Simulating Industrial Recycling of Biodegradable Irrigation Pipe Scraps into Sustainable Monopolymer Blends

Abstract

Recently, many industries are adopting closed-loop recycling models to recover and reuse production scrap in order to reduce waste, conserve resources, and minimize environmental impact. In this scenario, this paper aims to simulate such a model using biodegradable pipe scrap, with the objective of studying how the concentration of recycled biodegradable pipe scrap affects mechanical and rheological properties and to evaluate the effectiveness of this approach. Firstly, irrigation pipes were subjected to multiple extrusions to evaluate their thermal and mechanical stability under repeated processing. Subsequently, blends of virgin polymer and biodegradable irrigation pipe scraps (monopolymer blends) were prepared following an industrial approach. All systems were fully characterized through mechanical and rheological tests. The results obtained showed that multiple extrusions had a significant impact on the mechanical and rheological properties of the pipe, while the presence of reprocessed pipe in the blend only minimally affected the characteristics of the virgin biopolymer, demonstrating the effectiveness of this approach.

1. Introduction

In a global context increasingly oriented towards sustainability, today, recycling [1,2,3] represents an efficient solution to solve the problems of waste management and reuse conservation, becoming a fundamental pillar for the transition toward a circular economy.

In fact, every year, global plastic production continues to grow and shows no signs of slowing down: from 15 million tons in 1964, we have gone to approximately 400 million in 2023 [4]. As result, these data highlight how important it is to recognize the importance of adopting sustainable practices to prevent serious environmental consequences [5,6,7].

In the wake of this growing awareness, and beyond regulatory pressures, [8] many industries are increasing their efforts to adopt sustainable solutions, understanding that sustainability has become an essential priority. Although many of them are increasingly using biodegradable polymers to meet sustainability goals, it is clear that these materials also require appropriate recycling operations as volumes of biodegradable polymers today continue to grow, both in the agricultural sector and elsewhere. In this context, the literature [1,3,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] on this topic is starting to propose different recycling solutions that can be implemented on an industrial level. For example, biodegradable polymers such as Polylactic Acid (PLA), Poly(butylene adipate-co-terephthalate) (PBAT), and Poly(butylene succinate) (PBS), as well as their blends, are increasingly being mechanically recycled. Studies on PLA [21,27,28] have shown that PLA can be reprocessed multiple times, although its mechanical properties tend to degrade due to chain scission. PBAT [13] has also shown good potential for mechanical recycling, maintaining flexibility and tensile properties under controlled processing conditions. Similarly, PBS [12,13] has been mechanically recycled with promising results, although thermal degradation during reprocessing still remains an open challenge. Furthermore, blends of these polymers [13,16,17,18,29,30] have also been studied with results suggesting that the compatibility and mechanical performance of recycled materials depend on the blend composition and recycling conditions. Indeed, recent studies [15,20,31,32] have shown that processing conditions, such as extrusion temperature and screw speed, can influence the molecular weight and mechanical properties of biodegradable polymers during mechanical recycling. However, the addition of chain extenders [15,31] or compatibilizer agents [20,32] can help reduce property loss, especially in blends, where compatibility issues may occur.

Today, due to an incessant increase in the volumes of biodegradable plastic, many industries are adopting closed-loop recycling models, where manufacturers collect and recycle their production waste, ensuring that it is reused in their production processes. Therefore, while several studies have been reported on the recycling of biodegradable materials, to our knowledge, no study has yet focused on recycling an industrial product like pipes to assess their potential for recycling.

This paper aims to simulate a similar model using biodegradable pipe scrap, with the objective of studying how the concentration of recycled biodegradable pipe scrap affects mechanical and rheological properties and to evaluate the effectiveness of this approach.

2. Materials and Methods

2.1. Materials

The biodegradable irrigation pipe used in this study was manufactured by Irritec (Capo d’Orlando, ME, Italy) using a proprietary extrusion-drawing process. The pipe, exiting the extruder, was drawn to achieve the desired dimensions, with a thickness of approximately 200 μm at a die temperature of 180 °C. It was produced using Bioplast 105/20 BAR (Biotec^®, Emmerich am Rhein, Germany), an advanced polyester compound based on PBAT and PLA with a melt flow rate (MFR) of approximately 4.1 g/10 min at 190 °C under a load of 2.16 kg.

