Synthesis, properties and application prospects in biomedical areas of unsaturated polyester resin modified with iron(II) clathrochelate

Introduction

The unsaturated polyester resin is a liquid polymer which cured (cross-linked with styrene, organic peroxides, named hardeners), maintains the solid shape characterized by exceptional strength and durability as well as resistance characteristics, or aesthetic qualities in relation to the imitation of materials. ^{1 , 2} Unsaturated polyester resins are mostly used in combination with reinforcing micro- and nanomaterials. ^{3 , 4} It is a well-known thermosetting polymer that is widely used for various applications. Thus, the polyester resins represent one of the absolute compounds applied in the most important wide ranges of industries: wood paints, chemical anchoring, composite materials, automotive and bathroom fixtures, corrugated panels, ribbed panels and others. ^{2 , 5 , 6 , 7 , 8 , 9} Currently, numerous examples in the literature detail the production and use of unsaturated polyester resins derived from post-consumer PET recyclate. ^{3 , 4 , 10}

Clathrochelates of d-metals are coordination compounds of a new type in which the metal ion is in a three-dimensional ligand being isolated from external factors. ^{11 , 12} Symmetrical macrobicyclic clathrochelate metal complexes (Fe²⁺) are capable of forming bonds with polymer chain atoms through functional centers on the surface. In addition, iron(II) clathrochelates are reactive macrobicyclic precursors. ^{11 , 13 , 14}

Encapsulation of a transition metal ion leads to the formation of complexes that demonstrate unprecedented chemical and photochemical stability as well as unusual properties. ^{11 , 15} The chelate compounds can be used as molecular scaffolds for electrocatalysts for hydrogen production as well as a distinct metallo-macrocyclic framework and a heterometallic coordination polymer. ^{16 , 17}

The purpose of using clathrochelate metal complexes (Fe²⁺) in the polymeric structures is to establish the influence of macrobicyclic compounds which having an encapsulated metal ion in the rigid macrobicyclic environment, causes low toxicity, chemical and photochemical resistance of these compounds, as well as their propensity to form orderly molecular structures. ¹⁸

Iron (Fe²⁺) clathrochelate complexes were used to change the properties of synthetic polymers, such as epoxy resin. ¹⁹ The use of such a dye allows to expand the application of modified composites in various areas of modern industry. The interphase interactions of the filler with the polymeric structure change the physical and chemical properties and functional parameters, which is a current trend of modern materials science.

In the literature, there are several iron and vanadium compounds used as curing accelerators for unsaturated polyester resins. ²⁰ It is believed that the addition of iron in the clathrochelate form can change the characteristics of the resin and will allow to get a new type of polymeric systems with styrene copolymerization without the use of toxic cobalt(II) compounds. ^{21 , 22} Iron(II) clathrochelate can be effective for arranging the polymer structure by the interraction polymer chains through the functional centers.

In the presented work, polymeric matrices based on unsaturated polyester cross-linked with different contents of iron(II) clathrochelate and without this additive were used for studies. Moreover, the antimicrobial properties of the obtained functional materials were determined. The potential application of polyesters in biomedical areas were identified.

Materials

Chemicals

The unsaturated polyester resin (UPR) from recycled PET (poly(ethylene terephthalate)), Estromal 14PB-06 NZ (acidic value 13.4 mg KOH g⁻¹, viscosity at 23 °C 356 mPas, non-volatile content 61.2 wt%, and reactivity factor 1.53) was provided by LERG (Pustków, Poland). The blue-green syrupy liquid owes its colour to waste water bottles. Methyl ethyl ketone peroxide (MEKP, Luperox DHD-9) was purchased from Sigma-Aldrich (St. Louis, MO, USA) while N,N-dimethyl-p-toluidine (DMPT) from Merck.

Monochloromonodimethylamino clathrochelate FeBd₂(Cl(Me₂N)Gm)(BF)₂ was prepared by the nucleophilic substitution reaction of dichloride macrobicyclic precursor FeBd₂(Cl₂Gm)(BF)₂ (where Bd²⁻ and Cl₂Gm²⁻ are α-benzyldioxime and dichloroglyoxime dianions, respectively) with dimethylamine, according to the procedure (Fig. 1). ²³

Fig. 1:

Structure of 1,8-bis(2-fluorobora)-2,7,9,14,15,20-hexaoxa-3,6,10,13,16,19-hexaaza-4,5,11,12-tetraphenyl-17-chloro-18-dimethylaminobicyclo[6.6.6]eicosa-3,5,10,12,16,18-hexa-ene(2−)iron(2+), FeBd₂(Cl(Me₂N)Gm)(BF)₂ (monochloromonodimethylamino clathrochelate).

