Correlations Between Crystallinity, Rheological Behavior, and Short-Term Biodegradation for LDPE/Cellulose Composites with Potential as Packaging Films

Abstract

The need for renewable and biodegradable materials for packaging applications has grown significantly in recent years. Growing environmental worries over the widespread use of synthetic and non-biodegradable polymeric packaging, particularly polyethylene, are linked to this increase in demand. This study investigated the degradation properties of low-density polyethylene (LDPE), a material commonly used in packaging, after incorporating various natural fillers that are sustainable, compatible, and biodegradable. The LDPE was mixed with 2.5, 5, and 10 wt.% of sawdust, cellulose powder, and Nanocrystalline cellulose (CNC). The composites were melted and mixed using a twin-screw extruder machine with a screw speed of 50 rpm at 190 °C to produce sheets using a specific die. These sheets were used to prepare samples for rheological tests that measured the viscosity curve, the flow curve, and a non-Newtonian mathematical model using a capillary rheometer at 170, 190, and 210 °C. X-ray diffraction analysis was carried out on the 5 wt.% samples, and a short-term degradation test was conducted in soil with a pH of 6.5, 50% humidity, and a temperature of 27 °C. The results revealed that the composite melts exhibited non-Newtonian behavior, with shear thinning being the dominant characteristic in the viscosity curves. The shear viscosity increased as the different cellulose additives increased. The 5% ratio had a higher viscosity for all composite melts, and the LDPE/CNC melts showed higher viscosities at different temperatures. The curve fitting results confirmed that the power-law model best described the flow behavior of all composite melts. The LDPE/sawdust and cellulose powder melts showed higher flow index (n) and lower viscosity consistency (k) values compared with LDPE/CNC melted at different temperatures. The sawdust and powder composites had greater weight loss compared with the LD vbbPE/CNC composites; digital images supported these results after 30 days. The degradation test and weight loss illustrated stronger relations with the viscosity values at low shear rates. The higher the shear viscosity, the lower the degradation and vice versa.

1. Introduction

Most products on the market are packaged for a variety of uses, including presentation, preservation, storage, transit, and protection. Both the food and non-food sectors use a lot of packaging. Glass, paper, and petroleum-based polymers are just a few of the many materials that can be utilized for packaging. Petroleum-based polymers have been widely employed for a variety of uses due to their exceptional qualities and versatility. However, because plastics are non-biodegradable and recycling or reusing them is extremely difficult, they can only be used once before being discarded, which results in the production of a significant volume of non-biodegradable garbage [1,2].

One of the main issues from an environmental point of view is the accumulation of garbage in the environment. As a result, there is a continuous rise in demand for environmentally friendly items. The phrase “eco-friendly” describes a technology or product that does not negatively affect the environment or wildlife, but it does not imply that the product must be fully biodegradable. There are two methods for creating eco-friendly polymers: (a) making bio-based polymers [3,4] from renewable resources and (b) making biodegradable plastics [5]. Currently, significant efforts are being made to integrate these two factors. Natural organic fillers, or fillers derived from renewable and biodegradable resources, are becoming more and more popular in polymer composites due to growing environmental concerns and the demand for more adaptable polymer-based products [6]. Numerous issues occur, including decreased ductility and lower processability. In general, bacteria do not attack hydrophobic polymers, such as polyolefins. Fillers like starch, cellulose, and lignin are added to polymer mixtures to make them biodegradable. Natural fibers and wood flour are highly intriguing due to their low cost, dimensional stability, elastic modulus, and degradability [7].

Cellulose, a naturally occurring hydrophilic polymer made up of poly (1,4-danhydroglucopyranose) units, is the main constituent of plant fibers. Because of the hydroxyl groups in these units, cellulose can form robust hydrogen bonds. Wood is often the most widely used commercial natural resource including cellulose. Significant percentages of cellulose are also included in a few natural fibers, including cotton, flax, hemp, jute, and sisal [8].

Following the digestion of cellulose fibers by a regulated acid hydrolysis process, nanocrystals of cellulose (CNC) can be recovered from cellulose sources. The crystalline rod-like particle known as CNC is produced as a stable aqueous colloidal solution [9,10]. CNC has been shown to be a more effective nucleating agent than micro-sized cellulose fibers for improving the crystallization characteristics of a polymer matrix [11]. To facilitate a reduction in the free energy barrier and speed up the crystallization rate, CNC enables a heterogeneous nucleation process [12,13]. Adding CNC may result in a higher nucleation density, which would reduce the size of the crystal and increase the number of nucleation sites [14].

