Effect of EVA on the Unnotched Impact Toughness of PP/HDPE/EVA Blends

Duc Duy Huynh , Hoang Phuc Nguyen , Tran Quoc Thang Nguyen , Thi Hong Nga Pham ^*, Quan Anh Pham , Thi Van Anh Duong , Van Huong Hoang , Xuan Tien Vo , Vinh Tien Nguyen

1. Ho Chi Minh City University of Technology and Education, No. 1 Vo Van Ngan St, Thu Duc, Ho Chi Minh City, 721400, Vietnam

* Correspondence: Thi Hong Nga Pham

Academic Editor: Anissa Eddhahak

Special Issue: Recent Advances in Polymer Recycling, Experimental and Modeling Investigations of the Mechanical Behavior

Received: May 29, 2025 | Accepted: September 17, 2025 | Published: September 24, 2025

Recent Progress in Materials 2025, Volume 7, Issue 3, doi:10.21926/rpm.2503015

Recommended citation: Huynh DD, Nguyen HP, Nguyen TQT, Pham THN, Pham QA, Duong TVA, Hoang VH, Vo XT, Nguyen VT. Effect of EVA on the Unnotched Impact Toughness of PP/HDPE/EVA Blends. Recent Progress in Materials 2025; 7(3): 015; doi:10.21926/rpm.2503015.

© 2025 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

The caps of drinking water bottles are usually made from two primary materials, polypropylene (PP) and high-density polyethylene (HDPE) [1,2,3,4,5]. For recycling, waste from water bottles will be shredded, then separated by density. The label and light bottle body will float. The heavier-density bottle caps will sink. After separation, the waste from the cap and the waste from the bottle body will be washed with a specialized waste plastic washing solution, then dried and injected molding. Waste from bottle caps is often difficult to recycle because many types of additives are frequently added during manufacturing, depending on the manufacturer [6,7,8]. In this study, we used virgin PP and HDPE. This study is a preliminary study for future research. We will apply the best PP/HDPE/ethylene-vinyl acetate (EVA) ratio from this study to study the waste from bottle caps.

PP is one of the most important and widely used thermoplastics in the world today. PP possesses outstanding physical properties, low cost, and versatile applications [9]. In terms of mechanical properties, PP has relatively high stiffness and tensile strength compared to many common plastics. However, its main drawback lies in its relatively low impact strength, especially at low temperatures, which makes the material brittle. PP is extensively applied in a wide range of fields, including packaging, fibers, and thin films, as well as technical components such as automotive parts, household appliances, and medical equipment. The importance of PP lies not only in its versatility but also in its development potential through blending with other polymers, allowing for the flexible tailoring of mechanical properties for specific applications [10].

HDPE is a widely used thermoplastic with high global consumption and extensive applications across various industries [11]. The most common applications of HDPE include the production of bottles, household items, toys, trash bins, containers, … In terms of mechanical properties, HDPE has relatively low tensile strength, elastic modulus, stiffness, and flexural strength, indicating limited load-bearing capacity and structural rigidity. However, HDPE exhibits high impact toughness and flexural toughness, meaning it has good energy absorption and impact resistance. These traits reflect the flexible and ductile nature of HDPE, making it suitable for applications requiring materials that are not brittle and can deform without breaking. HDPE’s ability to be combined with other materials to form composites is also notable, as such blends can significantly enhance mechanical properties and broaden its application scope by creating superior materials [12].

Blending PP and HDPE through melt mixing has been widely employed due to the similarity in their melt flow indices [13]. According to Ahmed et al. [14], the 50HDPE/50PP blend demonstrated the best mechanical properties. This ratio improved both tensile and flexural strengths, with results showing that a 50% PP content increased the tensile strength of the composite by 29%, the highest value among the polyblends studied. The flexural strength across all polyblends was approximately 23 MPa and improved by up to 44%.

However, PP and HDPE are not fully compatible. Without compatibilizers, blends may exhibit inferior mechanical properties compared to pure materials, often due to issues such as phase separation or non-uniformity resulting from differences in melting temperatures and molecular structures. Many researchers have found that adding suitable fillers can improve the compatibility of PP/HDPE blends. For instance, Nina et al. [15] reported a significant increase in impact strength when EPDM was added to PP/HDPE blends. In Dikobe et al.’s study [16], incorporating wood flour into PP/HDPE blends enhanced stiffness and thermal stability but reduced impact strength due to the polarity difference between wood and the polyolefin matrix. In a study of Anjana et al. [17], nano kaolin clay had an apparent reinforcing effect on PP/HDPE blends; however, exceeding 5% content led to reduced performance due to agglomeration, making the material more brittle and decreasing its impact resistance. Wang et al. [18] have demonstrated that gamma irradiation is an effective method for enhancing the impact strength of both PP and HDPE.