Methods

Optical properties and visual characterization

UV–Vis spectroscopy

The UV-visible spectra of the specimens were analyzed based on absorption spectroscopy using a UV spectrophotometer, UV-2550 from Shimadzu (Kyoto, Japan) in the range from 200 to 900 nm at a scanning rate 200 nm min⁻¹.

Visual characteristics – colorimetric parameters measurements

The sample colour was measured in accordance with ASTM E308, for which the Ci4200 spectrophotometer X-Rite (Grand Rapids, MI, USA) was used. ³¹ The colour is described in the CIELAB colour space defined by the Commission Internationale de l’Éclairage (International Commission on Illumination), commonly referred to as the CIE, in the L*, a*, b* area. The CIELAB variables form three-dimensional orthogonal coordinates. The parameter L* axis correlates with lightness, luminance–brightness ranged from 0 to 100, representing the grey scale from black to white (0 is black and 100 is white). The parameter a* axis describes the colour from greenness (negative values) to redness (positive values) whereas the parameter b* axis, from blueness (negative values) to yellowness (positive values) (Fig. 13).

The difference between the two colours-two points in the 3D colour space is described by the relationship between a sample colour ( L 2 * , a 2 * , b 2 * ) and the standard colour ( L s * , a s * , b s * ):

(2) ∆ E a b * = ∆ L * 2 + Δ a * 2 + Δ b * 2

where: ∆ L * = L 2 * − L s * , ∆ a * = a 2 * − a s * , and ∆ b * = b 2 * − b s * represent the difference in colour parameters between the compared samples.

A standard observer can see a difference in colour when the ∆ E a b * parameter indicates the following relationships (Fig. 2):

1. 0 ∆ E a b *  ≤ 1 – the observer does not notice the difference,

2. 1 <  ∆ E a b *  < 2 – only an experienced observer can notice the difference

3. 2 <  ∆ E a b *  < 3.5 – an inexperienced observer will also notice the difference,

4. 3.5 <  ∆ E a b *  < 5 – a clear difference in colour is noticeable,

5. 5 <  ∆ E a b *  – the observer notices two different colours. ³²

Fig. 2:

CIELAB colour space grading system (Graphical interpretation of the colour difference and colour space different from the pattern of a value less than ∆ E a b * ).

Results and discussion

The purpose of using clathrochelate metal complexes (Fe²⁺) in the polymeric structures is to establish the influence of macrobicyclic compounds, having an encapsulated metal ion in a rigid macrobicyclic environment on their chemical and photochemical resistance as well as propensity to form orderly molecular structures (Fig. 1). Moreover, it was examined whether such a unique iron(II) compound could act as an effective accelerator of resin polymerization without the need to use toxic low-molecular-weight cobalt(II) compounds. ³⁹

Polymeric materials based on the unsaturated polyester resin (UPR) with various contents of iron(II) clathrochelate (0.005, 0.01; 0.03, and 0.05 wt%) were prepared (Fig. 3). As expected, the transition metal compound is an intensely coloured substance, causing coloration. ²⁰

Fig. 3:

Images of cured, pure unsaturated polyester resin (a) and its materials with different amounts of iron(II) clathrochelate: 0.005 (b), 0.01 (c), 0.03 (d), and 0.05 wt% (e).

The crystallographic study of the iron(II) clathrochelate started with the analysis of the crystal shape observed in the SEM images. In Fig. 4, a view of micro-crystals of FeBd₂(Cl(Me₂N)Gm)(BF)₂ can be observed. The morphology of clathrochelate is mainly styloid, rhomboid, and arrow shaped as confirmed by scanning electron microscopy.

Fig. 4:

SEM micrographs of micro-crystals of FeBd₂(Cl(Me₂N)Gm)(BF)₂ under various magnifications (a–f).

The styloid morphotype is characterized by the presence of elongated crystals with pointed ends in the front view or truncated on the lateral surface (Fig. 4a–f). Arrow-shaped crystals can be observed in Fig. 4b, where this morphotype, characterized by a sharp end with a V-shaped notch resembling the tip of an arrow, is very noticeable. The parallelogram with unequal adjacent sides in Fig. 4d is an example of a rhomboid crystal morphotype. In addition to large, single monocrystals, smaller ones, so-called crystal sand, can also be observed (Fig. 4a–f).

