packaging board because of coarser newspaper fibers as compared to fruit pomace fibers. Iewkittayakorn et al. [27] also observed that a coarse texture was obtained with pineapple leaves pulp with leaf residues showing even after compression molding. Increasing the cooking time to 180 min chemical pulping time would produce paper with fine structure and no residues of pineapple leaves. However, in this study, 90 min was sufficient to obtain fine fruit peels fibers for a smooth texture without compression, which can be advantageous in terms of processing parameters and heat energy requirement on an industrial scale.

With increasing peels pulp percentage, however, the number of cracks increased for the orange peels. This is because the orange peels fibers are shorter and weaker and tend to rip apart during the drying process. The paper cups cracked at 80% and 100% orange peels. For the pineapple peels, the cup containing the highest percentage of hemp pulp (80% hemp) cracked despite the hemp fibers are stronger than the orange peel and pineapple peel fiber.

This is because the hemp fibers were grinded, thereby shortening the length. Since short fibers are weaker than long fibers [23, 28,29,30], the hemp fibers in the mix ratio 80:20 could not bind properly. There were also more voids in between the fibers since less finer pineapple fibers were present to interlock with the hemp fibers. Since paper cups should contain no cracks as per the GB 18006 (2008) standard [16], those having cracks were considered unfit for purpose and were therefore eliminated. They were not used for subsequent tests, which require cup samples having no cracks.


Drop test

Sample cups with no cracks were dropped from a height of 0.8 m and were thereafter analyzed for cracks or splits after impacting on a level cement floor as given in Table 5.

Table 5 Crack analysis of composite cups after drop test

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All molded cups remained intact, with no deformation, cracks or split after the drop test as the control cup, except the 60:40 and 80:20 orange composite cups. The drop test gives an early indication of the strength properties of the molded cups. Those which cracked after the drop test shows that no big force was necessary to rupture the bonds between the lignocellulosic fibers. Since the GB 18006 (2008) standard [16] states that the cups should not crack after the drop test to be fit for purpose, they were therefore viewed as not strong enough to be used as drinking cups. These two composite cups were therefore eliminated.


Weight load test

Those cups conformant to the drop test were then subjected to a 3 kg load for the weight load test as given in Table 6, where w = the variation rate of the weight load of the sample (%), h0 = the height before weight is loaded (mm), h = the height after weight is loaded (mm).

Table 6 Variation rate of weight load test results on molded cup samples

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This test assesses the load-bearing performance of the cups, that is, the ability of the cups to hold the liquid load while being held in the hands of the user without being subjected to any deformation.

The variation rate of weight load, which represents the change in height of the cup due to compression by the 3 kg load, decreased to 0% at 60% pineapple peels fiber and increased to a maximum of 2.72% at 80% pineapple peels fiber (Table 6). This might imply that the 60:40 hemp:pineapple composite cup provides the best combination of the finer pineapple and coarser hemp fibers, which binds together sufficiently to result in a high-strength, high-bulk-density paper cup. The 60:40 mix ratio had the same variation rate of weight load of 0% as the control cup, whereas the remaining two compositions were subjected to a significant increase in variation rate of weight load. In order to justify the ultimate cup composition, however, the tensile and burst strength tests were performed.


Thickness determination

The average thickness of cup specimens conforming to the drop and weight load tests is given in Table 7.

Table 7 Average thickness of cup specimen which passed the drop and weight load tests

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The composite cups were observed to have a higher average value for thickness (all above 1.00 mm) than the control cup, with the pineapple peels composite cups have a thickness value closer to the control cup. The cup molding process involved no compression of the fibers, as a result of which there was no significant degree of compaction. The control cup, on the other part, was well compacted in the form of sheets prior to making the cup. Hence, the thickness of the cup specimens was higher. This observation is supported by the study Saxena et al. [31], who found that the maximum thickness of paper bottle formed through vacuum molding was 1.969 mm.

