Effect of PET functionalization in composites of rubber–PET–HDPE type

2.1 Materials

Rubber tire waste was obtained from the Radburg company (Romania). Rubber from truck tires was used to obtain the composites; this contains natural rubber (43%w) and synthetic rubber (styrene butadiene rubber and butadiene rubber < 22.8%w). The tire rubber contains vulcanizing agent (sulphur < 0.8%w), accelerators (MBT/TMT < 0.8%w), inorganic activators (calcium/zinc oxide < 3.3%w), other ingredients (carbon black < 27%w), plasticizers (mineral oil), antioxidants and antiozonants (<1.3%w) and other components (<1%w). The rubber tire density is 0.87 g/cm³ and the tensile strength is 34.67 N/mm².

The tires are processed by the company on specially designed machines and rubber mills. The procedure is essentially mechanical and runs four phases of grinding: cutting beads, shredding (pieces of about 100 mm), granulation (granules from 0 to 15 mm) followed by blades grinding to obtain granules and rubber powder in pieces ranging from 0.2 to 6 mm.

The plastic wastes, PET and HDPE, were purchased as small pellets with an average diameter of 1 cm from the Eco Green company (Romania), that processes plastic household wastes. The plastic wastes were further milled (centrifugal mill ZM 200, Retsch) to obtain grains with the average size of 1 mm with the following densities: for PET is 1.22 g/cm³ and for HDPE is 0.89 g/cm³.

In the composite development the tire rubber was used as matrix, PET as dispersed phase in the polymer–polymer composite and HDPE as additive to strengthen the PET/rubber interface.

2.2 PET functionalization

To increase the interfacial adhesion in the composite material, the component with the highest hydrophilicity (PET) was functionalized with 1% solutions of SDS (Alfa Aesar, 99%) and PEG 400 (Alfa Aesar, 99%), prepared using distilled water. Both surfactants are hydrophilic and have low toxicity; PEG 400 is a low-molecular-weight grade polyethylene glycol, Fig. 1a, and SDS is an anionic surfactant with the formula H₃(CH₂)₁₁OSO₃Na, Fig. 1b.

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Figure 1

(a and b) Structural formula for PEG 400 and SDS surfactant.

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For functionalization, the milled PET grains were inserted into the 1% surfactant solution (SDS, respectively PEG 400), for 1 h. Then, they were dried (200–300 ECV type thermostat) at 80 °C for 60 min, before using them to obtain the composites.

2.3 Composites synthesis

Blends of rubber–PET–HDPE at various compositions were prepared by compression moulding (thermostatic oven type ECV 200–300), in two series of experiments: at 220 °C (samples Type a) and 240 °C (samples Type b). Two moulding durations were investigated: samples Type 1: 60 min and samples Type 2: 30 min, respectively. Samples of 15 g were obtained as composites with mass ratio rubber:PET:HDPE = (60 − x):x:5, where x = 35; 40; 45; a volume of 20 mL surfactant solution 1% was used to functionalize the corresponding amount of PET. The following compositions were studied in: composite Type a: rubber: PET:HDPE = 60:35:5; composite Type b: rubber:PET:HDPE = 55:40:5, and rubber:PET:HDPE = 50:45:5 – composite Type C.

The ratio functionalized PET:solution was 5.25–6.75 g:20 mL for obtained samples (depending on the composite composition). The effect of the functionalizing additives was comparatively investigated with samples using PET that was not functionalized; thus, three series of rubber:PET:HDPE composites were obtained, using

1. PET functionalized in SDS solution 1%, samples Type S;

2. PET functionalized in PEG 400 solution 1%, samples Type P;

3. Un-functionalized PET, samples Type N.

The sample preparation data and their codes are given in Table 1.