Based on the above analysis of morphology using SEM, it can be assumed that the iron clathrochelate compound crystallizes in the monoclinic crystal system.

SEM is an appropriate technique to show particle morphology. Determining particle morphology using TEM is no longer straightforward because TEM shows only two-dimensional projected images of three-dimensional particles (Fig. 5).

Fig. 5:

TEM micrographs of FeBd₂(Cl(Me₂N)Gm)(BF)₂.

Scanning TEM (STEM) elemental mapping showed that iron, oxygen and carbon are uniformly dispersed in FeBd₂(Cl(Me₂N)Gm)(BF)₂ micro-crystals (Fig. 6).

Fig. 6:

STEM-EDS elemental mapping of FeBd₂(Cl(Me₂N)Gm)(BF)₂.

The crystallographic study was carried out using X-ray diffraction patterns of cured polyester resin and polyester material with 0.05 wt% content of the clathrochelate (Fig. 7). The diffraction patterns for both samples, pure material and that containing the clathrochelate additive, consist of two main crystalline peaks. The values of diffraction angles peaks of about 9° and 20° indicate amorphous and semi-crystalline features of the polymer matrix. The only significant difference observed during the XRD analysis was a decrease of the peak intensity and the shift in the diffraction pattern from 9.35° to 7.51° for the polyester with the iron compound.

Fig. 7:

X-ray diffraction (XRD) patterns and crystalline structure of the cured polyester resin and polyester material with the 0.05 wt% clathrochelate content.

The transmittance and absorbance data for pure polyester and polyesters modified by clathrochelate in the UV-vis-NIR range are presented in Table 1 and Fig. 8.

Fig. 8:

UV-vis-NIR spectra: (a) transmittance and (b) absorbance.

Light transmission for the pure polyester with the values of 63.4, 70.9 and 84.5 % appears at the wavelengths of 460, 550 and 850 nm, respectively (Fig. 8 a). For the materials with clathrochelate, a transmittance peak appears at λ range 350–500 nm (visible light) and decreases with the increasing additive concentrations: 0.005 (22.3 %, 407.5 nm), 0.01 (20.6 %, 402 nm), 0.03 (6.0 %, 393.5 nm) and 0.05 (1.2 %, 389 nm). It is worth noting that the clathrochelate-modified polyesters were characterized by higher transmittance above 700 nm (NIR range) compared with the pure polyester.

The comparison of the absorbance of pure polyester and polyester with different amounts of iron(II) clathrochelate is given in Fig. 8b. The spectrum of pure UPR (black line) showed almost zero absorbance in the VIS range (400–700 nm) while for the materials with clathrochelate a peak with the maximum at a wavelength of about 480 nm. There was observed a tendency that with the increasing concentration there was an increase in absorbance in the mentioned region.

The technique of spectrophotometry in both transmission and reflection modes is a versatile tool for studying optical properties of polymeric materials. Characterization of the material appearance using tristimulus values in the CIELAB colour system and related parameters provides useful information for describing physical properties of materials and their interactions with light.

The characteristics of polyesters using the CIELAB scale and coordinates are presented graphically (Fig. 9) and summarized in Table 2. As shown polymer compositions can affect the colour and trichromatic values of the final product.

Fig. 9:

Variability of the colour parameters L ^*, a ^* and b ^*.

Our colorimetric measurements have shown that lightness diminishes with the increasing amount of clathrochelate in polyester. For pure UPR and polymer with the largest contents of this additive, the values were 79.5 and 40.2, respectively. This may be due to greater heterogeneity of the composition.

Since the studied polyesters are colourful (Figs. 3 and 9), the values of their chromaticity parameters a* and b* vary towards yellowish and reddish, respectively. Considering the fact that the a* value for all clathrochelate-modified polyesters is positive, materials are in the redness region. The negative value for pure UPR may come from the blue-green colour of the resin.

Large values of the parameter ∆ E a b * indicating the following relationship 5 <  ∆ E a b * confirm evidently that the observer noticed two different colours without any problems. ³²

The effect of the amount of iron(II) clathrochelate on the thermal properties of the studied unsaturated polyesters was summarized and presented in Table 3. In the case of modified materials, there were observed changes in characteristic temperatures compared with pure UPR. The basic difference comparing the thermal research results is 15–20 °C lower than the temperature of 5 % mass loss for the resins cross-linked in the presence of iron(II) clathrochelate compared to the resin cross-linked in the presence of cobalt(II). Significant differences can be also observed for the 50 % mass loss and the maximum decomposition temperatures. These values obtained for the resins cross-linked in the presence of iron(II) are slightly larger than for the reference resin.

