The effect of UV additives on thermo-oxidative and color stability of pistachio shell reinforced polypropylene composites

Abstract

In this study, pistachio shell-polypropylene copolymer (PPc) biocomposites were prepared by using the melt compounding technique. Treated pistachio shell (TPS) (30%wt) was employed to increase mechanical and thermal properties of biocomposites. Maleation and filler pre-treatment processes improved the mechanical strength of the PPc-TPS composites. The UV additives, Chimassorb 944 (CHI) stabilizer, and the UV absorber Irganox 1010 (IRGV) were used to enhance oxidative and color stability of biocomposites after they were subjected to accelerated UV irradiation for 600 h. The mechanical properties of the UV additive containing composites were less affected after UV exposure. The oxidative induction time and discoloration analysis revealed that CHI and IRGV additives can be utilized in applications that require higher color stability while the former showed better performance for oxidative stability.

Keywords

ageing compatibilizer discoloration natural filler UV additives silanization oxidative stability

Introduction

Recently, composite materials have increased in importance in many applications such as food packaging, automotive, and electronics due to their advantages and diversity.¹ Today, as concepts such as recycling and sustainability come to the fore, the use of natural fillers in polymer composite is increasing rapidly.² Fillers based on natural resources of plant origin for use as reinforcement materials are being developed due to the biodegradability and recyclability, low density, high specific strength values, as well as sustainability.^3–5 However, the hydrophilic character of natural fibers, which causes incompatibility with the hydrophobic polymer matrix, and the requirement for low processing temperatures to avoid degradation, have limited the use of natural fibers in composite material production.⁶ The most widely used method to modify the fiber and polymer interface in natural fiber-reinforced composite materials is to apply surface treatments such as alkaline and silane, which can change the interaction between fiber and matrix. Binding agents provide optimum tension transfer at the fiber/matrix interface.⁷ Maleic anhydride grafted polypropylene (MA-g-PP) is used as a binding agent to strengthen the bond between lignocellulosic fiber and polypropylene. For natural fiber-reinforced composites to be used in outdoor products, they must have appropriate properties against outdoor conditions. When such composites are exposed to direct sunlight, covalent bonds are damaged, resulting in discoloration, an increase in surface roughness, a decrease in mechanical properties, and an increase in fragility. The desired properties of such materials as outdoor materials include resistance to UV rays, resistance to moisture absorption, and dimensional stability.^8,9 To improve the properties of natural fiber-reinforced composite materials against ultraviolet absorption, UV additives with absorbent properties are used. A review of published studies revealed that mechanical and thermal properties were improved by adding 2–6% MAPP to lignocellulosic fiber-reinforced polypropylene matrix.¹⁰ Zampaloni et al. found that for PP/kenaf composite the optimum addition was 3% MAPP.¹¹ In previous studies, they determined the general effect of photostabilizers on harmonizing the properties of minerals in outdoor applications of natural fiber-reinforced polypropylene composites.^10,12

This study aimed to examine the effect of UV stabilizer and absorbent on oxidative induction and performance of MAPP as a compatibilizing additive for PPc/TPS biocomposites. To our knowledge, this is the first study that evaluates PPc composites reinforced with pistachio shell. It also investigates the effect of two different UV additives on color and oxidative stability after UV irradiation.

Experimental

Materials

Commercial CB 6000 MO polypropylene copolymer (PPc) and MAPP were bought from Acran Kimya in Turkey. The density, melting point, and melt flow rate of PPc were 0.905 g/mL, 176°C and 60 g/10 min (230°C/2.16 kg), respectively. Pistachio shells (PS) were obtained from a local market located in Gaziantep province, Turkey. Tetraethoxysilane was used as silane coupling agent for PS pre-treatment. UV (Light Stabilizers-HALS) Chimassorb 944, and the UV absorber Irganox 1010 were obtained from BASF in Germany.