The thickness of the orange peels composite cups was found to be higher than that of the pineapple peels composite. This implies that more pulp was required to form one cup of the same capacity using the orange peels composite than the pineapple peels composite. Using the compression molding technique would result in a lower thickness for the same amount of pulp used since the bulk density would have increased due to compaction. Nevertheless, while compression molding resolves the problem of uneven fiber distribution and hence, uneven thickness, the study conducted by Iewkittayakorn et al. [27] revealed that more pulp would be required to obtain a compressed product without cracks as compared to a vacuum-molded one. This may have implications on the cost of raw material needed. As such, improving the vacuum molding process to enable the obtention of a product of even thickness could be a potential area of study.


Burst strength test

The bursting strength test indicates the ability of the paper cup to resist a rupture force applied perpendicular to it. It also provides an indication of the distribution matrix of the fibers within the paper cup. The distribution of fibers is usually affected by the fiber quality and length, addition of surface additives, blending time and the method of preparation [32]. The values of burst index (B) of the paper cups produced are recorded in Table 8.

Table 8 Recorded pressure values for burst test and corresponding burst index B

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The burst index reached a maximum at 60% pineapple peels fiber (Table 8), indicating that the addition of 40% hemp in the pulp mix produced the most burst-resistant paper cup, which needs 75.2 kPa to burst. The burst index increased as the percentage of pineapple fiber increased from 40 to 60%, but decreased significantly at 80% pineapple peels fiber. This decrease might be attributed to the uneven distribution of the pineapple peel fibers in the paper cup during the molding process or lower bonding strength between the fibers. The burst index for the orange peels composite cup was lowest compared to all pineapple peels composites, thereby implying that a much smaller pressure is needed to burst the hemp to orange composite cup specimen. For the same mix ratio of 40% peels and 60% hemp, the burst index was 0.08 kPa m2/g using orange peels, while it was 0.25 kPa.m2/g using pineapple peels. This reveals that the orange peels fibers have lower fiber and inter-fiber bonding strength than the pineapple peels fibers.

Therefore, inter-fiber bonding, which was controlled by the pulp density and porosity, influences the paper cups strength. This explains the immediate fracture of the material after maximum bursting strength instead of undergoing further plastic deformation with lower stress rate. The crack formation also does not exhibit any specific direction [33]. In addition, the fiber quality and length, which depends on its preparation method, pre-treatment options (grinding or refining) and/or blending time, affect the burst strength of the sample since shorter fiber length and lower fiber quality results in a lower burst index and vice versa.


Tensile strength test

The tensile strength test indicates the ability of the cup sample to resist a rupture force applied parallel to it. The mean tensile properties obtained for each test specimen (Table 9) were ultimately used to calculate the tensile index.

Table 9 Mean tensile properties of paper cup specimen

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The trend of tensile index was observed to be similar to that of the bursting index with respect to the percentage of pineapple peels fiber (Fig. 4). The tensile index reached a maximum at 60% pineapple peels fiber. The maximum tensile index may be the result of stronger interfacial bonding, lower lignin content, higher cellulose content, better proportion of hemp–pineapple fiber mix and lower micro-fibrillar angle present in the pineapple fibers [22].

Fig. 4

Test sequence to find the ultimate cup composition

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The tensile index increased as the percentage of pineapple fiber increased from 40 to 60%, but decreased significantly at 80% pineapple peels fiber. This decrease might be attributed to the irregular scattering of the pineapple peel fibers in the paper cup during the molding process or lower bonding strength between the fibers.

The tensile index for the orange peels composite cup was lower than all the pineapple peels composites, except for the 80:20 hemp–pineapple peels one. This implies that the 40:60 hemp–orange peels composite has a higher fiber and bonding strength than the 80:20 hemp–pineapple peels cup. For the same mix ratio of 40% peels and 60% hemp, the tensile index was 0.13 Nm/g using orange peels, while it was 3.30 using pineapple peels. This reveals that the orange peels fibers have lower fiber and bonding strength than the pineapple peels fibers at this mix ratio. The tensile index of the 40:60 hemp–pineapple peels composite was also found to be roughly 8 times lower than the control cup, which has a tensile index of 26.1 Nm/g.

