1. Introduction

Plastic materials are cheap, light, and durable and they can be used in a wide variety of industrial products and applications. As a result, plastic production has grown exponentially over the past 70 years. Rapid research and development in the field of petro-chemistry has led to an increasing number of polymeric materials that are available for a variety of applications [1].

Plastics can bring significant benefits in terms of lightweight, durability, and lower costs compared to many other types of materials. Unfortunately, the use and disposal of these products are causing more and more environmental problems.

Consequently, the International Regulatory Agencies have issued several directives [2], [3] which promote the concept of the Circular Economy and prioritize sustainable and non-toxic reusable products. The directives promote systems for reusing and reducing the amount of waste generated by them. The main regulations refer precisely to plastic products concerning this article which are summarized below:

-Plastic packaging, lids, and plastic closure systems [4], which are used for beverage containers, are among the most used plastic items on beaches. Therefore, beverage containers that are single-use plastic products should only be allowed to be placed on the market if they meet specific product design requirements that significantly reduce the impact on the environment. Mainly, technical solutions that do not allow the detachment of these components from the containers are searched out the products used in this case study (handles) match well the provisions of this directive because they remain firmly attached to the containers they serve.

-Recycling is currently one of the most important actions that reduce the impact of waste offering opportunities to reduce the consumption of crude oil, carbon dioxide emissions, and the amount of waste that needs removal or neutralization [5], [6], [7], [8]. Consequently, recycling is a pillar in the circular economy, i. e. the treatment of post-consumer plastics. Colombia has also produced regulations for one-time plastic usage [6]. Moreover, according to Euromap [5] in 2020 (estimate), each person in Colombia uses about 28,4 kg of plastics every year, whereas in Brazil 33,4 kg and Argentina 38,7 kg.

Due to the wide range of recycling activities, it is recommended to use common terminology. In this respect, several International Standards can be used (ASTM D5033, ISO 15270).

Finally, according to ASTM D-7611 [9], polymers are labeled as shown in Figure 1. This is done this way to ease the post-consumer treatment and recycling.

2. General considerations

An application often found in the food industry and not only is represented by products that facilitate the transport and handling of PET containers with volumes from 5 to 10 liters [Figure 2a) containing various liquids (drinking water, food oil, cleaning solutions, washable paint, various consumables in the automotive industry, etc.). These products are generally called "handles" [Figure 2b).

These handles must meet certain quality, mechanical resistance, and ergonomic requirements. Cost criteria, environmental requirements, and new legislative requirements in the field of plastic product usage and recycling are important elements that determine the design, materials used, and the product manufacturing and testing process. These handles can be applied manually but in recent years, bottling companies have been equipped with automatic applicators [10] which require a compatible design with the operating system of these applicators (center of gravity position, geometric symmetries, easy orientation, correct application, etc.).

Currently, there are such handles on the market in different constructive variations and their weights differ between 5,5 and 11 grams [10], [11]. The most important technical parameter in the design of these handles is the maximum Load when this product is going to break. Broken handles are continuously observed on the market.

The handles are injection molded products, manufactured from HDPE material that might come from:

Virgin resin

Post-Industrial Resin (PIR): trash from production (trimming and cut-off, colour changes, process setup, quality rejects, samplings, etc.),

Post- Consumer Resin (PCR): the resin has been used by end-consumers and is coming back to the production chain (so-called circular economy)

Mixtures of virgin and recycled resins (PIR or PCR)

This article is related to Post Industrial Resin. In the cases when mixtures of different re-grinded materials are used, the physical-mechanical properties resulting from these blends might be affected [12], [13], [14], [15], [16].

It is generally recommended to sort out this waste by material type with the same melting temperature, [12], [16].

After the selective collection phase, the high-density polyethylene (HDPE) waste can be ground (in flakes) or re-granulated (in pellets) and stored in sealed containers. As there is not yet a defined universal protocol to assess the handles’ performance, several methods to determine the tensile strength performance for this kind of product are employed by the bottlers, and we can mention even the “Drop Test” [Figure 3]. For the bottlers the most important requirement is to have no broken handles.

This is a very fast and simple method but for handle producers, it involves a lot of limitations in product design optimisation and recycled material usage. It is not considered a fully reliable method.

3.Materials and methods

In the first instance, it is important to know some of the Rheological and Mechanical Properties of the HDPE virgin material (ELTEX Superstress CAP 602) as reference. The specified values collected from the producer Material Data Sheet are presented in Table 1.