Pre-treatment of Pistachio shells

Pistachio shells were soaked in water for 24 h to remove any water-soluble impurities and were milled to 325–400 mesh (US Standard) after air-drying at 80°C for 72 h. The resulted PS powder was treated in an alkali medium and subsequently subjected to a silanization process.^6,13

Preparation of biocomposites

Based on existing literature, a PS/TPS filler content of 30% (w/w) of the composite was chosen.¹⁰ The amount of MAPP was kept at 3% (w/w), whereas the UV additives CHI and IRGV were fixed at 0.5% (w/w) (Table 1). A counter-rotating melt mixer was used to prepare the biocomposites at a temperature of 190°C and a rotor speed of 100 r/min. Pre-mixed PPc and MAPP were molten for at least 2 min and mixed for 5 min. Subsequently, the PS/TPS filler was introduced into the melt mixer and mixed for another 5 min. Lastly, UV additive was added to the melt mixer followed by 2 min of further mixing. Test samples were compression molded using a hot press at 190°C for 2 min followed by 2 min of cooling at 130 bar to obtain the desired sample dimensions for further analysis.

Table 1.

Specimen codes and formulations of biocomposites.

Specimen Code	PPc (w%)	PS (w%)	TPS (w%)	MAPP (w%)	UV additive (w%)
PPc	100	—	—	—	—
PPc/PS	70	30	—	—	—
PPc/TPS	70	—	30	—	—
PPc/TPS/MAPP	67	—	30	3	—
PPc/TPS/MAPP/CHI	67.7	—	30	3	0.5
PPc/TPS/MAPP/IRGV	67.7	—	30	3	0.5

Tensile analysis

The tensile properties of biocomposites were determined using a Zwick/Roell universal testing machine with 1 kN nominal force according to the ISO 527-1 standards. The test was performed with 60 mm/min cross speed. The test speed was 5 mm/min for using tensile bars. Test specimens were prepared according to ASTM D 638 standard.

Charpy impact tests

Charpy impact resistance tests were performed on unnotched specimens using an Instron CEAST 9050 test machine according to EN ISO 179-1 standard.

Morphological properties

The comparative study of morphology was taken using a scanning electron microscope (SEM) (Model Inspect S50 with an acceleration voltage of 7.5 kV). SEM specimens obtained from the fractured surface of specimens from tensile testing were sputter-coated with a layer of gold.

Oxidative induction time

The oxidative stability of biocomposite specimens was examined by oxidative induction time (OIT) measurement to observe the time to the onset of oxidative decomposition. The OIT of different PPc specimens was measured by differential scanning calorimetry (DSC) Seiko, Tokyo, Japan DSC 7020. The nitrogen flow rate was 50 mL/min. Samples were exposed to a nitrogen atmosphere at a heating rate of 10°C/min. After reaching a temperature of 200°C,¹⁴ the nitrogen flow was changed to an oxygen flow of 50 mL/min and a constant temperature was maintained until the sample was completely oxidized.

UV irradiation

Tensile bar specimens of the biocomposites were treated with UV irradiation. The accelerated aging process with UV light was conducted in an air-conditioned chamber at 50°C according to ASTM G113 standard.¹⁵ The species were prepared for tensile, colorimetry, and OIT analysis after 600 h of UV exposure.

Thermogravimetric analysis

The thermogravimetric analyses (TGA) (Seiko TG/DTA 6300, Seiko Instruments, Tokyo, Japan) were realized at a temperature range of 30°C–600°C under nitrogen atmosphere. The heating and flow rate were 10°C/min and 40 mL/min, respectively.

Color stability

Discoloration of the biocomposites were determined after 600 h of UV light exposure by comparing with the color of the initial specimens. Discoloration measurements were carried out with Color i5 X-Rite Color Spectrophotometer considering the CIELAB color parameters. Color differences (ΔE*) of the specimens were calculated as follows

Δ E^{*} : {[{(Δ L^{*})}^{2} + (Δ a^{*}) + {(Δ b^{*})}^{2}]}^{\frac{1}{2}}

L* is the color brightness, a* is the red-green color component and b* is the yellow–blue color component.

Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) was performed using a Dynamic Mechanical Analyzer (TA Instruments DMA Q800) instrument. Analysis was carried out at a heating rate of 3°C/min from −40 to 130°C in single cantilever mode (length: 35 mm, amplitude: 20 μm, and 1 Hz frequency). The analyses’ DMA results together with their discussions are presented in the Supplementary Material File.

