Tomato Plant Residues, a Sustainable Fiber Source for Cardboard Packaging

Abstract

Cardboard packaging is a good example of sustainable material use, yet even more sustainable solutions are available, such as replacing wood fibers with those from alternative sources like agricultural waste. In our research, we produced paperboard from fibers obtained from waste tomato stems using a pilot paper machine, and its basic, surface, and mechanical properties were then determined. Additionally, we produced corrugated cardboard from the tomato fiber-based paperboard and analyzed its performance under different environmental conditions. A comparative analysis was made with commercial corrugated cardboard to determine the differences in mechanical properties and the quality of the prints made using the flexographic printing technique. The results indicated that the properties of tomato fiber-based paperboard and corrugated cardboard are sufficient for many packaging applications. Analyses showed that the mechanical properties of both commercial and corrugated cardboard produced from tomato stems were affected by the environmental conditions, while the change in print quality was minor. Exposure to high humidity had a much greater effect than exposure to low temperature. High humidity lowered the tensile and bursting strength and resistance to compression, resulting in decreased strength and stiffness, while low temperature had a less severe effect. Nevertheless, corrugated cardboard made from tomato-based paperboard can be used for storage of fresh produce locally, as well as for transport packaging, provided the transport box is designed to address its poor stacking strength. The print quality of this cardboard is also suitable for transport packaging, and the readability of the UHF RFID tags remains satisfactory. The findings of this study have significant implications for the development of sustainable materials. The successful use of tomato stems, a waste product from agriculture, in the production of corrugated cardboard supports the circular economy by turning waste into a valuable resource.

1. Introduction

Sustainable development is becoming a fundamental aspect of product design, with a key focus on reducing environmental impacts throughout their life cycle [1]. This requires reflection on improving the product’s efficiency, with an emphasis on environmental protection and the usability of the product, lower consumption of energy and materials per unit of product, the use of safe and harmless substances in the design of the product, planning for recycling (material, decomposition) and re-use possibilities, and returning products to circulation [1,2,3]. When designing short-lifespan products such as packaging, it is particularly important to use materials efficiently, use materials from renewable resources, and design for recycling and reuse [4,5,6]. Packaging made from paper and cardboard is a good example of sustainable material use. One such solution is the replacement of wood fibers with fibers obtained from alternative sources, such as discarded biomass and agricultural residues. Potential sources of fibers for producing paper and cardboard include straw, grasses, reeds, stems, leaves, and seed fibers from annual and perennial plants. Among annual plants, cereal straws and fibrous residues of sugarcane are the most used, followed by reeds and bamboo [7]. An overview of non-wood fibers for paper production is provided by Pydimalla et al. [8] and Sapuan et al. [9].

A comprehensive review of the utilization of agricultural waste residues in various applications, including in paper and paper-based products, was given by Gupta et al. [10] The utilization of abundant agricultural and feedstock residues could lead to the revival of the paper industry in Kenya, as reported by Otieno et al. [11]. It is expected that the use of agricultural waste for pulp and paper making will increase due to economic and environmental factors. In addition to sugarcane bagasse, corn stalks, and rice and wheat straw, different agricultural residues are now being explored by non-wood pulp mills [12]. Creapaper GmbH uses grass fibers as a raw material to produce paper and cardboard. The paper contains 10 to 60% grass fibers and can be used for food packaging, brochures, and labels. Creapaper can be recycled like ordinary paper and is also suitable for composting [13]. Banana fibers, also called musa fibers, are among the strongest natural fibers, similar to bamboo fiber, and can be used in the production of paper and fabrics [14].

Using rice straw to make paper could benefit wood-deficient countries such as China, India, and Bangladesh. This also addresses the problem of rice straw waste, which is currently burned by farms, causing greenhouse gas emissions. Since rice straw contains less lignin compared to wood, it is easier to obtain fibers from it. Difficulties in making paper from this material are caused by its high silica content, which affects the efficiency of paper machines [15]. Wheat straw is an agricultural waste material that is produced in large quantities during the harvest of wheat. Similarly to rice straw, wheat straw also contains a lower proportion of lignin, meaning that less water is used in paper production. The lower density of wheat straw compared to wood fibers results in less energy being needed for their decomposition and pulp bleaching [16]. According to Plazonić et al. [17], straw pulp gained from wheat, barley, and triticale agricultural residues can enhance the strength of paper made from repulped recycled newsprint when added in amounts up to 20%. Román-Gutiérrez et al. [18] reported the viability of barley straw fibers for handmade paper production. The physical and mechanical properties of the sheets indicate that it is feasible to use either barley straw alone or a mixture of recycled wood fibers to produce paper with adequate properties. In a study by Jeetah et al. [19], cardboard was produced by combining rice husk with either waste paper or sugar cane waste residues. The highest tensile strength of the cardboard was obtained with a 20/80 ratio of rice husk to each of the other materials. Due to the lower lignin content of rice husk and sugarcane, bleaching was easier and required fewer chemicals than bleaching other agricultural waste materials and wood. In the research by Rajesh et al., agricultural waste (sugarcane bagasse, banana fibers, and rice husk) was used by utilizing various cellulose ratios to produce handcrafted paper. They found that a blend of 20% rice husk, 30% sugarcane bagasse, and 50% banana fiber gave the best quality of handmade paper [20]. The research by Gonzalo et al. [21] examined the possibility of waste from six different plants to produce pulp and paper. The plants included in the research were bell peppers, chili peppers, broad beans, asparagus, pea stems, and okra. The best mechanical properties were achieved with paper from okra residue fiber, which even surpassed those of commercial corrugated paper. An overview of different types of agricultural residues used in the production of paper was also given by Kumari and Rai [22]. Worku et al. [23] gave an overview of the characteristics of agricultural residues and their pulping process, with an emphasis on trends in the technological pitch of paper production and innovative applications such as membranes.

A few studies reported about utilization of cellulose fibers from tomato plants for different applications. In a review by Jeguirim and Khiari [24], various methods for converting tomato waste through thermochemical techniques are reported. They emphasize the significance of processing huge amounts of tomato waste for use in different recovery processes. In a study by Bascón-Villegas, agricultural plant residues (tomato, pepper, and eggplant) were identified as new sources for lignocellulose nanofibers that could be used as reinforcing agents in the paperboard recycling process [25]. In research by Covino et al. [26], paperboard sheets made from tomato plant agro-waste were thermoformed, without the addition of chemical binders. Enzymatic treatment of pulp substantially improved the tensile strength and stiffness of the paperboard compared to the untreated sample.