Iron compounds in the polyester materials caused a certain slowdown in the initiation of the polyester chains decomposition. Once this process had started, their decomposition was much faster and therefore ended at lower temperatures. The decrease in the decomposition temperature in the presence of Fe²⁺ ions suggests that the addition of iron compound affects the mechanism of thermal degradation of the polymer and in this case these compounds reduce the thermal stability of polymers by catalyzing their destructive reactions. ⁴⁰ This may be due to the presence of three decomposition maxima. The main cause of thermal instability is probably the effect of indirect, improved and more efficient heat transfer to the polymer matrix via iron ionic compounds dispersed in it.

The mechanical properties, hardness and impact strength of clathrochelate-resin materials are presented in Table 4.

Flexural strength refers to the ability of a material to withstand bending under the application of external force, which is determined by the largest stress the material can withstand before failure.

The flexural modulus (bending modulus or modulus of rigidity) represents a tendency of a material to resist bending. Taking into account the values obtained for the resins cured in the presence of iron(II) clathochelate, it should be stated that, regardless of its concentration, they are much lower than for the reference UPR cured in the presence of a divalent cobalt compound. In turn, the strain at break values, also known as elongation at break, are almost twice as large as those of the reference resin.

The results of the resin hardness test are interesting. Namely, the resin cured in the presence of the smallest amount of clathrochelate has a slightly smaller hardness than the reference resin but with the increase in the concentration of iron(II) compound, its decrease of the materials is visible.

Although for the resin with the 0.05 % clathrochelate addition, gelation occurred after 15 min, both this and the other resins cured in the presence of iron(II) clathrochelate are not completely cross-linked. In addition, the change in the iron oxidation state from II to III during curing indicated by the clearly visible brown colour of the cross-linked shapes, leads to the disintegration of the chelate structure and the released ligands can have a plasticizing effect on the resin. ^{41 , 42}

The resins containing iron(II) clathrochelate showed interesting thermomechanical properties. Unfortunately, in our study it was not possible to measure the Vicat softening temperature for all samples. This was due to the temperature limitation resulting from the fact that the apparatus was equipped with an oil bath with a maximum operating temperature of 150 °C. At this temperature the indenter immersed to 0.72 mm in pure UPR and to 0.74 mm in the polyester with the lowest clathrochelate content (0.005 %). In the case of materials containing 0.01 % iron(II) compounds, correct softening temperature measurements were already performed. Compared to the sample without clathrochelate, the polyesters with higher iron addition showed lower Vicat softening temperatures reaching 138.0, 116.5 and 85.3 °C for 0.01, 0.03 and 0.05 %, respectively which is a consequence of the already mentioned plasticizing effect of the ligands.

The impact strength of the pure unsaturated polyester resin was approximately 8.1 kJ m⁻², whereas the addition of the iron(II) clathrochelate significant enhanced the values of this parameter in the studied materials.

Increase of the iron content led to a decrease of impact strength, though still remaining above that of the pure UPR. This trend may be linked to the competing effects of network disruption and increased chain mobility. Higher iron concentrations likely promote aggregation or destruction of the metal complexes, which may create stress concentration points and reduce the material’s ability to dissipate energy under impact. Additionally, excessive plasticization can reduce crosslink density, negatively affecting the toughness.

Our studies show that the iron(II) clathrochelate addition offers a tunable approach to enhance the impact resistance of unsaturated polyester resins, with an optimal concentration around 0.005 % balancing increased ductility and network integrity.

The addition of iron(II) clathrochelate to unsaturated polyester resin significantly influences its network structure and thermomechanical properties (Table 5).

A comparative analysis of the thermomechanical data reveals distinct differences between the pure unsaturated polyester resin (UPR) and those modified with various concentrations of Fe²⁺-clathrochelate. The pure UPR exhibits the highest storage modulus at 20 °C (3279 MPa), glass transition temperature (T _g  = 133 °C), and crosslinking density (ν _e  = 1431 mol·m⁻³), reflecting a highly rigid and densely crosslinked network.