Results and discussion

Morphological properties

SEM image of the neat PPc was shown in Figure 1. a. PS, TPS, MAPP, and PPc composites with UV additive added were shown in Figures 1(b–f), respectively. It was seen that the fibers in the matrix were clearly exposed with composites without MAPP like as the study by Akindoya.¹⁰ In addition, the adhesion between the fiber and matrix is higher, as evidenced by the presence of shorter drawn fibers occurring on the surface of the polymer. When comparing PPc+PS and PPc+TPS composites with (Figure 1(e) and (f)) without UV (Figure 1(b) and (c)) additives, the adhesion between fiber and matrix decreased with the addition of CHI. Although the presence of compatibilizer (MAPP) caused a significant improvement in adhesion between phases (Figure 1(e)), the tensile stress values were negatively affected due to the gaps created by the addition of CHI to PPc/TPS/MAPP. It was found that the addition of IRGV absorbent (Figure 1(f)) did not affect mechanical properties.

Figure 1.

SEM images of the fractured surfaces of (a) PPc, (b) PPc+PS, (c) PPc+TPS, (d) PPc+TPS +3% MAPP, (e) PPc+TPS+3% MAPP+%0.5CHI and (f) (e) PPc+TPS+3% MAPP+%0.5 IRGV.

Mechanical tests

The tensile test results of the neat PPc and composite species before and after UV irradiation are shown in Figure 2. The experiments are designed to investigate the effects of pre-treatment of lignocellulosic filler, MAPP as a coupling agent, and the UV additives by comparing with pristine PPc+PS specimen. It is observed that (Figures 2 and 3) the tensile strength (TS) and modulus (TM) of the species decreased after UV irradiation in all cases. The addition of PS to PPc (PPc+PS) decreased the TS value, while TM values increased considerably for all biocomposites. Similar results were obtained by the study of Staffa et al. where incorporating 30% of coconut fiber into a PP matrix reduced the TS by 14.3% and increased the TM value by 28%.¹² It is assumed that since PS as a lignocellulosic filler possesses highly polar groups, the interphase adhesion of the composite is negatively affected, especially in the absence of coupling agents such as MAPP. The difference between polarities of the polymer matrix and the filler hindered proper load transfer and resulted in decreased TS values. On the other hand, regardless of the using coupling agent, the TM values of the composites were increased due to the mobility of the PP chains being restricted by the filler.¹⁶

Figure 2.

Initial and after UV irradiation of tensile strength of PPc, PPc/TPS biocomposites containing 30 wt% TPS, and different percentage of CHI, IRGV, and MAPP.

Figure 3.

Tensile modulus before and after UV irradiation of PPc, PPc/TPS biocomposites containing 30 wt% TPS and different percentages of CHI, IRGV, and MAPP.

Using pre-treated filler (TPS) with PPc in the absence of MAPP supplementation (PPc+TPS), both the TS and TM values increased compared to those of the PPc+PS sample as illustrated in Figures 2 and 3. These results showed the effectiveness of applying alkalinization and subsequent silanization procedures to PS filler for improving mechanical properties. Alkali treatment is the most widely applied method for lignocellulosic reinforcement materials. It removes impurities such as pectin and waxes and also reduces the hemicellulose and lignin content to some degree. Thus, the specific surface area of the natural material is increased and more reactive hydroxyl groups become available for further interactions .¹⁷ Implementation of a second pre-treatment step of silanization after alkali pre-treatment further improves the mechanical properties compared to alkali or silane pre-treatment alone.^18–20

Figures 2 and 3 show that the TS and TM values of the maleated composite sample (PPc+TPS+3%MAPP) are higher than those of the unmaleated composites (PPc+PS and PPc+TPS) irrespective of pre-treatment of filler material. It is also worthwhile to observe that the pre-treatment of PS and the addition of 3% MAPP compensated the decrease in TS of PPc+PS samples compared to neat PPc. In addition, the TM value increased by 41% compared to that of neat PPc. A coupling effect of maleation through the enhancement of adhesion between natural material surfaces and polymer matrix via the formation of covalent bonds is reported elsewhere.²¹ Although the addition of coupling agents facilitates processing and improves the dispersion of fibers in the polymer matrix, the composite should have an optimal coupling agent to fiber ratio to increase mechanical properties to a desirable level.²² Thus, it should be possible to increase the TS value of maleated composite (PPc+TPS+3%MAPP) by increasing the percentage of MAPP, compared to neat PPc.