Important factors that influence the applicability of cardboard packaging are environmental conditions. Contact with water or moisture can substantially deteriorate the mechanical properties of the cardboard, resulting in irreversible shape distortion [27]. In cold chain logistics, fresh vegetables and fruits are typically transported and stored in high humidity and low temperatures. An overview of climate conditions for storing and transporting different fruits and vegetables was given by Gast [28]. The conditions for storing tomatoes were temperatures between 12.8 and 21.1 °C and relative humidity between 90 and 95%. Under these conditions, tomatoes can be stored for 4–7 days.

Environmental conditions affect not only the mechanical properties of packaging but also the supply chain efficiency. To improve efficiency in supply chain logistics, RFID (Radio Frequency Identification) tags are commonly used. An RFID tag is a small electronic device that uses radio waves to transmit data. A UHF RFID (Ultra-High Frequency RFID) tag is a type of RFID tag that operates in the ultra-high frequency range. When attached to corrugated cardboard packaging, UHF RFID tags allow tracking and identification of products throughout the supply chain. In addition to their use in transportation and logistics, they are also used in retail applications, for inventory control, authentication, and security [29]. Application of UHF RFID offers some advantages, they enable real-time tracking of packages and store detailed information about the product, such as origin, handling instructions, and expiration. By improving supply chain efficiency, RFID tags can reduce spoilage in perishable goods and indirectly contribute to sustainability by minimizing waste. While environmental conditions can impact UHF RFID readability, we tested the tags’ readability after exposure to low temperatures and high humidity.

The objective of this study is to produce and evaluate the performance of paperboard and corrugated cardboard made from agricultural waste, specifically tomato stems, for packaging applications. This study aims to achieve the following:

• Develop corrugated cardboard from tomato fiber-based paperboard.
• Assess its print quality using flexographic printing.
• Integrate UHF RFID tags into the printed corrugated cardboard.
• Investigate the impact of environmental conditions, such as high humidity and low temperature, on the print quality, mechanical properties, and RFID readability of the corrugated cardboard.

2. Materials and Methods

2.1. Materials

A one-layer paperboard was made from a mixture of 75% wood fibers (25% hardwood, 45% bleached softwood and 5% unbleached softwood) and 25% tomato stem fibers. The tomato stems contain 14% lignin, 27% cellulose and 28% hemicellulose. The fibers from tomato plants were obtained from the air-dried biomass plant stems using the soda delignification process. The tomato stems chips are firstly delignified in a digester (reactor) to remove most of the lignin, which is an unwanted component in the paper industry. After cooking, the wood chips are washed and neutralized with water. This is followed by the disintegration process, where the aggregates of fibers are disintegrated into the individual fibers. The following process is screening, where the good fibers are separated from chips and impurities. The obtained fibers can also be milled to the desired length. The main pulp properties are presented in

Table 1. Main properties of fibers obtained from hardwood, softwood and tomato stems.

	Fiber Length [mm]	Fiber Width [µm]	Fines [%]	Fibrillation [%]
Hardwood	0.75	14.9	56.3	1.33
Softwood	1.41	25.3	66.9	1.15
Tomato stems	0.47	24.1	91.3	2.42

. The pulp was prepared together with filler (5% dry mass), cationic starch (0.75% dry mass), retention aid and surface sizing (1% dry mass). Paperboard was produced by the Pulp and Paper Institute, Ljubljana, Slovenia on a pilot paper machine (Andritz AG, Graz, Austria). The entire process is patented, the exact process of fiber preparation and paper production is described in the patent [30].

The tomato fiber-based paperboard served as a main component in the manufacturing of corrugated cardboard, that was produced using a standard production process on an industrial scale. Commercial corrugated cardboards are manufactured from a flat component, a liner and a corrugated component, a fluting, which always have different composition an contain different wood or recycled wood fibers. In our study, a commercially available single-wall E-flute corrugated cardboard from wood-based fibers (Sample 1) was used as reference. The reference was compared to a single-wall corrugated cardboard with an E-flute was made by using tomato fiber-based paperboard as a liner as well as a fluting (Sample 2).

2.2. Methods

As part of the testing, the properties of the paperboard were first evaluated. This was followed by an assessment of the corrugated cardboard under various climate conditions, testing of the print quality, and, finally, the UHF RFID tag’s response and functionality.

2.2.1. Testing of Paperboard

The tomato fiber-based paperboard’s basic, surface and mechanical properties were tested under standard climatic conditions: T = 23 °C and RH = 50% (ISO 187) [31]. The following analyses were performed using standardized testing methods:

• Basic physical properties: grammage (ISO 536) [32], thickness and specific volume (ISO 535) [33], moisture content (ISO 287) [34], ash content (ISO 2144) [35].
• Surface properties: water absorptiveness with Cobb method (ISO 535) [33], surface roughness according to ISO 8791-2 [36], and air permeance (ISO 5636-3) [37] with a Bendtsen roughness and air permeability tester (PTA Group, Paris, France)
• Mechanical properties: tensile strength and breaking strain (ISO 1924-2) [38], bursting strength (ISO 2759) [39], tearing strength using the Elmendorf method (ISO 1974) [40], and folding endurance (ISO 5626) [41]. The ring crush method (RCT) was used for the determination of the edgewise compressive strength of the paperboard according to ISO 12192 [42].

2.2.2. Testing of Corrugated Cardboard

The basic and mechanical properties of the tomato-based corrugated cardboard (Sample 2) and the commercial corrugated cardboard (Sample 1) have been measured under standard climatic conditions. Before the measurements, the samples were conditioned 24 h at T = 23 °C and RH = 50%. The tensile properties of the corrugated cardboard in both directions (with and across the flute) were measured with an Instron 5567 tensile testing machine. The samples were stretched at a rate of 20 mm/min, with continuous recording of the increasing load and elongation until breaking occurred. The elastic modulus, load, strength, strain, and energy at break were determined. Puncture resistance was determined according to ISO 3036 [43] and through a ball-puncture resistance test using a ball with a radius of 2 mm (ISO 12625-9) [44]. The bending resistance was determined with the three-point bending test (ISO 5628) [45]. Mechanical compressive strength tests of corrugated cardboard were determined by measuring edgewise crush resistance (ECT) according to ISO 3037 [46] and flat crush resistance (FCT) according to ISO 3035 [47].