Adding fillers to PP/HDPE blends may be essential depending on the intended application, as it offers significant advantages in tuning mechanical properties and processing performance. EVA is also one of the fillers that can be used when blending PP and HDPE. EVA is a copolymer of ethylene and vinyl acetate, primarily synthesized via free radical polymerization. EVA stands out for its suitable physicochemical properties and good compatibility with various additives, making it a popular choice in numerous industries, including insulation, cable jacketing, electronic packaging, waterproofing, anti-corrosion, and footwear manufacturing. In terms of mechanical properties, EVA has high elasticity, sound insulation, moisture resistance, and waterproof characteristics. However, it also has some limitations—most notably, low tensile strength, flammability, and the tendency to produce heavy smoke when burned, which limits its practical applications [19,20,21].

Based on previous studies, our group decided to study the influence of EVA on the unnotched impact toughness of PP/HDPE blends. This study employs a 50/50 PP/HDPE ratio, as this ratio offers the most stable mechanical properties. EVA is added to the PP/HDPE blend in increasing proportions from 5% to 25%.

2. Materials and Methods

Taisox (from Taiwan) supplies EVA (Figure 1A) with code 7350M (vinyl acetate content of 18.0 wt%). PP is supplied by Sabic from Korea (Figure 1B) with code 5704PR. HDPE (Figure 1C) is with code HDPE F120A, originating from Korea and manufactured by HANWHA. Samples were prepared and mixed according to the composition ratios listed in Table 1. The study was based on the most common mixing ratio, so 50/50 was chosen, and the change in EVA content was examined. The ratios of PP/HDPE/EVA blend are dried at 80°C before injection molding.

Figure 1 Types of plastics used in research. A) EVA, B) PP, C) HDPE.

Table 1 Blend ratio of PP/HDPE/EVA blends.

The specimens were fabricated via injection molding using a Toshiba IS 100E injection molding machine (Figure 2). The highest temperature is 170°C, the injection pressure ranged from 80 to 110 bar; the injection time was between 5 and 7 seconds; the injection speed was 50–60%; and the screw retraction distance was 30 mm. After injection molding, 10 samples were selected for testing (Figure 3). The measurements were conducted according to ASTM D256 under laboratory conditions with a temperature of 23 ± 2°C and a relative humidity of 50 ± 5%.

Figure 2 Dimensions of the test specimen for unnotched impact toughness testing.

Figure 3 Test specimens before and after unnotched impact toughness testing.

3. Results

3.1 Unnotched Impact Toughness Test

From Table 2 and Figure 4, it can be observed that the unnotched impact toughness increases with the addition of EVA to the PP/HDPE blend. Between 5% and 10% EVA, the unnotched impact toughness increases from 84.03 kJ/m² (5 EVA) to 85.06 kJ/m² (10 EVA); however, it then decreases to 70.3 kJ/m² at 15% EVA. After that, it increases continuously to 83.32 kJ/m² (20 EVA) and 89.42 kJ/m² (25 EVA). The discrepancy in value between the samples with the highest and lowest EVA content was 19.12 kJ/m². Overall, as the EVA content increases, the unnotched impact toughness also increases, except for the sample with 15% EVA, which shows a notable decrease.

Table 2 Unnotched impact toughness of PP/HDPE/EVA blend (kJ/m²).

Figure 4 Average unnotched impact strength of PP/HDPE/EVA blend.

3.2 Scanning Electron Microscope

Figure 5 shows the SEM microstructure of PP/HDPE/EVA blends. As can be seen from the figure, the 100% PP/HDPE sample has a clear phase separation between PP and HDPE, with an unstable bond structure, indicating complete incompatibility. This effect leads to low mechanical strength, particularly in terms of impact toughness. This result is similar to the result of Lin et al. [13] regarding the incompatibility between PP and HDPE, leading to incomplete improvement in mechanical properties.

Figure 5 The SEM photographs of the PP/HDPE/EVA blend at 200X magnification. A) PP/HDPE, B) 5 EVA, C) 10 EVA, D) 15 EVA, E) 20 EVA, F) 25 EVA.

The addition of compatibilizers such as EVA partly shows the improvement in the bonding ability between PP and HDPE. At 5% EVA content (5 EVA), phase separation between HDPE and PP still appears, but when the EVA content increases, the microstructure between PP and HDPE is improved. At lower EVA levels (such as 5%-15% EVA), the compatibility between PP and HDPE is not complete. There are still some incompatibility areas due to the lack of EVA, and the distribution of EVA may not be uniform and sufficient. This outcome may explain the uneven variation in mechanical properties, including the sudden decrease in non-notch impact toughness in the 15% EVA sample.