The main obstacle for the composites, which are made of hydrophobic polyolefins and hydrophilic cellulose, is their incompatibility. Because of the low interfacial adhesion inside the composites, the cellulose has very poor dispersion, which leads to a tendency for the cellulose to aggregate and produce poor material properties. Many strategies have been tried to solve this issue. However, compatibilizers and coupling agents are particularly appealing since these frequently polymeric molecules can be accurately created and adjusted in a controlled manner, allowing for a close examination and optimization of the effects of the molecular structure on the dispersion of cellulose. Low-density polyethylene-grafting maleic anhydride (LDPEgMA) is a popular compatibilizer used between LDPE and cellulose and has been used extensively. When peroxides are present, maleic anhydride (MA) can be grafted onto LDPE chains to create reactive sites where MA can graft, resulting in LDPEgMA [15,16].

The relationship between rheology and degradation in polymers is crucial in many industrial applications such as packaging, where rheology studies help monitor changes in the physical properties of a polymer as it degrades. It is possible to enhance production procedures and guarantee the sustainability of polymeric products in challenging environmental circumstances by forecasting how degradation influences a material’s behavior [17]. Since viscosity is highly dependent on the distribution of molecular weight, rheological tests are a very effective method of analysis for assessing deterioration. Numerous studies have investigated the connection between rheology and polymer degradation in recent years [18,19,20].

In this study, composites of LDPE/cellulose with contents of 2.5, 5, and 10 wt.% of sawdust, cellulose powder, and CNC were mixed and melted using a twin-screw extruder at 50 rpm and 190 °C to prepare samples for suggested tests. LDPEgMA was added as a coupling agent to improve the dispersion of LDPE and cellulose additives. The relations between rheological properties at 170, 190, and 210 °C; XRD; and degradation were investigated. This study established a direct link between shear viscosity and biodegradation, a relationship that has not been widely explored, offering potential alternatives to purely synthetic polymers in packaging applications.

2. Materials and Methods

2.1. Research Materials

The materials used in this work were as follows: Sawdust from poplar wood of Russian origin was obtained from sawmills and lumber markets using sieves with apertures of 180 µm (ρ = 250 kg/m³). Cellulose powder was purchased from Himedia company, Mumbai, India (particle size = 18.24 µm; ρ = 1.5 g/cm³). Nanocrystalline cellulose (CNC) was purchased from Nanografi company, Cankaya, Turkey, with the following characteristics: ρ = 1.49 g/cm³, crystallinity = 92%, and particle sizes of 10 to 20 nm (width) by 300 to 900 nm (length). LDPE, used as a matrix, was provided by Saudi Basic Industries Corporation (SABIC), Riyadh, Saudi Arabia and had the following characteristics: MFI (190 °C/2.16 kg; ISO 1133-1) = 2.1 g/10 min, ρ = 0.91 g/cm³, and Tm = 112 °C. Maleic anhydride-grafted polyethylene (LDPEgMA) (ρ = 0.92 g/cm³, Tm = 105 °C), used as a coupling agent, was purchased from Coace Chemical Company Limited, Xiamen, China.

2.2. Preparation of Composites

Prior to processing, the cellulose powder and sawdust were vacuum-dried in an oven at 60 °C for two hours, then sieved using a standard sieve (H-47325, USA) with apertures of 180 µm. The cellulose additives (CNC, sawdust, and cellulose powder) were mixed with 30 mL of distilled water and dispersed for 20 min at room temperature using an ultrasonic homogenizer (SJIA-1200W MTI). The samples produced were dried and mixed with LDPE granules and LDPEgMA coupling agents according to the proportions in

Table 1. The values of screw speed, temperature, and thickness and the proportions of cellulose, LDPEgMA, and LDPE in different LDPE/cellulose additives (wt.%).