2.4 Characterization

3.1 Functionalization of PET

The polymer chains interact with both the polar/ionic parts and the hydrocarbon chain of the surfactants; thus, PET–surfactant interactions can be of electrostatic and/or dispersive types. During functionalization with SDS (at a concentration higher than the critical micelle concentration, CMC = 2307 ppm = 0.23%), the polymer chains support the formation of SDS micelles/aggregates bounded on the PET chains, Fig. 2.

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Figure 2

Formation of SDS micelles on PET chain.

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Therefore, the PET macromolecular chains will be the subject of rearrangements and stretching, with opening of the SDS micelles and orientation in hydrophobic–hydrophobic PET–SDS interactions, that leave the hydrophilic part outwards, pushing hydrophilic areas at the surface of the system.

When using the PEG 400 surfactant, the similarities between the surfactant and the polymer may led to extensive dispersive or slightly polar interactions surfactant–polymer, in certain position on the PET chains.

Following functionalization, the PET surface state will change, the interfaces in the composite might contain different bonds and the transition temperatures will be modified; the functionalization process requires re-optimizing the composition and the technical parameters: the PET percentage in the composite, the processing temperature and its duration.

3.2 Composition and structural properties

The chemical structure/interface bonds of the PET component (un-functionalized and functionalized) and of the composites were investigated using Fourier Transform Infrared Spectroscopy analysis. The FTIR spectra of the composites contain several bands belonging to the basic components, but new bands appear as a result of mechanical adhesion during moulding, along with physical or chemical bonds formed between components because of their affinity or due to the oxidative processes, with development of new interfaces.

The FTIR spectra of un-functionalized PET and functionalized PET are shown in Fig. 3a–c. The variations in the spectra measured within the wave number range of 600–1000 cm⁻¹, are associated with the vibrations of unsaturated structures, mainly the bending vibrations of the C—H groups at the C⚌C bonds, corresponding to the aromatic nuclei.

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Figure 3

(a–c) FTIR spectra for un-functionalized PET, PET–SDS and PET–PEG.

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The FTIR spectra of un-functionalized PET show bands at 1018 cm⁻¹ (C—O stretching), and 873 cm⁻¹ (CH₂ rocking), which are attributed to the gauche/amorphous conformation. Other spectral features confirm the PET crystallinity, such as the strong band corresponding to the carbonyl stretching band at 1715 cm⁻¹. This spectral pattern is determined by the ordered structure in crystalline PET where the carbonyl groups are coplanar with the benzene rings.

There are displacements of the characteristic bands, as those at 944 cm⁻¹, corresponding to the ethylene glycol moiety in the trans conformation which also indicates crystallinity in PET. This band is shifted to lower wavelengths for functionalized PET (914 cm⁻¹ for PET–SDS and 923 cm⁻¹ for PET–PEG), outlining a crystallinity decrease in the functionalized PET. Displacement can be observed in the reflectance maxima of some bands for PET–PEG, such as those at 1448 cm⁻¹ displaced to 1456 cm⁻¹ corresponding to C⚌C vibrations and —CH₂ of ethylene unit planar vibrations, or those at 1088 cm⁻¹ moved to 1092 cm⁻¹, corresponding to the aromatic nuclei and vibrations of the gauche conformation. This means that some of the dipoles existing in the polymers undergo vibration alterations due to the change in their close vicinity, following the functionalization effect.

After the functionalization of PET with SDS structural and conformational changes occur. Characteristic bands of the group —SO₃H are between 1245–1155 cm⁻¹ and 1080–1040 cm⁻¹. In the PET–SDS spectra, the —SO₃H band appears at 1174–1117 cm⁻¹ and 1040 cm⁻¹ (these bands overlap with the specific aromatic nuclei and gauche conformers’ vibration). These results are consistent with contact angle analysis; thus, the SDS molecules bind to PET with the hydrophobic part, leading to smaller contact angle values.