The percent decrease of tensile properties upon UV irradiation of neat PPc and biocomposite specimens is shown in Table 2.

Table 2.

Percentage reduction in mechanical properties and ΔE* values of specimens after UV irradiation.

Specimen codes	Reduction in maximum tensile strength (%)	Reduction in impact strength (%)	Reduction in maximum tensile modulus (%)	ΔE^*
PPc	17	27	17	4.90
PPc+PS	17	22	16	16.67
PPc+TPS	11	18	13	4.13
PPc+TPS+ 3%MAPP	10	11	11	25.09
PPc+TPS+ 3%MAPP+ 0.5%CHI	2	1	4	2.61
PPc+TPS+ 3%MAPP+ 0.5%IRGV	5	3	7	18.57

Although the tensile properties of all specimens was reduced after UV irradiation, the decrease in TS and TM values of the UV additive supplemented composites are among the lowest, as is seen in Table 2. TS and TM reduction (17% and 16%) of PPc+PS samples after UV irradiation was higher than that of the PPc+TPS and PPc+TPS+3%MAPP specimens. For PPc+PS, this result might be attributed to improper interfacial interaction between the matrix and the filler, which results in elevated susceptibility of the composite to UV exposure.²³ It is noticed in Table 2 that there is also a reduction of TS and TM values of PPc+TPS+ 3%MAPP samples after UV irradiation. Similar results were obtained in a study by Akindoyo et al¹⁰ in which PP and MAPP were blended with palm fiber. They concluded that the TS reduction could be attributed to the formation of enhanced UV degradation sites originating from the increased number of bonds within the composite due to the addition of coupling agent. Overall, melt mixing of MAPP and TPS with PPc in the presence of CHI slightly reduced TS and TM values by 2% and 4%, respectively, after UV irradiation. On the other hand, the impact strength (IS) values of the biocomposites before and after UV exposure depicted in Figure 4 are in good agreement with the tensile values of the species. As it is seen in Figure 4, while the IS results of the neat polymer and the biocomposites without UV additives are decreased dramatically after UV irradiation, the biocomposites with UV additives were less affected. The decrease in impact values can be attributed to the deterioration of interactions at the composite interface due to UV irradiation. It is recognized that the oxidation of polymers can make them more brittle, and variants without UV additives have low IS values.

Figure 4.

Impact strength of PPc, and PPc/TPS biocomposites containing 30 wt% TPS, and different percentage of MAPP, CHI, and IRGV.

Color Stability

The results obtained from the color change analysis performed on the specimens are shown in Table 2. Color changes in samples before and after UV irradiation are presented in Figure 5. Color change (ΔE*) in biocomposites is significantly lower than in neat PPc. The reason for this can be explained in two ways; a) pistachio shell may exhibit UV protective effects, b) the initial color of the PPc may make the difference in color after UV exposure less noticeable. However, the reduction in tensile strength, impact strength, and tensile modulus after UV exposure presented in Table 2 clearly show that the PPc matrix undergoes photo-degradation. As shown in Table 2, specimens of PPc+PS (b), PPc+TPS+3%MAPP (d), and PPc+TPS+3%MAPP+%0.5 IRGV (f) biocomposites revealed ΔE* values of 16.67, 25.09, and 18.57, respectively, which is higher than those seen for PPc+TPS (c) and PPc+TPS+3%MAPP+%0.5CHI (e) biocomposite specimens at 4.13 and 2.61, respectively, indicating a higher sensitivity of the specimens to UV irradiation. The lower discoloration in the presence of CHI and IRGV (Table 2, specimens e and f) suggests protection from the negative effects of UV radiation. This may be attributed to the UV stabilizing effect of CHI and IRGV as reported in a similar published study.²⁴ In addition, the least color change was observed in the CHI-doped biocomposite sample, with a ΔE* value of 2.61. Among the biocomposites, the sample with MAPP (without CHI and IRGV) (Figure 5(d)) showed the greatest change in color. This suggests that MAPP is highly sensitive to UV radiation.¹⁰ Sensitivity to UV irradiation is due to the absorption of UV irradiation by the polymer. Photo-degradation results in surface oxidation, chain scission, and break-down of tie molecules. Micro-cracks occur as a result of chain scission and surface oxidation.²⁵ As can be seen in Table 2, the mechanical properties of composite samples without UV additive were greatly reduced after exposure to UV irradiation due to mechanical degradation caused by photo-degradation. It is seen in Figure 5 that a significant color change occurred in the biocomposite bars without UV additive after 600 h of UV irradiation. It has been stated in the literature that the color changes are caused by photo-degradation of the PPc matrix.²⁶ As seen in Figures 5(e) and (f), no color change was observed in PPc biocomposites supplemented with UV additive. The color of these biocomposites remained almost the same as before they were exposed to UV irradiation.