As can be seen in Table 1, the speed of testing used to determine the Tensile Strength at Yield is 50 mm/min.

3.1. Polymer samples

HDPE reground material, Eltex Superstress Cap 602, in flakes, has been used [Figure 4a).

As described in the article regarding “Influence of recycled material on the tensile strength of HDPE products”, the most important condition to mix HDPE

grades is the melting temperature [12], [13], [14], [15], [16]. For our case study, it is allowed to mix various colours as the colour of the final product is dark. Up to 100% of reground HDPE materials with different MFR can be used without compromising the product specifications.

Melt Flow Rate measurements for the reground material have been performed using a Gottfert MI-3 Plastometer [Figure 4b).

A total of 28 measurements of the MFR of reground material were performed, as seen in Figure 5 with an average MFR of 0.76.

As can be observed, the measured values are well within specification.

3.2. Polymer samples

Five sets of handles are injected in the same mould, with the same design, and with the same material: 100% recycled (flakes) of HDPE Eltex Superstress Cap 602.

The process parameters are described below.

Mold data:

• Number of cavities: 16 (4x4)

• Single tip control

• Hot runner type, no sprue

• Cycle time: 13 seconds

• Clamping force: 220 KN

• Mold cooling temperature: 13-14 °C

Injection molding machine data:

• Producer: Klockner-Ferromatik

• Machine type: FM 250.

• Melt temperature: 240-245 °C

• Number of injection stages: 4

• Injection pressure: 150/130/80 bar

• Injection hold on pressure: 55 bar

• Injection speed: 300/200/170/50 [mm/s]

3.3. Mechanical testing

For material performance testing, the standards ASTM D 638-14 have been considered as benchmarking [17], [18], [19].

A Tinius Olsen H25KT universal testing machine was used to measure the tensile strength performance [Figure 6].

The environmental conditions in the laboratory were set as follows:

• temperature: 22.6 °C

• humidity: 47.50 %

For the testing speed of 50 mm/s, given by the resin producer, the proposed reference for the resistance to breakage (max Load force) was defined at 500 N and the minimum allowed breaking force must be no less than 400 N. The maximum value of the load force is less important, but it can be roughly estimated at 700 N.

To test the handle's resistance, a dedicated device was built, as shown in Figure 7, to reproduce the grip and the real stress as accurately as possible.

Furthermore, the grips were designed and built to mimic the force exerted by a hand when picking up just one PET bottle.

The handles were subjected to tensile stress with a force (Load) set to a progressive increase between the minimum limit of 0 N and the maximum limit of 700 N, with different values of traction speed (between 50 and 50,000 mm / min) until the complete break was achieved. [Figure 8].

The handle in the flat configuration, shown in Figure 1b, is moving to the spatial configuration by applying the load force along the vertical axis. The active parts are staying now perpendicular to the holding ring.

Thus, in addition to the longitudinal tensile stress, the material is subjected to transversal stress also leading to the bending effect [Figure 9].

In this way, the longitudinal tensile strength performance of the handle is affected, and the stress should be estimated.

The stress, σ, in an axially loaded component is estimated by equation (1).

σ - Tensile strength in N/mm²

P - Load, in N

A - Original cross-section area [Figure 10]

Cross sectional area can be determined according to the equation (2):

The critical area has the value: A = 22.7 mm²

If there is also an applied bending moment M, such as the case of an eccentrically applied force, the normal stress is computed through equation (3).

where I am the second area moment and c the distance from the neutral axis to the evaluation point. In case c represents the furthest away point, the stress is then the maximum on the cross-section.

The acting loads and the geometry for the top part of the handle are depicted in Figure 11 in a free body diagram (FBD). The handle has a 3º inclination. The acting force P is exerted by the universal testing machine, which is decomposed in the forces Fx and Fa. Therefore, in interest, Fx produces a bending moment (named M1) and Fx produces axial tensile stress.

Besides that, the handle is normally flat, so bending the handle to turn it vertically induces a moment, named M2. The directions of bending moments, acting around interest, are shown in Figure 12 and all loads produce normal stress. One can see the stresses act simultaneously, so they can be superimposed using Equation (1).

Figure 12 are also shown the signs of the acting normal stresses. In the notation used, tensile is positive and compression is negative. Fa is always positive (shown in yellow), M₁ (shown in blue) produces tension on the left side of the neutral plane and compression on the right side, whereas M₂ (shown in red) produces tension at the top of the neutral plane and compression at the bottom of it. Furthermore, M₂ produces a constant stress value whereas the stress produced by M₁ is proportional to the applied load. However, it fades when the handle is aligned with the load, as shown in Figure 4, which happens before rupture. Therefore, in the end, only axial normal stress is in play.