In the composites spectra with PET–SDS and PET–PEG there are missing bands at 2916 cm⁻¹ (—CH₂—), 2839 cm⁻¹ (CH₂⚌CH—), 2128 cm⁻¹ (intramolecular OH gr. C⚌O), 1715 cm⁻¹ (C⚌O)O— terephthalate units), and 1267 cm⁻¹ (—CO—). The absence of these bands indicates the formation of new physical–chemical interfaces between the components in the composites. There are interactions at the molecular level between PET and SDS thus increasing the ability of PET to form the polymer–matrix interfaces. Oxidative degradation (absence of band 2839 cm⁻¹) also takes place, obtaining a new radical that initiates chemical reactions in composite, involving the C⚌O or the intramolecular —OH groups (band 2128 cm⁻¹), and depolarizing the surface. The new bands in the spectra of functionalized PET (2915 cm⁻¹ and 2855 cm⁻¹ for PET/SDS; 2886 cm⁻¹ for PET/PEG corresponding to CH₂⚌CH—) confirm the surface changes in the PET structure.

The FTIR spectra of the samples obtained at 30 min moulding have similar bands as samples obtained in 60 min moulding but with higher intensity.

A comparative analysis of the FTIR bands for (un)functionalized PET and for the three composite samples is presented in Table 2.

As a result of oxidative degradations on macromolecular chains from the rubber–PET–HDPE composites, new bands appear at 3150 cm⁻¹ (—OH), 1867 cm⁻¹ (C⚌O, acid) and 1162 cm⁻¹ (gauche conformers). Another important spectral range significantly influenced by the amorphous/crystalline state of the composites includes the bands at 1162 cm⁻¹ (C—O stretching), 924 cm⁻¹ and 813 cm⁻¹ corresponding to trans and gauche conformers. These bands are responsible for the crystallinity in the composites.

The PET crystallization and high stereoregularity in the macromolecules are due to nuclei position of phenylene and coplanar ester groups, Fig. 4a. PET characteristic peaks were identified at 2θ = 16.8° and 25.2°, with an overall crystallinity degree of 56.8%. In PET functionalized with SDS and PEG, this band moved to lower wave numbers (at 1713 cm⁻¹ and 1712 cm⁻¹ respectively). The crystallinity percentage in the composite type 1-S and type 1-P, decreases with 8.9% respectively 4.4% (Fig. 4b), as a result of structural changes affecting the trans conformers during functionalization (the band at 944 cm⁻¹).

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Figure 4

XRD diffraction for: (a) rubber, HDPE and (un)functionalized PET; (b) composites with (un)functionalized PET.

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In the Type 1 composites series, the samples 1b-BS and 1a-AP recorded the highest crystallinity degrees (34.1% and 34.7%), as presented in Table 3, while for the Type 2 samples the values slightly decrease by 1–1.5%.

The data in Table 3 outline that SDS effect can be well observed at the higher processing temperature (240 °C) and at a high PET percentage (40% and 45%), acting as a possible plasticizer and yielding ordered structures with increased crystallinity. The SDS molecules will change orientation: with the ionic group towards the PET polar zone and the hydrocarbon chain at the surface of the hydrophobic components in the system, particularly those who impose crystallinity to the composite (HDPE). PEG functionalization allows a similar trend but with lower overall effect, as a possible result of a lower ordering effect (consequence of weaker interactions with PET).

According to the experimental results, the composites crystallization was suggested to be due to PET functionalization (with SDS and PEG 400) and the nuclei migration across the interface or altered chain mobility at the interface. This modification provides nucleation sites for crystallites formation with a consequent increase in the tensile modulus and in the mechanical strength of the compounds. The increase in the PET content slightly lowers the crystallinity degree and a similar effect is observed at longer processing duration. For both processing durations (60 and 30 min) the highest crystallinity degrees correspond to the samples containing 40% of PET obtained at 240 °C, functionalized with SDS (sample 1b-BS, 2b-BS) and with PEG (1b-BP and 2b-BP).