Figure 5.

Images of specimens (a) PPc, (b) PPc+PS, (c) PPc+TPS, (d) PPc+TPS+3%MAPP, (e) PPc+TPS+3%MAPP+%0.5CHI, and (f) PPc+TPS+3%MAPP+%0.5 IRGV.

Oxidative stability

As described in the discoloration sections, the decrease in mechanical properties of the specimens is due to the effects of photo-degradation. The oxidative stability of specimens was evaluated by OIT analysis. The interaction between composite components can be explained by a dual mechanism of action. CHI and IRGV may function either as UV absorber or as UV stabilizer. As is well known, the energy of UV irradiation is greater than the chemical bond energy between polymer molecules. As a UV additive, CHI and IRGV can help prevent the polymer by trapping radicals generated by UV irradiation.¹⁰ The OIT curves of the biocomposites are shown in Figure 6(a). The OIT test is a method for evaluating the thermal stability of a polymer in an oxygen atmosphere at a temperature higher than the polymer’s melting point. Musajan et al. reported the effect of antioxidants on the thermal stability of polypropylene (PP). The antioxidants in the sample increased the oxidation induction time and then delayed the thermal oxygen degradation of the sample. Compared to Irganox 1010 (IRGV), all-natural antioxidants slow the oxidation and break-down of PPc and also play a certain role in anti-oxidation. Musajan et al. suggested that the removal of PPc radicals by natural polyphenols mainly occurs via hydrogen transfer from the phenolic hydroxyl group to reactive free radicals and quench the active radicals to stabilize the degradation of PPc.²⁷

Figure 6.

Oxidative induction time curves of composite specimens (a) MAPP, PPc, CHI, and IRGV (b) Initial: Oxidative induction time curves of PPc composite specimens without UV Irradiation c) Oxidative induction time curves of PPc composite specimens after UV irradiation.

As can be seen from Figure 6, the oxidative decomposition of composites with MAPP begins faster (3.41 min) compared to neat PPc (8.76 min). However, CHI and IRGV appear stabilize against UV-induced oxidation (Figure 6(a)).

Oxidative induction time curves of PPc, MAPP, CHI, and IRGV biocomposites before and after UV irradiation are shown in Figures 6(a–c), respectively. As seen in the figures, it can be seen that the presence of MAPP tends to reduce the OIT values of biocomposites, while the presence of CHI and IRGV helps to increase oxidative stability. However, it is worth noting that after UV irradiation composites (PPc+TPS and PPc+TPS+3%MAPP) exhibit higher OIT compared to PPc. The antioxidants contained in natural fibers contribute to the stabilization of composites during prolonged exposure to UV rays. It can also be seen that in the presence of UV protectors, oxidation reactions in the composites are significantly slowed down. The effect of UV protectors (especially CHI) has also been observed in color measurements and OIT results.¹⁰

According to the OIT results observed after UV irradiation treatment, the addition of pistachio shell to PPc increased OIT time. The addition of TPS further increased OIT time of PPc. The advantage of alkali treatment and silanization of the pistachio shell was also seen in the OIT results. The OIT result of the CHI-doped biocomposite was 6.09 min, higher than the OIT value of the IRGV-doped biocomposite at 1.52 min. In terms of OIT test results, CHI-doped composites gained more oxidation stability compared to the other composites.