Testing of corrugated cardboard was performed before and after the exposure to different climate. In order to evaluate the influence of environmental conditions on corrugated cardboard’s properties, two sets of samples were exposed to two different climate conditions:

• Low-temperature conditions (6 days at T < 0 °C) in refrigerator;
• High-humidity conditions (6 days at T = 14 °C and RH = 90% in climate chamber Binder KMF (Binder, Germany)).

2.2.3. Testing of Print Quality

Corrugated cardboard was printed using the laboratory IGT F1 printing tester for flexographic inks (IGT Testing Systems, Almere, The Netherlands). Prints were made using commercially available Doneck water-based flexo printing ink. The printing speed was set to 0.6 m/s and the anilox force to 60 N. Printing conditions for both samples are presented in

Table 2. Printing conditions for commercial single-wall corrugated cardboard (Sample 1) and corrugated cardboard made from tomato paperboard (Sample 2).

	Sample 1	Sample 2
		Condition 1	Condition 2	Condition 3	Condition 4	Condition 5
Ink volume (mL/m2)	8.5	4	4	4	8.5	8.5
Printing force (N)	90	90	100	125	150	175

, where Sample 1 achieved the best printing quality when a printing force of 90 N and ink transfer volume of 8.5 mL/m² were used. To determine the best printing conditions for Sample 2, different printing conditions were used.

The color testing chart, featuring a cyan color field with 100% color application intensity, as well as lines and letters, was created using Adobe Photoshop 2024™ (Version 25.0) image software. Lines of different thicknesses were printed as positive and negative images. One widely used typeface, a sans serif typeface, was printed in different sizes, i.e., 8, 10, and 12 pt. This papers presents an analysis of the letters e and g in size 12 pt, in both positive and negative formats, as well as the letter H and lines (positive and negative). An analysis was performed on samples of the same size (800 × 300 pixels) using ImageJ 1.53t software. Before the measurements, the image of prints was taken with the Leica s9i stereo microscope (Leica Microsystems, Wetzlar, Germany). Subsequently, each image was converted into a binary image by the software and the coverage of the surface with printed elements was determined.

The CIELAB values of color print samples were measured according to the ISO 13656 [48] standard using an iOne (X-Rite, Grand Rapids, MI, USA) spectrophotometer with D50 lighting, 45°/0 measurement geometry, 10° standard observer, and a 4 mm measuring aperture. The color difference ∆E*ab was calculated according to Equation (1).

$$∆E^{\ast }ab=\sqrt{{(\mathsf{\Delta }L\ast )}^2+{ (\mathsf{\Delta }a\ast )}^2+{(\mathsf{\Delta }b\ast )}^2}$$

(1)

For the difference calculation, the cyan color values from the color chart attached to the spectrophotometer were used as the standard.

2.2.4. Testing the Readability of UHF RFID Tag

A commercial UHF RFID antenna design was printed on both types of corrugated cardboards (Sample 1 and Sample 2) using silver conductive printing ink (Sun Chemical, Parsippany, NJ, USA). The antenna was printed using a RokuPrint SD 05 semi-automatic screen-printing machine, with a screen mesh density of 70 L/cm (theoretical ink volume around 10 cm³/m²). The printed antenna was dried at 100 °C for 225 s in the drying tunnel to achieve the appropriate conductive properties. The NXP strap chips were glued on the printed antennas using electro-conductive adhesive.

The readability of printed RFID tags was determined using an IDS-R902 reader equipped with a Patch A0025 antenna (Poynting GmbH, Dortmund, Germany) that also measures the strength of the modulated signal backscattered from the tag.

2.2.5. Statistical Analysis

The physical and colorimetric properties were measured according to ISO standards. Following the prescribed procedure in each standard, the number of test specimens was 5 or 10, depending on the requirements of each standard. For evaluation of measured data, descriptive statistics were used—the mean, standard deviation and confidence intervals were determined. Results are displayed as the mean ± standard deviation (SD). The obtained data were evaluated for statistical significance using Student’s t-test. The differences observed between samples were considered significant when p < 0.05.

3. Results and Discussion

While numerous studies have investigated the use of alternative fibers in the production of paper and cardboard, there is a notable gap in research specifically focusing on the production of corrugated cardboard using these alternative fibers. Although companies like DS Smith are conducting trials on converting alternative fiber-based paper into corrugated cardboard, published research detailing the processes, challenges, and outcomes of such applications remains limited. Our study aims to address this gap by manufacturing corrugated cardboard from paperboard produced using fibers obtained from tomato stems. First, we present the properties of the paperboard, followed by the properties of the corrugated cardboard and evaluation of print quality. Another objective in this study was to determine how environmental conditions influence mechanical properties, print quality and readability of UHF RFID tags. The discussion concludes with the implications of our findings for the sustainable use of materials.

3.1. Paperboard Properties

The basic paperboard properties, such as grammage, thickness, density, and moisture and filler contents, are crucial for evaluating the quality and usability of the material for various applications. Kraftliner, a type of paperboard commonly used as the outer layer in corrugated cardboard, typically has a grammage ranging from approximately 120 g/m² to 440 g/m², depending on the specific requirements of the packaging, whereas for the testliner, a recycled liner board, the grammage is in the range of approximately 100 g/m² to 250 g/m².

In

Table 3. Basic and surface properties of tomato fiber-based paperboard, given as the mean values and standard deviation (SD).