3.3 Thermal properties

The thermal characterization (DSC analysis) is in good agreement with the FTIR measurements, indicating the modification in the thermal behaviour of PET due to its functionalization. Differential scanning calorimetry results showed that the PET–SDS sample recorded the lowest melting temperature (T_m = 200.39 °C), Table 4 and Fig. 5. The decreased transition temperatures and the melting enthalpies of functionalized PET allowed the assumption that rubber–PET–HDPE composites may be obtained at lower temperatures than 240 °C (temperature previously optimized for these types of composites with lower PET content (Cazan et al., 2014)).

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Figure 5

Differential scanning calorimetry for un-functionalized PET, PET–SDS, and PET–PEG.

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3.4 Surface properties

By adding the functionalized PET in composites, the control of the interfacial tension is aimed at, to promote adhesion between the blended components, resulting denser structures in the composites. However, PET functionalization will also influence the composites surface; therefore, contact angle measurements were done to evidence the nature of changes and the surface energy of the composites.

In a first step, the polar and dispersive contributions of the surface energies were calculated for the PET component, based on the initial contact angle values, and the data are given in Table 5.

The functionalization of PET (with SDS and PEG 400) increases the surface energy, due to the development of an area dominated by polar components, with a significant increase of ${\sigma }_{\mathit{SV}}^p$ . This represents over 69.36% of the total surface energy for PET–SDS and 62.97% for PET–PEG. Thus, the PET sample functionalized with SDS shows the best hydrophilic properties, and can be best wetted by NaCl solutions, confirming the uniformly distributed polar groups at the surface. The opposite effect, represented by low surface energies (with the majority dispersive component) is recorded for sample with un-functionalized PET. For the PET–PEG component, the highest value of the contact angle was recorded; however, the highest surface energy was found for this sample (with predominant polar component, 37.23 mN/m, given the amount of oxygen present in PEG), confirming the PEG effect that promotes a higher homogeneity of the surface charge.

These results are consistent with FTIR analysis, and indicate that functionalization can be a tool to control the hydrophilicity/hydrophobicity of the composites surface.

Further on, surface energy experiments for the composite sample Type 1 (moulding duration of 60 min) were done and the results are presented in Table 6.

In the N type composite series (un-functionalized PET), the contact angle values decrease with increasing the PET percentage, supporting a hydrophilic character of the surface. A similar effect is observed for composites with PET–SDS (composite type S) proving an increased wettability, thus a higher surface contribution of PET (more polar than the rubber matrix). By comparing the N and S sample types, the SDS effect is obvious, as the un-functionalized samples are more hydrophilic; this could be a result of the surface composition or of the surface morphology. In the P-type composites (PET–PEG series) an opposite trend is observed, with increased hydrophobicity, thus a predominant hydrocarbon-rich surface or a less density of highly polar groups, as result of a possible increased stability to side oxidation effects during processing.

The moulding temperature also influences the surface energy. Basically, higher processing temperatures are responsible for two main effects with opposite consequences: (1) a better interface adhesion and (2) a higher oxidation likelihood, supporting the development of polar groups (carbonyl, carboxyl) or reactive polar bonds as —OH (confirmed by FTIR). The results show that the PET composition and functionalization have a significant influence in balancing these two effects: higher functionalized PET content supports a lower surface energy, with a more evident effect in the P-type composites, confirming the higher oxidation stability.

To investigate the effect of the moulding duration, similar investigations were done for the samples of Type 2 (processing duration: 30 min) and the results are given in Table 7.

The values corresponding to the samples moulded for a shorter duration show a more significant effect of the processing temperature that leads to higher surface energy for all the samples (except one, 2b_AN); this may be the result of faster oxidations that, at longer processing durations (60 min) can further promote interfacial bonds and a surface re-distribution of the components. This assumption is also supported by the reversed variation of the surface energy with the PET composition, registered for both functionalized PET series of samples (Type S and Type P).