Sample Code	Cellulose wt.%	LDPE wt.%	LDPEgMA wt.%	Screw Speed (rpm)	Temperature (°C)	Thickness (µm)
LDPE	0	98	2	50	190	25
L1Sawdust 2.5 L1CNC 2.5 L1Powder 2.5	2.5	95.5	2	50	190	27
L2Sawdust 5 L2Powder 5 L2 CNC 5	5	93	2	50	190	28
L3Sawdust 10 L3Powder 10 L3CNC 10	10	88	2	50	190	25

. The obtained samples were melted using a twin-screw extruder.

The films were prepared using a twin-screw extruder (SLJ-30A.chine). Details of the preparation, including the formulations, the operating parameters adopted, the proportions of the materials used, and the final thicknesses of the films, are listed in Table 1. L1, L2, and L3 indicate LDPE.

2.3. Characterization

2.3.1. Rheology Test

An improved single-bore capillary rheometer, the SR20, was used in this investigation. The polymeric substance was dried before being put into the barrel for each test. Hardened steel, tungsten carbide, and Stellite were used to make the barrels, with a die held up at the bottom to preserve geometric integrity. In this case, the die’s length-to-diameter ratio was 20:10. For each of the shear rates, a piston with a diameter of 15 mm was seeded to propel the polymer melt through the die at varying velocities. In order to make the polymer soft to flow in the die [21], the preheating time was equal to 150 s. The pressures at the die’s openings were measured during each test using suitable pressure transducers (500 bar). The tests were conducted at temperatures of 170, 190, and 210 °C within the apparent shear rate range of 10 to 1000 s⁻¹ to produce viscosity curves.

2.3.2. XRD Test

X-ray diffraction (XRD) profiles were used to examine the crystallinity index (CrI) values of the composites. An X-ray diffractometer (Rigaku Diffract meter) fitted with nickel-filtered Cu Kα radiation (0.1542 nm = 0.1542) was used. At 40 kV, 30 mA, and a scan rate of 2° min⁻¹, the X-ray pattern was captured in the 2θ range of 10–70°. The CrI was determined using Equation (1).

CrI = [(I₂₀₀ − I₁₁₀)/I₂₀₀] × 100%

(1)

where I₁₁₀ is the intensity of the diffraction peak of (110) at 2θ = 20.94◦. I₂₀₀ refers to the intensity at about 2θ = 23.1° [22].

2.3.3. Degradation Test

Pure LDPE and LDPE/cellulose composites underwent biodegradation at 27 °C in soil with a pH of 6.5 and 50% humidity. With initial weights of roughly 2 mg, the dried samples were divided into tiny pieces (1 × 1 cm²). For 30 days, each sample was positioned in a soil container at a depth of 3 cm or 5 cm. The samples were cleaned using distilled water at regular intervals, then dried for a full day in a vacuum oven. Ultimately, the samples were weighed, and the mass loss was calculated using the methodology outlined in [23].

M_loss = [M_dry,0 − M_dry,t\M_dry,0] × 100%

(2)

A digital microscope (model AM4815T Dino-Lite edge made in Kyoto, Japan) with zoom (average 20×–220× using optics and a digital camera) was used to obtain images of the surfaces of the samples.

3. Results

3.1. XRD

The crystal structures of the samples were examined using X-ray diffraction (XRD). The XRD spectra of the LDPE and LDPE/5 wt.% CNC samples are shown in Figure 1. The general XRD patterns of both samples were similar and comparable to those of sawdust and powdered cellulose, as reported in [24]. The 2θ peaks appeared at 20.94° (110) and 23.1° (200), which are characteristic diffraction peaks of low-density polyethylene (LDPE) [25]. Furthermore, with the addition of CNC, the crystallinity of the LDPE and LDPE/CNC composites maintained an orthorhombic structure. The crystallinity index values of the samples were determined based on the XRD data, and they are listed in

Table 2. The crystallinity index values of the LDPE and LDPE/5 wt.% CNC samples.

Sample	2θ (200)	2θ (110)	CrI%
LDPE	23.1	20.49	75
LDPE/5 wt.% CNC	23.1	20.5	83.3

. The results indicate that the crystallinity of the LDPE composites increased with the incorporation of cellulose. This increase was attributed to the role of cellulose as a nucleating agent, which enhanced the crystallinity index and improved the crystallization of the LDPE/CNC composites [26]. The cellulose exhibited behavior consistent with a more amorphous material, as it became increasingly organized and crystalline upon incorporation into LDPE. The crystalline nature of the cellulose contributed to the formation of packaging materials with more structured and ordered morphologies. This, in turn, influenced the physical and chemical properties of the final composite materials.