2.2. Recycling Method

2.2.1. Multiple Extrusions and Sample Preparation

Multiple extrusion cycles were conducted on the biodegradable irrigation pipe using a single-screw extruder (Thermo Scientific HAAKE PolyLab QC, Karlsruhe, Germany) to assess its thermal and mechanical stability under repeated processing. The temperature profile was set at 150–160–170–180 °C, with the screw rotational speed set at 60 rpm. After each extrusion, the materials were subjected to compression molding using a press Carver laboratory hydraulic press (Carver, Wabash, IN, USA) at 180 °C under a mold pressure of 100 psi; holding time: 3 min; cooling time: 10 min.

2.2.2. Simulating Industrial Recycling

To simulate industrial recycling conditions, blends of virgin polymer and biodegradable irrigation pipe scraps (monopolymer blends) were prepared as described in ref. [33] and reported as follows:

$$\mathrm{R}\text{-}{\mathrm{Tbs}}_{\mathrm{Cycle}\text{-}1}=\alpha \mathrm{B}105+(1-\alpha ){\mathrm{Tbs}}_{\mathrm{Cycle}\text{-}1}$$

(1)

$$\mathrm{R}\text{-}{\mathrm{Tbs}}_{\mathrm{Cycle}\text{-}2}=\mathsf{\alpha }\mathrm{B}105+(1-\alpha ) [{\mathrm{Tbs}}_{\mathrm{Cycle}\text{-}1}+(1-\alpha ) {\mathrm{Tbs}}_{\mathrm{Cycle}\text{-}2}]$$

(2)

where $\alpha$is the weight fraction of the assumed polymer.

Table 1. Detailed composition of the blends developed in this study, highlighting the specific proportions of each component.

Sample Code	B105, %	TbsCycle-1, %	R-TbsCycle-1
R-TbsCycle-1-10	90	10
R-TbsCycle-2-10	90	-	10 a
R-TbsCycle-1-25	75	25
R-TbsCycle-2-25	75		25 b

^a R-Tbs _Cycle-1-10, ^b R-Tbs _Cycle-1-25.

summarizes how the compositions were obtained.

B105 (Bioplast 105) is the virgin polymer used; Tbs_Cycle-1 (Pipe) refers to the pipe extruded once, while R-Tbs _Cycle-1 represents the recycled pipe fraction obtained by mixing the recycled pipe with a virgin polymer. Subsequently, a fraction of this system was further reprocessed together with the virgin polymer (e.g., R-Tbs_Cycle-2)

This approach follows an industrial recycling model in which scraps are continuously recycled by mixing a fraction of them with the virgin polymer to maintain good performance. As already described in ref. [34], during successive reprocessing cycles, the composition of the system gradually changes, with a gradual decrease in the fraction of recycled material that has been reprocessed several times.

The equipment used and the conditions are the same as those reported above; see Section 2.2.1.

Figure 1 shows the experimental approach of the current study.

2.3. Characterizazion

Rheological characterization in shear flow was performed using a rotational rheometer model AR-G2 (TA Instruments, New Castle, DE, USA) with parallel plate geometry. Samples were prepared directly on the machine. The pellets were placed on the rheometer’s base plate, and heated and melted at 180 °C for approximately 3 min. The measurement gap was set between 1.5- and 2-mm. Excess sample was removed through a trimming procedure using a spatula. All tests were conducted at 180 °C, with an angular frequency range of 0.1 to 100 rad/s. Before testing, all samples were allowed to dry in a vacuum overnight at 70 °C.

To also assess processability in the drawing step of the production of the pipes—where non-isothermal elongational flow is involved—after successive recycling steps, non-isothermal elongational flow tests were performed using a capillary viscometer (Rheologic 1000, CEAST, Turin, Italy), equipped with a tensile module, operating at the same temperature as described above. The melt strength was directly measured as melt strength (MS), while the breaking stretching ratio (BSR)—defined as the ratio of the drawing speed at break to the extrusion speed at the die—was calculated as described in our previous paper [34], according to the following equation (Equation (3)):

$$\mathrm{Breaking} \mathrm{Stretching} \mathrm{Ratio} (\mathrm{BSR})={\displaystyle \frac{{\mathrm{V}}_{\mathrm{roll}}}{{\mathrm{V}}_{\mathrm{p} }· \frac{{\mathrm{D}}_{\mathrm{p}}^2}{{\mathrm{D}}_{\mathrm{c}}^2}}}$$

(3)

where V_roll is the collecting speed; V_p is the capillary piston speed; D_p is the piston diameter; and D_c is the diameter of the capillary.