When the EVA content is increased to 20%-25% EVA, the higher content allows EVA to participate more in improving the compatibility between PP and HDPE, resulting in a nearly monolithic surface, as seen in the 25% EVA sample. The interphase adhesion is significantly enhanced at higher EVA content. At 25% EVA, relative homogeneity between PP and HDPE is shown. This consequence shows that EVA is entirely feasible as a compatibilizer for PP and HDPE. Adding EVA to PP/HDPE (5 EVA, 10 EVA, 15 EVA, 20 EVA, 25 EVA samples) gradually improves the homogeneity of PP and HDPE.

In summary, EVA significantly enhances the compatibility and microstructure of PP/HDPE blends, reduces phase separation, and creates a more homogeneous structure, especially at high EVA content. This improvement directly leads to changes in the material’s mechanical properties.

4. Discussion

The unnotched impact toughness is 84.03 kJ/m², 85.06 kJ/m², 70.3 kJ/m², 83.32 kJ/m², and 89.42 kJ/m², corresponding to samples 5% EVA, 10% EVA, 15% EVA, 20% EVA, and 25% EVA. The unnotched impact toughness of the specimens can be partly attributed to the mechanical properties of EVA and the microstructural morphology of the samples. PP and HDPE are two common thermoplastic polymers, but they are not completely chemically compatible. When blended without a compatibilizer, PP/HDPE blends can produce products with lower mechanical properties than the virgin material and exhibit segregation or unevenness due to differences in melting temperatures and molecular structures. EVA, a copolymer of Ethylene and Vinyl acetate, is added to PP/HDPE blends to act as an effective compatibilizer. EVA has good compatibility with both PP and HDPE due to its polar group structure and suitable physical properties. The addition of EVA improves phase dispersion in the polymer blend and enhances interactions between incompatible polymers. This influence is expected to have a positive effect on mechanical properties such as impact strength. According to previous studies [22,23] on the incorporation of EVA and varying EVA content in polymer blends, the positive effect of EVA on the impact toughness of the final product has been consistently reported. As the EVA content increases, impact toughness also increases, which aligns with the results of Eduardo et al. [24] study on the effect of EVA on PLA. The impact toughness gradually increased; the sample without EVA reached 27 J/m, the PLA/EVA 80/20 sample was about 145 J/m, and the PLA/EVA 70/30 sample reached 180 J/m. Compared with the measurement results at 0% EVA, the PP/HDPE blend reached 51.45 J/m and increased to 157.7 J/m at 20% EVA and 183.27 J/m at 25% EVA. It can be seen that with the similarity, when increasing the EVA content, the impact toughness was also significantly improved.

However, there are some inconsistencies with this general trend: at 10% and 15% EVA content, unnotched impact toughness decreases. Microstructural analysis reveals that the low-toughness groups (PP/HDPE/10% EVA and PP/HDPE/15% EVA) exhibit a poorly developed microstructure characterized by numerous defects, voids, and insufficient interfacial adhesion. In formulations showing high unnotched impact toughness (5% EVA, 20% EVA, and 25% EVA), the microstructure is more fully developed, interfacial bonding between phases is more stable, and the surface is smoother, resulting in enhanced unnotched impact toughness.

5. Conclusions

This study investigated the effect of EVA content on the unnotched impact toughness of PP/HDPE blends. Test specimens were fabricated via injection molding with varying EVA concentrations. The experimental results demonstrated that increasing EVA content generally enhances the unnotched impact toughness of the blend. Specifically, the unnotched impact toughness increased with a maximum difference of 19.12 kJ/m² between the sample with the highest EVA content and the sample with the lowest EVA content. Notably, the sample containing PP/HDPE/15% EVA exhibited a significant drop in toughness, deviating from the overall trend.

Acknowledgments

We acknowledge Ho Chi Minh City University of Technology and Education, and Material Testing Laboratory (HCMUTE). They gave me an opportunity to join their team, accessed the laboratory and research machines. Without their appreciated support, it would not be possible to conduct this research.

Author Contributions

Conceptualization, P.T.H.N.; Methodology, H.D.D.; Software, N.H.P.; Validation, N.T.Q.T.; Formal Analysis, H.D.D., N.H.P. and N.T.Q.T.; Investigation, D.T.V.A. and V.X.T.; Resources, H.V.H. and P.Q.A.; Data Curation, N.V.T.; Writing – Original Draft Preparation, H.D.D., N.H.P. and N.T.Q.T.; Writing – Review & Editing, P.T.H.N.; Visualization, P.T.H.N.; Supervision, P.T.H.N.; Project Administration, P.T.H.N.

Funding

This work belongs to the project grant No: SV2025-378 funded by Ho Chi Minh City University of Technology and Education, Vietnam.

Competing Interests

The authors have declared that no competing interests exist.

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

AI-Assisted Technologies Statement

During the preparation of this work, the authors used Grammarly to improve readability and language. After using this tool/service, the authors reviewed and edited the content as needed and took full responsibility for the publication's content.