Property	Mean Value with SD
Grammage (g/m2)	113.8 ± 1.5
Thickness (µm)	191 ± 4
Density (kg/m3)	595 ± 18
Moisture content (%)	5.7 ± 1.3
Ash content (%)	1.5
Roughness Bendtsen (mL/min)	928 ± 85
Air permeability (mL/min)	1094 ± 56
Cobb value (g/m2)	159 ± 9.4

, basic and surface properties of the tomato fiber-based paperboard are presented. With a grammage of just under 120 g/m², the tested paperboard is considered a lightweight liner and can be used for applications where high strength of packaging is not needed. The thickness of the material also has a significant effect on its stiffness and strength too [49]. A thickness of 191 µm, matches the thickness of both types of commercial lightweight liners. With a density of 595 kg/m³, the material is light and porous, which affects its absorption capacity and mechanical properties. Compared to both types of commercial lightweight liners (kraftliner and testliner), the tested tomato fiber-based paperboard is much less dense and more voluminous. Its low density is also related to the low ash content (1.5%). Fillers are added to paperboard to improve certain properties such as its brightness, smoothness, and printability [50]. The filler content in commercial kraftliner is usually less than 5%, whereas in testliner, it ranges from 10% to 20%, due to balancing cost and surface properties while maintaining acceptable strength. In our case, only a minimal amount of fillers was added (5%) to avoid compromising the strength properties of the tomato fiber-based paperboard. The determined ash content (1.5%) suggests that fillers were lost during the production of paperboard. With a moisture content of 5.7%, the moisture content of tested tomato fiber-based paperboard is at the lower limit of the typical range for moisture content in commercial kraftliners. A lower moisture content results in improved dimensional stability, better printability and an improved shelf life, due to the paperboard being is less prone to mold growth and degradation over time, though it can also lead to brittleness and reduced flexibility [7].

As seen from Table 3, the high air permeability value confirms the high porosity of the tested tomato fiber-based paperboard. A high air permeability in paperboard can enhance ink absorption at printing. Bendtsen roughness measures the smoothness of a material’s surface. With a value of 928 mL/min, the paperboard is quite rough, which can affect its printability too. The standard deviation is also relatively high and may cause variations in surface uniformity and affect print quality. The Cobb value measures the ability of a material to absorb water. The very high Cobb value determined confirms the porosity and high surface absorbency of the tested tomato fiber-based paperboard, suggesting its tendency to absorb more moisture, including ink, consequently influencing its printability and print quality. This also means that it can withstand a certain amount of moisture without losing its mechanical properties.

The mechanical properties of tomato fiber-based paperboard presented in

Table 4. Mechanical properties of tomato fiber-based paperboard in machine- (MD) and cross direction (CD), given as the mean values and standard deviation (SD).

Property	Mean Value with SD (MD/CD)
Tensile index (Nm/g)	32.10 ± 0.20/18.98 ± 0.24
Breaking length (km)	4.72 ± 0.26/2.79 ± 0.31
Breaking strain (%)	1.47 ± 0.15/3.83 ± 0.48
Bursting index (kN m2/g)	1.27 ± 0.04
Tearing index [mN m2/g]	5.05 ± 0.94/5.98 ± 0.94
Folding resistance MIT (2 kg) (no. double folds)	491 ± 213/122 ± 40
RCT (kN/m)	0.78 ± 0.13/0.52 ± 0.16

are influenced by its basic properties. The paperboard has approximately twice the tensile index and breaking length in the longitudinal (MD) direction than in the transverse (CD) direction, while the elongation at break is twice as high in the transverse direction. The tomato fiber-based paperboard has a fairly low burst strength and tear resistance. The cracking and tearing index is lower than in the case of commercial liners. The tomato fiber-based also has a low resistance to folding, especially in the transverse direction. Measured properties presented in Table 4 show that the tensile and bursting strength, as well as the tearing and folding resistance, are within the lower range for testliner paperboard [51,52]. Compared to paperboard made from waste rice husk it has similar bursting index and higher tensile index [19]. The obtained results suggest that the mechanical properties of the tested tomato fiber-based paperboard are sufficient for packaging applications, where moderate strength and rigidity are acceptable. It was reported that paperboard made from invasive plants exhibits similar mechanical properties and has been used in the production of boxes for jewelry and cosmetics [53,54].

Using agricultural residues in papermaking aligns with EU environmental directives and represents a significant step toward a circular economy, in accordance with the UN’s Agenda 2030 and Sustainable Development Goal 12 on Responsible Consumption and Production. According to Dungani et al. [55], agricultural waste represents one of the most significant problems that must be resolved in order to ensure the conservation of the global environment. Utilizing agricultural waste supports the goals of sustainable development within a circular economy [56]. Among other applications, it can be used to produce eco-friendly packaging. The use of agricultural waste fibers for paper and cardboard production is an area of growing interest. Current research in exploring alternatives to traditional wood pulp has examined materials such as rice and wheat straw, corn stalks, and bagasse, among other agricultural residues [12,14,22,56]. The literature overview showed that mainly hand sheets, handmade paper, and lightweight paper, such as newsprint, have been produced from these materials [17,18,20]. Like other reported alternatives to wood, our study demonstrates that tomato stem fibers can be effectively remanufactured into paperboard.

The seasonality of plant cutting and preparation typically has an impact on fiber properties, as environmental conditions can vary throughout the year. However, in this study, the tomato plants used for fiber obtaining were grown in a greenhouse, where environmental factors such as temperature, humidity, and light are controlled. This controlled environment minimizes fluctuations in growing conditions, leading to more consistent plant growth and fiber characteristics. As a result, we assume that the influence of seasonality on the properties of the fibers was minimal, ensuring that the quality and performance of the fibers remained stable throughout the production process.

The current economic viability of producing paperboard from tomato stems is negative, primarily due to the manual labor involved in sorting the biomass and the small production volumes. This project was designed as a practice example for promotional purposes, showcasing the potential of alternative fibers for paper and cardboard. Due to the low level of automation and limited production scale, the costs are significantly higher than standard industrial practices. However, there is potential for a positive economic balance under certain conditions. If the process were automated, particularly in the sorting and preparation of the biomass, labor costs would decrease substantially. Additionally, establishing the necessary infrastructure for larger-scale production would enable higher output, reducing the cost per unit through economies of scale. Moreover, sourcing the tomato stems locally would minimize transportation and logistics costs, further improving the economic outlook. With these improvements—automated processes and increased production volumes—we believe that the economic balance could become positive, making paperboard from alternative fibers economically competitive with traditional wood-based paperboard.

3.2. Corrugated Cardboard Properties

The properties of corrugated cardboard are influenced by the grammage as well as the type of paperboard used for the liners and flutes. The grammage impacts the cardboard’s strength, stiffness and structural integrity. The quality and type of paperboard significantly affect the strength and durability of the corrugated cardboard. The moisture content of the paperboard affects its flexibility, strength, and resistance to deformation. Since both types of corrugated cardboards tested in our study are single-wall boards with E-flute, the grammage of boards is similar, as seen from

Table 5. Basic and mechanical properties of corrugated cardboards given as the mean values with standard deviation, and p-value from t-test.