The surface energy data outline the sensitivity of the surface composition and morphology to the processing parameters; therefore, further investigations were done on SEM images (morphology) and EDX analysis (surface composition) and the results are presented in Fig. 6. Analyses were collected from four or five different zones, to outline the specifics of the composite components; the components below 0.5%w were not mentioned as their contribution proved to be quite randomly distributed at the surface. Three representative samples were discussed but similar results were registered for all the samples of types N, S and P.

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Figure 6

SEM images of: (a) sample 2b-AN; (b) sample 2a-CS; (c) sample 2a-BP.

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The data allow to distinguish the phases and interfaces in the composite and between the components. As expected, the composite is a mix of large aggregates (where chemical strong interfaces could be developed) and aggregates composed of smaller grains among which mechanical interfaces are observed.

According to the surface composition, zones 1 and 2 are rich in rubber (also confirmed by S from vulcanizing and by Ca and Si used as oxides in fillers) and HDPE. The presence of oxygen in these areas shows improved oxidation stability (zone 1, with % O < 14%) or higher oxidation sensitivity (zone 2 with % O > 21%). These aggregates may also be responsible for the crystalline part of the composite, due to the HDPE content.

By marking zone 3, the mechanical interfaces were highlighted, involving rubber (with traces of Si, Zn, S) and HDPE; the data, confirmed also by the FTIR results, show a slight/null involvement of PET, as this zone is oxygen poor.

Zone 4 indicates the PET scrap, as recording a high per cent of oxygen and the amounts of Na (sample 2a-CS) following PET functionalization with SDS. The elemental analysis on zone 5 shows that the PET functionalized composite also includes large voids, particularly larger for the Type P composite; this confirms that SDS promotes a better attraction among PET and the hydrocarbon components, thus denser composites.

Corroborating these observations with the surface energy data, it may be concluded that the surface energy mainly depends on the morphology and less on the surface composition; however, the surface oxygen content, thus the oxidation stability of rubber and HDPE during moulding is significantly influenced by functionalization and increased stability is attained on smaller aggregates when using functionalized PET–PEG (zone 2), confirming the improved homogeneity of the surface charge.

3.5 Mechanical properties

The mechanical properties are of interest, allowing to identify the best suited application for the tailored composites. The results of the mechanical testing of the composites obtained at 60 min of moulding (Type 1) are shown in Table 8.

Literature data show that PET functionalization improves the mechanical properties of composites. The highest value of tensile strength was 2.01 N/mm² for the 1a-CP composite, but lower than CNT/PMMA composite (4.1 N/mm² for the 12 wt% CNT/PMMA composite) (Weng et al., 2014) and grapheme/LDPE composite (12.5 N/mm²) (Han et al., 2014). However, functionalization does not always improve the mechanical properties, as for nanocomposites of waterborne polyurethane (WPU) reinforced with functionalized graphene sheets (FGSs) (Choi et al., 2012), where the tensile strength and elongation at break, measured at high deformation, decreased with increasing FGS in the nanocomposite.

The results in this paper show that for a composite that only contains wastes, surfactant functionalization of PET represents an effective and low-cost path to improve the interfaces and thus the mechanical properties.

For the un-functionalized (type N) samples, the best mechanical properties (tensile strength and compression) are obtained for the 1b-AN composite; by increasing the PET percentage, the samples become increasingly brittle, and the mixture containing 45% of PET did not actually lead to an aggregated composite structure.

After PET functionalization, a larger PET amount (⩾45%) could be incorporated in the composite and the mechanical strength is significantly modified, for both samples type S and P (PET–SDS and PET–PEG).

The S-Type samples (containing PEG–SDS) obtained at 220 °C have improved mechanical properties in terms of compression and impact compared with those obtained at 240 °C in good agreement with crystallinity degree. Sample 1a-CS shows remarkable compression resistance as due to the higher amount of PET–SDS incorporated in the matrix, stronger interfaces are developed (as confirmed by FTIR analysis). The PET functionalization improves the composites’ processing by decreasing curing temperature, as the thermal analysis shows.