3.2. Rheology

The addition of cellulose, depending on its dispersion and its interaction with LDPE, may increase or decrease viscosity, affecting processability and long-term stability. In Figure 2, shear-thinning behavior is dominant on the viscosity curves of all composites. This means that the cellulose additives had no effect on the usual flow behavior of LDPE. The shear viscosity values of all composite melts increased exponentially with an increase in the ratio of the additive, especially in the low-shear-rate zone (0–50 1/s), while slight increases were observed from 50 to 450 1/s. The viscosity was arranged from higher to lower values at the low shear rate with 5, 10, and 2.5% ratios for all cellulose additives at 170, 190, and 210 °C. On the other hand, the shear viscosity values at the low shear rate, arranged from higher to lower values according to the cellulose type, were 10^3.996, 10^3.951, and 10^3.829 Pa.s at 170 °C; 10^3.969, 10^3.921, and 10^3.896 Pa.s at 190 °C; and 10^3.93, 10^3.877, and 10^3.782 Pa.s at 210 °C for CNC, sawdust, and powder, respectively. A higher viscosity (due to a higher polymer molecular weight or strong interactions with cellulose) can indicate a more stable structure that resists degradation, while a lower viscosity may suggest polymer chain scission or thermal degradation during processing, leading to faster breakdown. A higher viscosity induces fewer biofilm attachment sites, which reduces microbial colonization and slows the hydrolysis of cellulose due to limited water penetration. Molecular entanglement increases and the free volume decreases in the polymer matrix. In addition, using PEgMA as a compatibilizer resulted in effective adherence between CNC and the LDPE matrix, leading to improvements in the mechanical properties of the composite. The reactive interaction of hydrogen bonding between the hydroxyl groups on the CNC surface and the maleic anhydride moieties in the PEgMA facilitated this enhanced adhesion. Stronger hydrogen bonding between cellulose hydroxyl groups (-OH) and LDPE chains reduces hydrolytic and microbial degradation.

Figure 3 shows the viscosity behavior of different LDPE/cellulose melts at different ratios and different temperatures.

The LDPE/CNC melts had higher viscosity values at the low shear rate for all ratios compared with the other melts. Crystalline Nanocellulose (CNC) has a highly ordered, rod-like structure with a high aspect ratio, which increases its surface area and enhances interactions with the polymer matrix. These rigid CNC particles create a physical barrier that restricts the movement of polymer chains, particularly under low-shear conditions, increasing melt viscosity.

3.2.1. Rheological Model

The flow index (n) and viscosity consistency (K) values of all melts were determined using a rheology app (Rheology Lite for Windows, an app for rheological analysis of the flow behavior of materials). This program utilized the power-law, correct Herschel–Buckley, Caisson, and Bingham models. The flow behavior of all composite melts matched the power-law mathematical model where the flow index (n) is less than 1, as shown in Figure 4.

• Power-law model:

η = K (γ)ⁿ⁻¹

(3)

τ = K (γ)ⁿ

(4)

where η is the viscosity (Pa.s), τ is the shear stress (Pa), γ is the shear rate (1/s), n is the flow index, and K is the viscosity consistency.

The flow index (n) level describes the melting behavior of compounds during polymer extrusion, while the viscosity consistency (K) describes the melting behavior and melting force near the end of the process. The flow index (n) indicates a value lower than one at 5% for all cellulose additions due to shear-thinning behavior. The stability of a melt depends on the viscosity consistency (K), which represents the viscosity and molecular weight values of the composite melt at low shear rates at 170, 190, and 210 °C.

Decreasing the flow index (n) to less than one leads to better dispersion of cellulose and increased surface exposure to microbes and moisture. A lower n value (stronger shear-thinning behavior) can lead to a higher degradation rate, as cellulose is more exposed and microbial colonization is enhanced. K represents the viscosity at low shear rates and depends on the molecular weight and interactions within the polymer matrix. A higher K value means a higher viscosity at a low shear rate, which indicates stronger polymer–cellulose interactions and a structure that is more resistant to microbial attack. A lower K suggests that the composite is more prone to degradation, as water and microbes can diffuse more easily. A lower n and a higher K produce more structured matrixes, resisting microbial breakdown. A higher n and a lower K lead to more cellulose exposure, accelerating microbial colonization and biodegradation.