Property	Sample 1	Sample 2	p-Value
Grammage (g/m2)	369.3 ± 1.4	374 ± 7.2	0.194
Thickness (mm)	1.48 ± 0.006	1.78 ± 0.008	p < 0.001
Density (kg/m3)	249 ± 1	210 ± 4	p < 0.001
Moisture content (%)	5.7 ± 0.3	6.6 ± 0.2	p < 0.001
Ultimate force (N): MD/CD	218 ± 24/204 ± 8	227 ± 17/216 ± 8	0.518/0.043
Breaking strain (%): MD/CD	1.1 ± 0.08/4.17 ± 0.29	1.3 ± 0.14/5.6 ± 0.34	0.007/p < 0.001
Energy at break (mJ): MD/CD	0.25 ± 0.03/1.17 ± 0.12	0.33 ± 0.06/1.62 ± 0.17	0.040/p < 0.001
Elastic modulus (MPa): MD/CD	895 ± 76/568 ± 7	612 ± 39/367 ± 8	p < 0.001/p < 0.001
ECT (kN/m)	3.22 ± 0.63	1.36 ± 0.39	p < 0.001
FCT (kPa)	569 ± 113	302 ± 13	p < 0.001
Puncture resistance (J)	2.20 ± 0.07	2.22 ± 0.04	0.652
Ball-puncture force (N)	36.1 ± 3.11	32.5 ± 1.78	0.056
Bursting strength (kPa)	220 ± 35.8	310 ± 64.9	0.026
Bending resistance (mN)	8975 ± 166/5938 ± 664	6810 ± 667/4289 ± 483	p < 0.001/p < 0.001

. This is also confirmed by the p-value (p = 0.194), which is higher than 0.05, indicating that the null hypothesis, stating that there is no significant difference in the mean values, cannot be rejected. A larger standard deviation of Sample 2 suggests more variability in its weight. Tomato-based corrugated cardboard (Sample 2) has a higher thickness and lower density (around 15%) resulting from bulkier and thicker tomato fiber-based paperboard liner. The higher moisture content of Sample 2 is also connected to the more porous structure of paperboard and the properties of fibers, which are shorter and thicker compared to wood fibers, as well as much higher content of fines, as seen from Table 1. The characteristics of fibers and their influence on paper properties are more thoroughly presented in [57]. An independent two-sample t-test was conducted to compare the mean values of properties between Sample 1 and Sample 2. The assumption of equal variances was tested and not violated, so the t-test for equal variances was applied. The results showed a significant difference in the mean values between Sample 1 and Sample 2 for basic properties (thickness, density, and moisture content), with p-values less than 0.05 (Table 5), confirming a significant difference between samples.

The tensile strength of both tested corrugated cardboards is very similar in both directions, as seen from the measured ultimate force in Table 5. The machine direction (MD) in corrugated cardboard is perpendicular to the main axis of the fluting and parallel to the paperboard fiber alignment, whilst the cross direction (CD) is parallel to the fluting. Tomato-based corrugated cardboard (Sample 2) exhibits higher bursting strength, suggesting that it is more resistant to pressure and internal stresses. Sample 2 also has a slightly higher breaking strain, indicating that it can stretch more before breaking, particularly in the corrugated direction (CD). Consequently, it also has slightly higher energy at break, meaning that it can absorb more energy before failing, indicating better toughness. Commercial corrugated cardboard (Sample 1) has an elastic modulus more than 30% higher in both directions, indicating greater stiffness and less deformation under stress, which was confirmed by the results of compression and bending resistance testing. Sample 1 exhibits significantly higher edgewise crush resistance (ECT), indicating better stacking strength and resistance to compression along the edges. It also has a higher flat crush strength (FCT), suggesting that it can better withstand forces perpendicular to its surface. Higher bending resistance in both directions indicates better stiffness and resistance to bending. Both samples have similar puncture resistance, as seen from Table 5, which means that they have comparable performance in resisting penetration by sharp objects, whereas Sample 1 shows slightly higher ball-puncture resistance (around 10%) meaning slightly better puncture resistance to small, rounded objects. A significant difference in the mean values between Sample 1 and Sample 2 for most mechanical properties was determined. The p-value is less than 0.05, suggesting that the alternative hypothesis is supported, confirming the difference in mechanical properties between corrugated cardboards. The results show that properties of tomato-based corrugated cardboard are in the range of corrugated cardboards made from recycled fibers, though it has inferior mechanical properties compared to commercial one when exposed to compression and bending. Our findings suggest that tomato-based corrugated cardboard has adequate mechanical properties for potential packaging applications, particularly where use of sustainable material is prioritized over the mechanical performance of packaging under compression stress. In addition to using fibers obtained from agricultural waste, the sustainability of the material is further demonstrated through the composition of the paperboard. Unlike commercial products, it is made with fewer functional additives and in lower quantities. Tomato-based corrugated cardboard is produced without bleaching, using fewer chemicals and less energy for fiber preparation.

3.3. Assessment of Print Quality

The conditions used for printing are presented in Table 2. By changing the printing force and volume of applied ink for tomato-based corrugated cardboard (Sample 2), we wanted to find a combination that would ensure a similar print quality to commercial corrugated cardboard (Sample 1). Print quality was evaluated subjectively through visual assessment and objectively via image analysis of different printed elements. Two letters evaluated were e and g at 12 pt size (negative and positive letters), along with the letter H (italic), lines, and a full-color field. In Figure 1, the captured image of the evaluated elements is shown.

The binary images of the printed elements served as the basis for measuring the surface coverage of prints, which are presented in

Table 6. Surface coverage of printed elements, expressed in percentages: reference (PDF of printing form), commercial corrugated cardboard (Sample 1), and tomato-based corrugated cardboard (Sample 2) printed at different conditions.