Upon the introduction of 0.005 wt% Fe complex, a notable decrease in stiffness (E ^′ = 1815 MPa) and T _g (110 °C) is observed, accompanied by the highest tanδ (0.70) and broadest phase transition (FWHM = 55 °C). These features suggest plasticization effect of the network, and increased structural heterogeneity. However, as the Fe²⁺ content increases from 0.01 % to 0.05 %, a gradual recovery in thermomechanical performance is evident. The storage modulus, T _g , and crosslinking density all increase, while tanδ and FWHM progressively decrease.

At 0.05 wt% Fe²⁺, the modified UPR nearly approaches the properties of the pure system, with E ^′ = 2705 MPa, T _g  = 125 °C, and ν _e  = 1280 mol·m⁻³. This suggests that Fe²⁺ at higher concentrations acts as a crosslinking promoter, reinforcing the polymer network and restoring mechanical integrity.

Droplet images and contact angles of water and diiodomethane for the polyesters are shown in Table 6. It was also found that the water contact angles, measured at about 65° for the pure UPR. However, as expected for those with clathrochelate, the water contact angle increased to about 81°, which is a consequence of increased hydrophobicity of polyesters. Diiodomethane contact angles were measured to about 39° for pure polyester. The values for clathrochelate-modified polyesters decreased to about 31°, indicating increased dispersion interactions on the polyester surfaces.

Table 6:

Contact angle images of pure UPR and polyesters with iron(II) clathrochelate.

Sample	Water droplet	θ w	Diiodomethane droplet	θ d
Pure UPR		64.6		38.8
UPR 0.005 % Fe2+		68.9		38.1
UPR 0.01 % Fe2+		74.9		37.7
UPR 0.03 % Fe2+		77.4		35.8
UPR 0.05 % Fe2+		80.5		30.8

Surface free energies of the polyesters were calculated from the measured contact angle values and they are shown in Table 7. The decrease in the surface free energy from 49.8 mJ m⁻² for the pure polyester to 45.9 mJ m⁻² for that polyester with the largest amount of clathrochelate can be attributed to the decrease in polarity caused by the increased clathrochelate content. The polar part decreases from 9.6 to 3.1 mJ m⁻² with the increasing clathrochelate content. According to the Table 7, the increase in the dispersive part from 40.2 to 42.8 mJ m⁻² confirms the previous suggestion that clathrochelate caused an increase in dispersive interactions.

When determining the characteristics of materials for the medical, pharmaceutical, cosmetology or food industries, it is extremely important to determine the resistance of these materials to the adhesion of bacterial biofilm. For this purpose, the biofilm formation test was carried out on the surface of pure UPR and with the addition of Fe²⁺. The obtained results were compared with the control biofilm growth formed at the bottom of the well of a 24-well polystyrene plate. Bacterial strains responsible for infections in the medical and food industries were used in this test: S. aureus ATCC 25923 or E. faecalis PCM 896, or E. coli ATCC 25992. Planktonic bacterial cells were cultured twice so long to form a mono-species biofilm, i.e. 48 h under appropriate temperature and media.

Bacterial cells present in the environment under the favourable conditions, can form organized biofilm structures from planktonic forms, which pose a health threat. The data (Figs. 10 and 11) indicate that the addition of iron from only 0.03 % and more has a beneficial inhibitory effect on the adhesion of bacterial biofilm. The data show that the presence of 0.01 % iron in the polyester does not change composition, porosity or charge of the material significantly, because in relation to all tested bacterial strains, this material behaves like a pure control material.

Fig. 10:

Visualization on the CLSM images of biofilm formed by Staphylococcus aureus ATCC 25923, Enterococcus faecalis PCM 896 or Escherichia coli ATCC 25992 on the surface of the tested materials. (Magnification 200×, green fluorescence – viable cells, red and yellowish fluorescence – dead bacteria cells).

Fig. 11:

Quantification of the bacterial cells adhered to the tested materials compared to the control polystyrene surface (24-wells polystyrene plate).