4. Outcome

The average values of the load and elongation for each set of measurements are presented in Table 2.

The evolution of the Loads and Elongations until the moment the breaking occurs can be seen as an example, in Figure 13.

Figure 13 depicts the product comportments along with the tensile test:

• In the first region, the load and elongation are increasing up to the point YP.

• The points YP, usually called yield point, is the first point at which an increase of elongation (strain) occurs without an increase of the load (stress). The handle still does not break. At this point, the maximum Load has been recorded.

• After the YP, the material continues to elongate up to point C, the cross-section continues to reduce but without any fatal rupture.

• At the BP point, called rupture point, the complete rupture of material occurs, and the handle is breaking. At this point, the total/maximum deformations (elongations) have been recorded.

The distribution of measured values of Load force (tensile strength) and elongation, are shown in Figure 14 and Figure 15, respectively [20].

It must be emphasized that the calculated stress is tensile strength, as shown in Equation (1) and the elongation is the value given by the universal testing machine.

From Figure 14 and Figure 15 we can observe:

• The best results of the Load force (tensile strength) were obtained at the speed of 50 mm/min which is the reference value of the tensile strength, in the material specification sheet.

• By increasing the speed from 50 to 500 mm/min, the tensile strength performance of the handles decreased.

• By increasing the speed over 500 mm/min, the tensile strength stabilizes at approximately the same value, until the speed of 50,000 mm/min is reached. These high- speed values correspond to a very hard stress regime and simulate the real “shaking" phenomenon happening on the market.

• The elongation decreases continuously by increasing the testing speed.

• The outlier observed in Figure 14 is defined as an unusual value, as long is over than upper whisker this might be ignored.

• The reference value of tensile strength σu which is given in the material specification with a value of 26 MPa. as seen in Table 1. We notice that in all cases, the measured tensile strength was far below this reference value. As already described above, this aspect can be explained by the fact that by bringing the handle into the working position, it suffers an additional bending stress.

• A slightly lower value on cavity no.6 at the test speeds over 10,000 mm/min has been observed. This is not leading to product rejection, but it might be a warning sign for the machine operator that the process capability starts to deteriorate. Mold filling and injection parameters should be investigated for this case (wearing on injection nozzle, injection pressure to low, dust on the injection gate of cavity 6, etc.).

• All handles break in the opposite side of the injection point, this is the area where each individual cavity of the mold, is filling last. The flow of melted material from different directions creates an effect of “welding” when the streams of material join [Figure 16].

As observed, the part always breaks in the same place, this may be indicative of mold design issues since the flow lines would not be compensated concerning the geometry of the mold.

One possible solution is to relocate the injection point to improve the flow in the cavity [Figure 17].

Two injection points simulated by CADMOULD software might be the best technical solution required by the complex geometry of the piece [Figure 18].

The flow lines are compensated, the material solidifies after filling the cavity completely and the cooling time is also improved. This improvement might solve also the issue detected at cavity 6. But this solution needs to be validated by economical study, as the costs for the new Hot Runner configuration will increase significantly.

It is difficult to compare handle behaviour with a standard test specimen comportment, but we found this method as the best available tool to evaluate the mechanical performance of these products.

5. Conclusions

For the qualitative determinations of the tensile strength performance, for this type of products, it is recommended to use the test speed of 500 mm/min. The use of this limit speed is since above this, the tensile strength values do not vary significantly. In this way, it will be easy to correctly perform a real benchmarking between similar products on the market.

The most important parameter is the Breaking Force referred to in this study as maximum Load. The nominal (target) value should be defined at 500 N and the minimum value at 400 N, at a testing speed of 500 mm/min.

Such models can be very useful in increasing the degree of recycling of plastic waste at the source, either by capitalizing on products with similar properties or by downgrading to different products where mechanical performance is less restrictive.

Depending on the specifications of the finished product, obtained from recycled materials, one can establish which are the mechanical properties and the necessary parameters to be tested. In this sense, the parameters tested in this study are not limitated.

Acknowledgments

The authors are grateful to the company Obrist Eastern Europe Romania for the materials and finished products available for testing. The experimental research was carried out in the laboratories of the Politehnica University of Timișoara and Technical Die Casting Timisoara.

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