Printed Element	Reference	Sample 1	Sample 2
			Condition 1	Condition 2	Condition 3	Condition 4	Condition 5
Letter e—positive	46.7	54.4	47.0	49.3	49.6	59.7	53.8
Letter e—negative	50.0	42.9	47.2	52.1	46.6	47.9	50.4
Letter g—positive	47.2	53.0	46.0	46.7	48.1	50.0	52.3
Letter g—negative	48.2	42.5	42.2	51.9	51.2	51.2	48.2
Letter H	61.6	65.1	50.5	55.9	54.7	53.1	52.5
Line—positive	22.7	25.2	27.5	22.7	22.7	23.3	22.7
Line—negative	58.9	56.5	55.4	49.9	48.9	51.8	52.7
Full-color field	100	92.7	84.4	88.3	90.4	90.9	92.1

. As a reference, the surface coverage of the digitally prepared elements in the PDF of the printing form was used. Printed surface coverage depends on the substrate properties, ink viscosity and printing pressure. The resolution and screening techniques used in flexography may differ from the digital reference (PDF). This can cause discrepancies in how fine details and gradients are reproduced, potentially increasing the perceived coverage. It is seen from Table 5 that the surface coverage of printed elements at Sample 1 is higher than reference PDF of the printing form due to dot gain, which is influenced by the properties of substrate. On a less absorbent substrate, ink can spread more, causing the printed area to expand beyond the intended boundaries, increasing the perceived surface coverage.

The results of image analysis have shown that the surface coverage of printed elements is higher when printed on commercial corrugated cardboard (Sample 1) except at negatives of letters e and g, where, as expected, it was lowest. Based on the element, the difference ranges from 20% to a few percent, depending on the printing conditions. Printing Condition 5, with the highest printing force of 175 N, resembles most (letter e, g positive, full-color field) in the case of lines (positive) that was Printing Condition 4 (150 N). As both of the maximum printing forces applied resulted in excessive compression of the corrugated cardboard and crushing of the corrugated middle layer, they cannot be applied. For negative letters and letter H Printing Condition 2, with a printing force of 100 N, gave the best results. Compared to the reference digital elements of the printing form, the match in surface coverage was again dependent on the printing conditions. However, the best match in most cases was achieved with Printing Condition 2.

The visual evaluation confirmed that the quality of print is better on the commercial corrugated cardboard. The print on the tomato-based corrugated cardboard is more uneven, the ink is missing in between the letters, breaks occur for thin lines and the filling is less even for wider lines, lines are not sharp, and bleeding occurs. In the image of the negative lines print, there is an overlap, and the two thinnest lines are only partially visible. Uneven color coverage is even more visible in the full-color field.

The reduced printability of tomato-based corrugated cardboard is connected to the high porosity and high surface absorbency of tomato fiber-based paperboard. High air permeability and excessive moisture absorption contributed to dot gain, where printed dots spread more than intended. This resulted in reduced image sharpness, loss of detail, and lower color accuracy, causing unevenness of prints. High paperboard roughness affected the wicking and bleeding of the printed letters and lines, and the sharpness of the edges of the letter images was reduced. Similar conclusions were obtained by other studies dealing with ink paper interactions [57,58,59].

The color of the corrugated cardboard plays an important role in determining the final print quality. Inks are translucent to some extent, and the color of the corrugated cardboard can influence the final printed color. White cardboard provides a neutral background, allowing printed colors to appear more vibrant. As seen in Figure 2, the color of both types of corrugated cardboards differs: commercial cardboard is white, whereas tomato-based paperboard is yellowish.

The colorimetric properties of a print represent an important criterion of print quality. When determining the color differences, the cyan color values from the color chart were used as a standard. The measured L*a*b* values show that all prints are noticeably redder compared to the standard, with most also exhibiting a bluer hue (

Table 7. Colorimetric values of the cyan full-color field and color difference to ideal cyan ink (L* = 46.2, a* = −27.7, b* = −24.8).

Value	Sample 1	Sample 2
		Condition 1	Condition 2	Condition 3	Condition 4	Condition 5
L*	39.5	40.9	46.8	44.7	40.6	42.1
a*	4.9	1.1	0.3	0.4	1.3	0.5
b*	−43.3	−33.8	−26.8	−29.0	−33.3	−32.9
∆E*ab—ink	38.1	30.7	28.1	28.5	30.7	29.7
∆E*ab—Sample 1	/	10.3	18.6	15.9	10.9	11.6

). The prints are generally slightly darker than the standard, except for the print made on tomato-based corrugated cardboard with a printing force of 100 N, which is lighter. The largest color difference (ΔE*ab) to the reference (cyan ink) is seen at print on commercial corrugated cardboard. The color difference on tomato-based corrugated cardboard is smaller, the smallest being for Printing Condition 2 (printing force of 100 N). From Table 6. it is evident that different printing conditions have only a minor influence on ΔE*ab. The highest difference is 2.6 units. The color difference is visible, but small and only perceptible under close observation; it is thus, acceptable in most printing standards [60,61]. The difference of 2.6 is below the commonly accepted perceptibility threshold of 3 and is only noticeable upon close inspection. In the ISO 12647-2:2013 standard, the color tolerance (∆E*ab) for CMYK solids for production print was set to 4 and in the ISO 12647-7:2016 proofing standard (∆E*00), the tolerance (∆E*ab) is up to 3 [62,63]. High color difference ΔE*ab between prints on both substrates (∆E*ab—Sample 1 in Table 7), confirms that the influence of paperboard color on print quality is evident. The results from t-test confirmed the significant difference between Sample 1 and Sample 2, with p-value being much less than 0.05. A very large color difference is seen in paperboards with distinctive shades and flexographic printing, which are absorbent, especially in paperboards that are not coated.

The issue of print quality observed in tomato-based corrugated cardboard, due to its high porosity, absorbency, and yellowish color, differs from the more uniform printing results typically achieved with white commercial corrugated cardboard. This challenge is consistent with the experiences reported in other studies using non-wood fibers, where high absorbency often leads to dot gain and reduced image sharpness [64,65]. Uneven print quality could limit the material’s appeal in applications where aesthetics are essential. In this case, the development of surface treatments or coatings that reduce porosity and improve ink adhesion may be necessary to enhance the printability of these sustainable materials, thereby enhancing the visual quality of the packaging. When visual quality is less important (e.g., in transport boxes), the print quality is acceptable, and the yellowish color of the packaging provides a ‘natural look’, which customers often associate with eco-friendly materials [66].

3.4. Influence of Environmental Conditions on Corrugated Cardboard Basic and Mechanical Properties

How corrugated cardboard is handled, stored, and transported can impact its performance. Stacking, compressing, and exposing it to varying humidity and temperature conditions can affect the cardboard’s strength and dimensional stability and lead to deformation and reduced strength [27,67,68,69]. In our research, both types of corrugated cardboards were exposed to high humidity (90% RH at 14 °C) and low temperature (below 0 °C) conditions. Figure 3 illustrates the difference in basic and mechanical properties, expressed as percentage changes compared to standard climatic conditions.