The results show that PET functionalization with SDS has a stronger effect than with PEG, by possible electron conjugations as e⁻ (p–π) and e⁻ (p–p), and by changes of trans–gauche conformers, making it more reactive. In this case, new physical–chemical interfaces PET–rubber, PET–HDPE and rubber–HDPE were developed, mainly based on stronger electrostatic interactions promoted by the anionic SDS as compared to the polar PEG. These results are also supporting the assumptions made based on the SEM and EDX data which show denser structures for the S-type composites.

It is important to outline that the sample with the highest PET content in the P-type composites series also shows the highest mechanical/tensile strength, due to new mechano-physical interfaces between the hydrocarbonated part of functionalized PET and unsaturated part of rubber/HDPE.

These results show that PET can be incorporated in a larger amount if functionalized and the final application (requiring tensile strength and/or compression and/or impact resistance) will indicate the type of functionalizing agent to be selected.

To obtain these types of composite in a lower energy intensive process, the processing duration was reduced at 30 min and the results of mechanical testing are shown in Table 9.

The samples obtained at 30 min with un-functionalized PET have recorded poorer strength compared to the samples obtained during 60 min of moulding, outlining that, without functionalization, 30 min of processing is not enough to develop suitable interfaces.

The PET-functionalized composites show significant increase in the compression strength and impact resistance, while also preserving good tensile strength. The interface plays an important role in the mechanical properties of these composite materials, as here the load transfer from the matrix to the dispersed phase occurs. The blends with functionalized PET showed considerable improvement in mechanical properties over the blends with un-functionalized PET, as a result of a better adhesion when using functionalized PET. Additionally, this can be the result of lower degradation/oxidative side-processes that can affect both the components and the interfaces. These samples also have higher values of the elasticity modulus, because the functionalized PET has a higher modulus that provides less means of absorbing or dissipating energy.

The sample 2a-CS from type S has impressive compressive strength and good tensile and impact strengths, being the best in the S-Type series (and the best among all in terms of compression strength), because rubber is better maintaining its elastomeric properties during the shorter processing duration.

In type P samples the mechanical strength is lower compared to the Type S composites; however, in this series an amount of 45% PET allowed obtaining the higher mechanical resistance and the sample 2b-CP also has a high value of the Young’s modulus.

The significant influence of the functionalized component supports the assumption that SDS or PEG is inserted between macromolecular chains, modifying not only the surface charge but also promoting interface relaxing. This assumption is confirmed by the high compressive strength compared with the reference specimens (with un-functionalized PET). The composite with the highest dimensional stability (2a-CS) was analysed on the fractured surface after the tensile test, using SEM, and the result is presented in Fig. 7.

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Figure 7

SEM images of the fracture surface after tensile failure (1a-CS).

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The scanning electron microscopy images of the fractured surface show that the large aggregates of zone 1 are preserved; thus, stress failure occurs on the interfaces dividing the smaller aggregates. As stress–strain fractures lead to a variety of structures, eight zones were investigated by EDX. All the phases (except zone 4) have similar elemental content, thus derived from the same components (rubber, PET, HDPE), but in different per cent. Zone 4 only contains C and Si elements from rubber and HDPE. Based on the oxygen content, the PET occurrence in the fractured surface can be approximated and can be corroborated with the shape of the ruptured zones to make an assumption on the stress failure order; thus, it is possible that during the tensile test, the zones rich in PET (mainly 2 and 6 but also 3) are firstly affected, followed by 1, 5 and 7 (containing PET and oxidation compounds), where the tensile force promotes sliding among the components, before fracture; the least affected zones are 6 and 7. Zone 8, containing only carbon could be allocated to the black carbon aggregates from rubber. These results outline the need for focusing on PET as being the component mostly responsible for the mechanical strength in the composites.