Mechanical tests were conducted at room temperature (about 25 °C, and RH50%) using an Instron universal testing machine (Instron, mod. 3365, High Wycombe, UK) according to ASTM D638. Rectangular specimens (length = 90 mm, width = 10 mm, and thickness = 0.5 mm) were tested at a deformation rate of 1 mm/min until 3% deformation. Subsequently, the speed of the crosshead was increased to 100 mm/min until the specimen failed. Mechanical test results (elastic modulus, E; tensile strength, TS; and elongation at break, EB) are expressed as the mean (±standard deviation) of ten measurements. Data were analyzed using ANOVA, and a Student’s t-test was employed for further comparisons, with a significance level of p < 0.05.

3. Results and Discussion

3.1. Multiple Extrusion Tests

Figure 2a shows the viscosity curves obtained by a rotational rheometer (closed) and capillary viscometer (open) of B105 and Tbs subjected to repeated processing.

Firstly, it can be observed that the curve of the virgin biopolymer (B105) obtained by the capillary viscometer does not match the corresponding one obtained by the plate-plate rheometer (the Cox–Merz rule is not respected). This result is in agreement with other research reported in the literature and is attributed to the heterogeneous nature of these polymeric systems [17,35,36]. Certainly, it is also evident that the pipes display lower viscosity compared to the virgin biopolymer, likely due to thermomechanical stress incurred during processing.

With regard to the systems subjected to multiple extrusion (Tbs_Cycle-1 and Tbs_Cycle-2), it can be observed that the flow curve exhibits a more significant decrease in viscosity compared to both B105 and Tbs as the number of steps increases. This behavior is clearly attributed to a reduction in molecular weight resulting from repeated processing cycles, as already reported in other studies [17,29], which led to thermomechanical stresses and, consequently, slight thermodegradation of the material.

To assess non-isothermal elongational flow involved in the drawing step during pipe production, melt strength, MS, and breaking stretching ratio, BSR, were monitored as shown in Figure 2b and Figure 2c, respectively.

As visible in Figure 2b, as the number of reprocessing steps increases, there is a decrease in melt strength, MS, which aligns well with the shear viscosity results. Clearly, as expected, the breaking stretching ratio, BSR, curves mirror that of melt strength; however, the BSR values increase with the number of extrusions, which can be attributed to a reduction in molecular weight, leading to a more deformable melt.

Table 2. Elastic modulus, E, tensile strength, TS, and elongation at break, EB, of B105, Tbs, and Tbs after multiple extrusion cycles.

Sample Code	E, MPa	TS, MPa	EB, %
B105	111 (2.4) a	11.5 (1.3) a	416 (28) a
Tbs	127 (2.7) b	9.9 (1.1) b	348 (19) b
TbsCycle-1	142 (3.8) c	9.0 (0.9) b	325 (13) c
TbsCycle-2	149 (4.6) c	8.7 (0.5) b	284 (10) d

Different letters in the same column indicate significant differences (p < 0.05) when analyzed by multiple Student’s t-tests.

shows average values with the respective standard averages of the tensile test results.

Regarding the tensile test results, the decrease in molecular weight, as discussed above, observed by the decrease in complex viscosity has effects on the mechanical properties as visible in Table 2. Specifically, the decrease in elongation at break—a highly sensitive parameter to molecular and morphological alteration—becomes particularly significant after the second extrusion cycle, leading to a substantial decrease in ductility of approximately 46% less than virgin polymer. This behavior, as found in other similar work [17,37], is attributed to chain scission, which not only decreases molecular mobility but also contributes to an increase in the stiffness of the reprocessed material and serves to balance the decrease in polymer crystallinity [29,38].

3.2. Simulating Industrial Recycling

Aiming to investigate the effect of biodegradable irrigation pipe scraps on the rheological properties of sustainable monopolymer blends, the results for shear and non-isothermal elongational flow are reported in Figure 3 and Figure 4.