Corrugated cardboard is sensitive to environmental moisture, which can significantly alter its physical properties. High-humidity conditions drastically increase the moisture content in cardboards, as expected (Figure 3a). Low-temperature conditions led to a slight increase for commercial corrugated cardboard (around 6%), possibly due to the presence of some moisture in the low-temperature environment, whereas for tomato-based corrugated cardboard, a decrease of 6% was determined. Consequently, due to higher moisture content in cardboard, the grammage, thickness, and density increased. High humidity increased the grammage and density of both samples, due to the absorption of moisture, which added to the mass. Low temperature has a minor effect; the change is in the range of −0.5 to −1.6%. Both high humidity and low-temperature conditions cause a minor increase in thickness. The increase is more pronounced in high humidity, indicating that moisture enters the voids in the cardboard structure and causes fibers to swell.

The change in basic properties of corrugated cardboard influences its mechanical properties, as seen in Figure 3. High humidity reduces the puncture resistance and ball-puncture strength for both samples (in the range from −5% to −13%), indicating a weakening of the cardboard’s resistance to puncture. Low temperature has a minimal impact on puncture resistance, with almost no change observed, and even increases the ball-puncture strength, suggesting an improvement in resistance to bursting when a small, rounded object (diameter = 2 mm) acts with the increasing compression force on the cardboard. As seen from Figure 3b, the impact of high humidity and low temperature on the bursting strength is different for commercial and tomato-based corrugated cardboard. While Sample 1 shows a significant increase, especially after being exposed to high humidity, Sample 2 shows a substantial decrease. The absorption of moisture reduces the brittleness, enhances the deformability and flexibility of cardboard and improves the bonding between fibers due to the presence of water, which results in strengthening of the overall fiber network and an increase in the bursting strength, which could be the case with commercial corrugated cardboard. On the other hand, the absorption of moisture increases the bulk and porosity and can cause the hydrogen bonds between fibers to weaken. Fibers swell and lose their stiffness; water also reduces the internal friction between fibers, which helps resist deformation, and by interfering with hydrogen bonds between fibers, weakens the overall structure of the cardboard, making it more prone to deformation and bursting under pressure [70]. Tomato-based corrugated cardboard contains shorter, more fibrillated fibers and higher percentage of fines, it is thicker, less dense, and much bulkier than commercial corrugated cardboard. Consequently, it can absorb more water, which could explain the reduction in bursting strength after exposure to high humidity and low temperature.

The influence of environmental conditions on the tensile properties of corrugated cardboards is shown in Figure 3c,d. The changes from the standard conditions to high humidity and low temperature affect the ultimate force, breaking strain, elastic modulus, and energy at break of corrugated cardboard in both directions (MD) and (CD). The environmental conditions, particularly high humidity, significantly affect the tensile properties of corrugated cardboard. High humidity weakens the cardboard, which is seen from the decreased ultimate force and elastic modulus, but also increases its flexibility (higher breaking strain). The change is high, from 20 to 50%, indicating a high loss of strength and stiffness. On the other hand, low temperature has a minor impact, with a slight decrease in tensile force, breaking strain, elastic modulus, and energy values.

A paired t-test was conducted to compare the mean values of the determined properties of corrugated cardboard before and after exposure to different environmental conditions. The results indicated a statistically significant change in properties when samples were exposed to high humidity, with the p-value below the 0.05 significance level, allowing us to reject the null hypothesis, as seen from

Table 8. p-values for paired t-test determined after exposure of corrugated cardboards to different environmental conditions.

	High Humidity		Low Temperature
Property	Sample 1	Sample 2	Sample 1	Sample 2
Grammage (g/m2)	p < 0.001	p < 0.001	0.100	0.351
Thickness (mm)	p < 0.001	p < 0.001	0.014	0.226
Density (kg/m3)	p < 0.001	p < 0.001	0.006	0.482
Moisture content (%)	p < 0.001	p < 0.001	0.017	0.002
Ultimate force (N): MD/CD	0.013/p < 0.001	p < 0.001/p < 0.001	0.434/0.131	0.118/0.339
Breaking strain (%): MD/CD	p < 0.001/p < 0.001	0.002/p < 0.001	0.161/0.045	0.061/0.076
Energy at break (mJ): MD/CD	0.012/0.004	0.014/0.003	0.178/0.289	0.061/0.162
Elastic modulus (MPa): MD/CD	p < 0.001/p < 0.001	p < 0.001/p < 0.001	0.398/0.050	0.496/p < 0.001
FCT (kPa)	/	/	0.442	0.472
Puncture resistance (J)	0.004	0.017	0.500	0.380
Ball-puncture force (N)	0.104	0.055	0.103	0.001
Bursting strength (kPa)	0.002	0.025	0.126	0.105

. These findings suggest that exposure to high humidity had a significant effect on both the basic and mechanical properties of the samples. Conversely, when both samples of corrugated cardboard were exposed to low temperatures in a refrigerator, the p-value for mechanical properties was greater than 0.05, indicating that the null hypothesis—stating that there is no effect of exposure on properties—cannot be rejected.

The loss of strength and stiffness make corrugated cardboard less capable of withstanding compressive forces, both edgewise and perpendicular, when exposed to high-humidity environments. The edgewise crush resistance (ECT) and flat crush strength (FCT) could not be determined for either sample. When exposed to compression at ECT measurement, the test samples were immediately squeezed and folded; when FCT was measured, the corrugated part of the cardboard was immediately crushed and flattened. In the bending resistance test, cardboard offered no resistance, and the test specimen immediately folded. Because of an overall reduction in structural integrity at high humidity, the stacking strength and resistance to compression along the edges of corrugated cardboard and perpendicular to its surface are lowered substantially. This could imply that the stacking strength of tomato-based corrugated cardboard is not sufficient to give enough strength for transport packaging in these extreme environmental conditions (storage at high humidity). These issues could be overcome with proper design of packaging that can facilitate logistical handling at high humidity, while still protecting the produce from mechanical damage or with an additional surface moisture barrier coating to prevent moisture absorption.