The 40:60 hemp–pineapple peels composite cup therefore provided the best combination of variation rate of weight load, burst and tensile strength properties, which were nearer to those of the control cup. It was thus chosen as the ultimate cup composition for waterproof coating.


Water leakage test

In order for the paper cup to be fit for purpose, the waterproof material should be such that it prevents water or any other liquid from leaking. Therefore, the beeswax coating thickness was varied to test the water resistance of the cup.

The logical trend observed from Fig. 5 is an increase in the time the cup takes to leak with increasing beeswax coating thickness. At low thickness coating, it was observed that the amount of lipids in beeswax was not enough to prevent the water from penetrating the fiber network and the paper cup would leak. At 0.7 mm coating thickness, the cup held water at the time specified by the GB 18006 (2008) standard (30 min). The thickness of beeswax coating was thus limited to 0.7 mm since beyond this value, the wax remained as solidified lumps on the cup surface. This is because the cup was already saturated with the melted wax, which filled all pores of the cup.

Fig. 5

100% pineapple peels paper cup (left) and 100% orange peels paper cup (right)

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While biodegradable paper cups are usually coated with polylactic acid as substitute to polyethylene for waterproofing [34], using beeswax presents several advantages. Beeswax is considered food grade material since it is used as additives in pharmaceuticals and foods [27]. It is also rich in lipids, which can enhance water repellence in a coating due to their strong hydrophobicity [27]. It was in fact more water resistant than the sodium alginate–gellan composite used by Zhang et al. [35] to bio-coat paper cups for hot drinks, that has calcium ions as a crosslinking agent and plasticizer to enhance water resistance. However, beeswax does not contribute to added mechanical strength [27, 35] and cannot sustain hot liquids due to its low melting point of 60–70 °C [36]. As such, the coating was applied to both sides of the cup since the latter is intended to be used as a cold cup.


Biodegradability test results

The rate at which a substance biodegrade depends on several factors, including moisture content, temperature, nutrients and microorganisms content of the medium in which it is buried, as well as the size, surface area and inherent biodegradability potential of the substance itself. In this study, the biodegradation of 5 × 5 cm samples occurred in two different environments, namely active soil and damp sand, at a measured average temperature of 34 °C and average pH of 7.4 for the seawater used to keep the sand medium damp. The decrease in mass of the samples after each week was recorded and thereafter plotted as in Fig. 6.

Fig. 6

a Hemp:pineapple peels composite paper cups and b hemp–orange peels composite paper cups in the ratio 20:80, 40:60, 60:40 and 80:20 (from left to right)

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The control cup is observed to biodegrade at a slower rate as compared to the composite cup in both environments. The control cup has a polyethylene coating which delays the biodegradation process of the paper part of the cup. Since the coating is on one side only, the surface area exposed to the soil and sand environment for biodegradation is one-sided. After the 4 weeks period, the polyethylene film detached itself from the paper cup exposed to the damp sand environment. The film was, however, intact after 4 weeks, which implies that the film will take a much longer time to degrade, potentially into microplastics. The rate of biodegradation of the paper control cup is therefore expected to increase after the 4 weeks period (Figs. 7, 8).

Fig. 7

Variation of mean burst and tensile indexes with pineapple peels fiber

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Fig. 8

Variation of water resistance of 40:60 hemp–pineapple peels cup with coating thickness

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The wax-coated hemp–pineapple composite cup, on the other part, was subjected to a greater loss in mass, since both the waterproof coating and fiber making up the cup are biodegradable. In this respect, it can be inferred that using the composite cup will impact negatively the environment to a lesser extent than when using the control cup. The composite cup disappeared after 5 weeks in soil, whereas traces remained in the damp sand until after 6 weeks. As observed from Fig. 9, the rate of biodegradation of the composite cup is greater in soil than that in sand. This is because the nutrients content required by microorganisms to function was lower in sand than in the active soil, considering that the temperature and moisture were almost same for both environments since they were subjected to the same weather conditions.

Fig. 9

Decrease in mass of cup specimen as it biodegrades for a period of 4 weeks

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