As expected, viscosity values, both shear and elongational flow, decrease as the percentage of biodegradable pipe scrap in monopolymer blends increases, regardless of the number of cycles. This behavior can be attributed to a reduction in the viscosity of the reprocessed component in the blend, leading to a lower molecular weight. Similar trends have already been previously reported in our studies of polypropylene systems [33,39] subjected to multiple extrusion cycles. Clearly, as the number of reprocessing cycles increases, the effect becomes more pronounced. This is because the scraps undergo further processing, which further reduces its viscosity as can be clearly seen from the R-Tbs_Cycle-2-25 curve. (see Figure 3b). In fact, as is well known, PLA only undergoes chain scission. However, the presence of branching and/or cross-links is not excluded as already reported in our previous work. [17] Indeed, PBAT, during thermomechanical processing, can undergo free radical attacks on the carbon of the methyl group near the C=O group, leading to the formation of a radical that can cause both branching and/or gel fraction and chain scission. The experimental data clearly put in evidence that chain scission predominates over cross-linking formation.

Such evidence is also visible in Figure 4, which shows melt strength (Figure 4a,a’) and breaking stretching ratio (Figure 4b,b’). While the curves do not show significant differences in behavior, as observed earlier with multiple extrusions (see Figure 2b,c), they highlight how the percentage of scraps influences the values. This is consistent with previously discussed results and aligns with findings from other studies [30,34].

Table 3. Elastic modulus, E, tensile strength, TS, and elongation at break, EB, of B105, Tbs, and monopolymer blends after recycling.

Sample Code	E, MPa	TS, MPa	EB, %
B105	111 (2.4) a	11.5 (1.3) a	416 (28) a
Tbs	127 (2.7) b	9.9 (1.1) b	348 (19) b
R-TbsCycle-1-10	127.8 (3.8) b	9.8 (1.9) b	357 (13) b
R-TbsCycle-2-10	128.3 (4.6) b	9.9 (1.2) b	348 (22) c
R-TbsCycle-1-25	133.2 (5.2) b	9.4 (1.1) c	331 (18) c
R-TbsCycle-2-25	134.2 (7.8) c	9.3 (0.6) c	322 (14) d

Different letters in the same column indicate significant differences (p < 0.05) when analyzed by multiple Student’s t-tests.

shows the tensile test values for B105, pipe, and the monopolymer blends at different scrap concentrations, while Figure 5a–c illustrate the trends for comparison between the two recycling methods used: multiple extrusion and simulating industrial recycling.

As noticeable in Table 3, while the tensile test data showed a decrease in mechanical properties as the amount of scrap increases in the blends, the trends for the two methods show a more pronounced reduction with the multiple extrusion method; see Figure 5a,c. This indicates that the detrimental effect to repeated processing is more pronounced in this test. In more detail, the elastic modulus of the pipe subjected to multiple extrusion increases by 6.3% in the first cycle and 10% in the second cycle, while the elongation at break decreases by 2% in the first cycle and 13% in the second cycle compared with the 25% monopolymer blend. Similarly, the elastic modulus increases by 9.8% in the first cycle and 14% in the second cycle, while elongation at break decreases by 9.8% in the first cycle and 22% in the second cycle compared with the 10% monopolymer blend. This trend is directly related to the presence of virgin material in these systems, which undergoes less thermomechanical processing as reprocessing cycles increase.

4. Conclusions

The aim of this study was to explore the use of biodegradable irrigation pipe scraps in sustainable monopolymer blends by simulating industrial recycling. The investigation focused on analysis of the rheological behavior and mechanical properties of the biodegradable irrigation pipe and the monopolymer blends subjected to recycling. During the multiple extrusion test, conducted to assess thermal and mechanical stability under repeated processing, a significant reduction in properties occurred, especially in the tensile test, where the elongation at break showed a decrease of approximately 46% compared to the virgin polymer. This result is due to the thermal degradation caused by continuous multiple extrusion, which is highly detrimental to these systems as thermomechanical stresses contribute to molecular chain scission as evidenced by a notable reduction in viscosity. In contrast, simulating industrial recycling involving the mixing of virgin polymer with biodegradable pipe scraps to develop sustainable monopolymer blends shows that although as the amount of recycled material increases, leading to slight changes in rheological and mechanical properties, the overall performance remains good. This result highlights the effectiveness of this approach in terms of processing and achieving mechanical properties very similar to those of virgin polymer up to two recycling steps.