Thermal properties

Thermogravimetric analyses was performed to characterize the thermal behavior of secondary, tertiary, and quaternary PPc biocomposites. As seen in Figure 7, the neat PP curve is different from the curves of biocomposites. Although there is one degradation stage in the PP curve at 462°C decomposition temperature, the degradation of biocomposites occurred in two stages at decomposition temperatures between 451 and 461°C. The characteristic parameters related to the thermal decomposition of the biocomposites are shown in Table 3. The T₅₀ value of samples is known to be a good indicator for evaluating structural stability.²⁸ Although a single degradation stage occurs at 462°C in the PP curve, the initial rate of degradation in composites started at about 360°C with a very slow degradation rate. The main decomposition of the biocomposites occurred in the temperature range of 451–461°C. PS (Figure 8, red line) began to decay at about 265°C, and its second decay began at 340°C. After pre-treatment (TPS, Figure 8, green) PS gained temperature resistance. The thermal decomposition temperature of the biocomposite increased from 454°C to 461°C with the addition of pre-treated PS (TPS) to PPc, The thermal decomposition temperatures of biocomposites with UV additives were 460°C and 459°C for CHI and IRGV, respectively. In addition, the degradation rates of biocomposites with UV additives were lower than those of the other biocomposites.

Figure 7.

TG thermograms of PPc and PPc biocomposites.

Table 3.

Thermal degradation of PPc and biocomposites containing 30 wt% TPS, and different percentage of UV additives and MAPP.

Specimens	T₁₀ (°C)	T₅₀ (°C)	T_d (°C)	Residue (%)(at 600°C)
PPc	431	456	462	1
PPc+PS	311	443	454	5.9
PPc+TPS	339	453	461	4.5
PPc+TPS+3%MAPP	335	440	451	3.8
PPc+TPS+3%MAPP+0.5%CHI	328	449	460	3.9
PPc+TPS+3%MAPP+0.5%IRGV	326	447	459	5.9

Figure 8.

TG Degradation rate of PPc and PPc biocomposites.

When the deformation temperatures of PPc and composites are compared in Table 3, although the deformation temperature decreased with the addition of PS to PPc, the deformation temperature reached that of neat PPc again with the addition of alkali and silane treated PS (TPS) and UV additives.

Alkali treatment of PS breaks the intra-intermolecular hydrogen bonds between cellulose, hemicellulose, and lignin, causing defibrillation of the fibers. Due to this defibrillation, fiber bundles are converted into smaller fiber segments, resulting in an increased effective surface area.²⁸ As a result of the alkali treatment and silanization, the surface interaction between TPS and PPc increased and thermal stability of biocomposites was achieved.

Conclusion

The UV stability of the composite was enhanced by adding CHI and IRGV as UV additives. Although the morphological analyses showed slightly improved surface interaction between PPc and TPS with the use of UV additives, the mechanical properties of these composites fell slightly. It should be noted, however, that the inclusion of CHI and IRGV does not provide a significant improvement in the thermal stability of PPc/TPS composites, as demonstrated by TGA analysis. However, OIT and discoloration analyses showed that UV degradation as well as discoloration of composites were prevented in the presence of UV additives. This indicates that those can be used in applications requiring higher color stability in the presence of UV additives. Furthermore, the DMA results showed that compared to ternary composites without UV additives (PPc+TPS +3% MAPP) or MAPP (PPc+TPS+0.5% CHI), the composites containing both CHI, and MAPP exhibited lower loss moduli. UV additives doped TPS filler reinforced PPc biocomposites can be used in outdoor applications.

Supplemental Material

sj-pdf-1-ppc-10.1177_09673911221081700 – Supplemental Material for The effect of UV additives on thermo-oxidative and color stability of pistachio shell reinforced polypropylene composites

Supplemental Material, sj-pdf-1-ppc-10.1177_09673911221081700 for The effect of UV additives on thermo-oxidative and color stability of pistachio shell reinforced polypropylene composites by Mithat CELEBI, Muslum ALTUN and Sabih OVALI in Polymers and Polymer Composites

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) (Grant No: 218M916).

Supplemental material

Supplemental material for this article is available online.

ORCID iDs

Mithat Celebi

Muslum Altun

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