Corrugated cardboard exposed to low temperatures behaved differently under surface compression and bending forces. For both samples, the flat crush strength (FCT) remained unchanged, while the bending resistance decreased more than 20% for commercial corrugated cardboard and 10% for tomato-based corrugated cardboard. Low temperatures have a minimal impact, causing only a slight decrease in tensile properties and bending resistance of corrugated cardboard, suggesting its suitable for transport packaging in cold chain environment.

The implications of findings obtained in this study for the development of sustainable materials are significant. The successful use of tomato stems, a waste product from agriculture, in the production of corrugated cardboard supports the circular economy by turning waste into a valuable resource. This contributes to reducing the environmental impact associated with both waste disposal and the use of virgin materials. Furthermore, the specific characteristics of tomato-based corrugated cardboard suggest its suitability for certain types of packaging, particularly where lower compression strength and rigidity are acceptable. This could lead to its adoption in niche markets, where sustainability is prioritized over mechanical performance. The results of this study are very important for the company, a local tomato producer. The company is highly focused on sustainability, utilizing organic farming, geothermal energy for growing, and rainwater for irrigation. By using paper and cardboard packaging made from waste tomato stems to store and transport tomatoes, the company will further enhance its sustainability efforts by embracing a circular economy. Packaging from corrugated cardboard is suitable for storing and transporting tomatoes to nearby shops and tourist farms. Distributing tomatoes locally reduces the distance food travels from farm to table, decreasing greenhouse gas emissions and energy use associated with transportation and refrigeration as well as supporting the local economy, which is a key aspect of sustainability.

3.5. Influence of Environmental Conditions on Print Quality

To evaluate how environmental conditions affect print quality, an image analysis was conducted on selected printed elements. The differences in print quality after exposure to high humidity and low temperature, compared to prints before exposure, are shown in Figure 4.

For commercial corrugated cardboard at high humidity, mixed results were obtained, with positive elements (e and lines) generally showing a small increase and negative elements showing more variation. For Sample 2, lower surface coverage of the evaluated printed elements was determined, particularly for negative elements like the letter e (negative) (−6%). After exposure to low temperature, greater variations in surface coverage are seen; negative elements in particular show significant deviations (e.g., the letter e (negative): −6.6% at Sample 1 and −8.9% at Sample 2). For the full-color field, both samples in both environmental conditions show a general decrease in print quality, with low temperature showing a slightly greater negative impact compared to high humidity.

High humidity generally results in increased ink absorption due to the swelling of fibers, which can cause both positive and negative deviations, depending on the interaction between the ink and the moist fibers. At low temperatures the negative impact on print elements is more pronounced, particularly for negative print elements, indicating potential issues with ink adhesion or condensation affecting the print quality. However, visual assessment did not show a noticeable change in print quality after exposure to high humidity or low temperature.

3.6. Assessment of UHF RFID Tag Readability

The functionality of products and box tracking in logistics or in the supply chain can be improved with the integration of UHF RFID tags into the packaging. The analysis of the UHF RFID tag’s readability was determined under standard climate conditions, in high-humidity conditions, and after exposure to low temperature. The received signal strength (power) was determined at a distance of 50 cm between the reader antenna and printed tag, from the front and back sides of the corrugated cardboard. In Figure 5, the signal power and reading distance of the printed UHF RFID tag are shown. The difference in signal power between measured sides is minor, below 5%, which is inside the measuring deviation range (CV ranges from 5.5% to 9%). Additionally, the difference between Sample 1 and Sample 2 is minor, meaning that the type of printing substrate and its thickness do not influence the signal power.

The influence of environmental conditions on UHF RFID tag readability is minor. In the case of refrigeration, the change in signal power and reading distance is negligible, from 0.5% to 4%. After exposure to high humidity, some deviation between measurements were observed. The change in signal power was minor, a few percent, regardless of the measured side of corrugated cardboard. For one test sample for each type of corrugated cardboard, the reading distance at which the signal was detected was substantially lowered. For Sample 1, the reading range was less than half the reading range, from 147 cm to 70 cm on the top side, and even more on the back side. The change in reading distance was most noticeable for tomato-based corrugated cardboard, where the signal was detected only at 10 cm. With time, the reading range improved; after 10 min in the standard climate, the signal was detected up to a distance of 90 cm.

Environmental conditions had a minimal impact on print quality and RFID readability, with low temperatures causing negligible changes. However, high humidity significantly reduced the reading distance for some samples, as water absorption that interfered with UHF radio waves, thereby reducing the read range and reliability of RFID tags. The reading range improved after returning to standard climate conditions, which must be considered in logistical handling. The significant reduction in reading distance under high humidity could hinder the adoption of tomato-based cardboard in smart packaging applications.

4. Conclusions

This research presents a novel approach to sustainable packaging by utilizing waste tomato stems to produce paperboard and corrugated cardboard. This study significantly contributes to the field of eco-friendly materials by demonstrating the potential of agricultural waste in packaging applications. The lightweight paperboard made from tomato fibers, though less dense and more porous than commercial liners, shows adequate mechanical properties for packaging applications that require moderate strength and rigidity. Despite some challenges, such as lower burst strength and tear resistance, and issues with print quality due to high porosity, the material holds promise for specific uses where these factors are less critical. Notably, the tomato producer involved in this study has already implemented paper bags made from this paperboard and has been awarded for innovation and the environmentally friendly nature of their products. This recognition highlights the potential of extending this innovative approach to other packaging solutions, further promoting sustainable practices in the company.

This study also reveals important insights into the impact of environmental conditions on the physical and mechanical properties of tomato-based corrugated cardboard. High humidity poses a significant challenge by reducing the material’s strength, stiffness, and overall structural integrity. This finding underscores the need to carefully consider environmental factors in the design and application of tomato-based corrugated cardboard, potentially necessitating the development of surface coatings or alternative design solutions for packaging box to enhance its resistance.

While this study highlights the potential of tomato stem fibers as a sustainable material for corrugated cardboard production, it also underscores the need for further research and development. Addressing the challenges related to moisture sensitivity, print quality, and RFID performance is essential for broader adoption. The mechanical properties of the tomato-based corrugated cardboard, particularly its sensitivity to humidity, restrict its use in certain packaging applications, especially those requiring high stacking strength and resistance to compression. Additionally, this study’s focus on specific environmental conditions limits the generalizability of the findings to other climates or use cases.

Nevertheless, the contribution of this research to sustainable materials is clear, offering a viable pathway for utilizing agricultural waste in environmentally friendly packaging solutions.