Figure 4 and

Table 3. Flow index (n) and viscosity consistency (LDPE K) values for LDPE and LDPE/5% cellulose additives melted at different temperatures.

Sample	Flow Index (n)	Consistency Index (K)
LDPE (170 °C)	0.393	20,389
LDPE (190 °C)	0.336	17,514
LDPE (210 °C)	0.526	8955
Sawdust (170 °C)	0.275	43,583
Sawdust (190 °C)	0.293	42,946
Sawdust (210 °C)	0.320	32,902
CNC (170 °C)	0.267	52,680
CNC (190 °C)	0.285	49,290
CNC (210 °C)	0.279	42,570
Powder (170 °C)	0.285	43,135
Powder (190 °C)	0.298	39,747
Powder (210 °C)	0.321	36,618

show that the higher n and lower K values obtained for the powder and sawdust cellulose composites fit the curve according to the power-law model finding, while lower n and higher K values are indicated for the CNC composites. These findings were confirmed by the degradation results for the same composites.

3.2.2. Degradation

The biodegradation properties of LDPE/cellulose additives can be determined by soil burial testing. As can be seen in Figure 5, degradation started due to water absorption from the soil, and after 30 days the weight loss of all compounds increased. The weight loss percentage in LDPE was much lower than that of LDPE/cellulose additives, as the water absorption was higher for the composites due to the hydrophilic nature of cellulose. Therefore, adding cellulose additives (sawdust, CNC, and cellulose powder) to LDPE increased the biodegradability of all LDPE/cellulose composites compared to LDPE at depths of 3 and 5 cm. Furthermore, sawdust and powdered cellulose showed higher rates of weight loss than CNC. This was attributed to the highly crystalline nature of CNC compared to sawdust and powdered cellulose, which results in greater strength and a denser structure with lower porosity and fewer voids, leading to reduced water diffusion. Swelling and slow attacks by microorganisms reduce hydrolysis and biodegradation and are compatible with the rheological findings. The burial depth did not appear to have an important role in biodegradation, while environmental factors such as temperature, humidity, and biological activity will affect the rate of degradation.

Figure 6 presents microscope images of LDPE and LDPE/cellulose additive composites with thicknesses of 3 mm containing 2.5, 5, and 10 wt.% cellulose. The samples containing cellulose appeared transparent with uniform surfaces. Following soil burial, slight brown spots emerged on the surfaces, becoming more prominent as the cellulose content increased. The surface changes of the LDPE/CNC composites were notably less distinct than those observed on the sawdust and powdered cellulose composites. This behavior aligned with the data from the rheological and mathematical models.

4. Conclusions

The degradation of LDPE/cellulose additive composite packaging is significantly influenced by extrusion process conditions, including the temperature, screw speed, extruder specifications, and material ratios. In soil burial testing, weight loss measurements were used as indicators of degradation for LDPE composites containing 2.5, 5, and 10 wt.% CNC, cellulose powder, and sawdust. Weight loss increased with higher additive ratios, with the samples containing powder or sawdust exhibiting greater losses over 30 days compared to the LDPE/CNC samples. Digital images at 1600× magnification revealed more pronounced pitting on the surfaces of the samples containing powder or sawdust after 30 days than on the LDPE/CNC samples. The crystalline nature of cellulose plays a key role in creating packaging materials with more structured and organized morphologies.

The viscosity curves of the LDPE/cellulose melts exhibited non-Newtonian shear-thinning behavior. The power-law model provided the best fit for describing these flows. The samples containing powder or sawdust demonstrated higher flow index (n) and lower viscosity consistency (K) values across different temperatures compared to the LDPE/CNC samples. The samples with 5% ratios exhibited higher viscosities at low shear rates across all tested samples, while the LDPE/CNC samples demonstrated higher viscosities at various temperatures. At lower shear rates, the viscosity was directly proportional to the molecular weight and the cohesive structure. Lower viscosity, molecular weight, and consistency index (K) values were associated with increased degradation through weight loss and vice versa. These findings revealed a strong correlation between weight loss and viscosity at low shear rates across all samples.