Such observations can be justified because many factors influence the process of cell adhesion to materials. Namely, the material surface and microstructure can have different properties (smoothness, porosity, roughness, hydrophilicity), which may influence biofilm adhesion to varying degrees. ^{43 , 44 , 45} As well as electric charge because materials with the opposite charge to bacteria (usually bacteria are negatively charged) could attract biofilm aggregates. According to Renner and Weibel, van der Waals forces and electrostatic interactions are the main forces of bacterial adhesion to material surfaces. ⁴⁶ The composites mechanical properties, such as hardness and elasticity, also have their consequences because biofilm settles and grows faster on flexible materials than on stiff, inflexible ones. Additionally, the chemical composition is important, i.e. the presence of organic compounds or ions that can be used as an energy source by bacteria if they are available. On one hand, low concentrations of magnesium or iron ions can stimulate the biofilm adhesion to materials. Microorganisms can use iron ions as a source of nutrients, which can increase their metabolic activity and promote surface colonization (Table 8).

On the other hand, high concentrations of such ions can be harmful to microorganisms because they can lead to formation of reactive oxygen species that can damage their cellular structures. High iron concentrations can also inhibit the metabolic activity of microorganisms and hinder formation of a stable biofilm.

Depending on the type of microorganisms and environmental conditions, the optimal concentration of iron ions can vary. Therefore, controlling the concentration of iron ions and other metal ions is important in the context of managing biofilm formation on materials.

Cytotoxicity tests showed that the obtained materials are not-toxic to skin fibroblasts. The WST-8 test proved that both native and enriched variants (pure or iron(II) clathrochelate-modified polyesters) have no negative effect on human normal fibroblasts after 24-h and 48-h incubation of cells with 24-h extracts. Due to the fact that cell viability was not reduced below 70 %, it is noticeable that according to ISO 10993-5 all tested materials are non-toxic (Fig. 12). ³⁶

Fig. 12:

BJ cell viability after 24 and 48 h of incubation at 37 °C using the WST-8 test.

Cytotoxicity of the obtained polyester-based materials was also determined by the indirect contact method using Live/Dead fluorescent staining of the BJ fibroblasts grown with the 24-h extract of tested materials. The CLSM images presented a healthy monolayer of viable cells with typical spindle-shaped morphology after exposure to all tested extracts, confirming the non-toxic character of the materials (Fig. 13). Importantly, the viability and morphology of the cells exposed to the extracts were comparable to control cells grown in the presence of fresh culture medium.

Fig. 13:

Live/Dead assay of fibroblast in contact with the PS control, pure UPR and polyesters with different amounts of iron(II) clathrochelate.

A leaching study was used to assess the stability or potential release of iron(II) clathrochelate over time (Fig. 14).

Fig. 14:

Immersion test of the clathrochelate-resin materials.

Considering the fact that the iron compound is colored, its release from the polymer matrix to the solution should be easily and unambiguously observed. For this purpose, crushed polyester materials were immersed in liquid chemicals and left for 1 month in the dark, determining only the solvent factor.

As expected, the only changes were observed for samples with iron complex immersed in acetone. This aggressive solvent is responsible for the destruction of the polyester matrix, which helps in the release of the chelate or ligand into the liquid. ⁴⁷ However, it was noted that the colour change of the liquid is insignificant, which also confirms that leaching from the matrix is nevertheless negligible (Fig. 15).

Fig. 15:

Leaching of clathrochelate from resin materials after 1 month.

Due to the potential use of iron(II) clathrochelate polyesters, these materials will be most exposed to contact with water or aqueous solutions of acids, bases and salts. This is why immersion test in such solvents is most important. No colour changes were observed in these samples.

The use of iron compounds is often limited due to their colour-enhancing properties. ²⁰ However, iron(II) clathrochelates impart unique characteristics to polymeric materials, making them promising candidates for biomedical applications.

Conclusions

The resin curing process was carried out without the presence of a cobalt salt catalyst. It was noticed that its role was taken over by an iron(II) clathrochelate.

Cured resins with different colours were obtained. The addition of clathrochelate did not significantly change the thermal, and mechanical properties of the cross-linked products. However, with the increase in iron content, the water contact angles increased.

Studies have shown that iron(II) clathrochelate acts as both a plasticizer and a crosslinking promoter, depending on its concentration.

Importantly, the performed cytotoxicity tests showed that the obtained materials were not-toxic to skin fibroblasts. It was also observed that the addition of iron to resins from 0.03 % and more has a beneficial inhibitory effect on the adhesion of bacterial biofilm. This suggests that such polyesters may have potential applications in biomedical areas.

Replacing cobalt salts with iron(II) clathrochelate in cross-linking UPR resins derived from recycled PET further reduces the process’s carbon footprint. It reduces dependence on high-impact critical raw materials and lowers embedded CO₂ emissions from the curing process.