Prolonged exposure to ultraviolet (UV) radiation represents a well-known problem for polymeric materials [1
]. It determines color loss, macroscopic fragmentation, and progressive reduction of molecular weight; hence, bettering understanding of the involved mechanisms and developing new methods to slow down this type of degradation are extremely active fields [4
]. Modern additives for polymers are capable of absorbing the UV radiation or blocking the free radicals and peroxides produced during the degradation process [7
]. These substances are generally produced from non-renewable sources, and bio-based materials should not rely on them to improve their performances [9
]. On the contrary, molecules obtained from renewable resources are a desirable alternative [12
]. Tannins, flavonoids, and other macromolecules classified as polyphenols are very interesting candidates from this point of view [13
]. They are present in many types of plants and play a fundamental role in their protection against pests and diseases. In particular, tannins are one of the most abundant groups of compounds of vegetable origin and are readily available throughout the world [14
]. Tannins can be classified as either hydrolyzable or condensed. Hydrolyzable tannins are made of simple phenolic products, being esters of gallic acid and their dimers (digallic acid, ellagic acid), and monosaccharides (especially glucose). They have already been used as partial substitutes for phenol in the manufacture of phenol formaldehyde resins [15
]. Condensed tannins consist of flavonoid units with different degrees of polymerization. They have been associated with their precursors: catechins (flavans-3-ols), leucoanthocyanins (flavanes-3,4-diols) [17
], and carbohydrates. This class of tannins is produced in the normal metabolism of plants, which explains why they are considered physiological and are widely present in plants across the planet [19
]. Condensed tannins are used in many industrial applications (i.e., as glues for wood products, dyes for leather, etc.) [19
] and constitute more than 90% of the world production of tannins. Due to its chemical structure (Figure 1
), which constitutes a high amount of multiple phenolic hydroxyl groups, they could be used as a platform for producing reactive polyhydroxyl chemicals [24
] and, hence, for precursors in polymer synthesis.
Among the several applications of tannins [26
], their potential use as a protecting additive for polymers against the effect of UV rays has been proposed only in recent years. Samper et al. [29
] reported the use of natural phenolic compounds derived from flavonoids, such as chrysin, quercetin, silibinin, and others, in the stabilization of polypropylene against thermo-oxidative degradation and UV radiation. The results show that these compounds provide the best performance in stabilizing against both oxidation and UV radiation.
Polyurethanes are among the most used polymers due to their wide range of properties. Their versatility allows the synthesis of different materials such as foams, coatings, adhesives, sealants, and elastomers. Applications can be found in fields like automotive industry, footwear, in construction as insulators, and, most recently, in medical devices. Polyurethanes are synthesized by reaction of an isocyanate, generally a polymeric isocyanate, with a polyol. As the synthesis of isocyanates is more complex than that of polyols, investigations usually focus on new bio-based polyols to reduce the PU carbon footprint while relying on synthetic isocyanates [30
]. Various foams based on bio-based materials have been prepared and characterized in the last years [32
]. Bio-based polyols offer a wide range of properties and, thus, the properties of the final material strongly depend on the polyol used [35
]. The environmental sustainability [37
] and the low cost render these materials environmentally friendly competitors of current synthetic polymers [39
]. However, bio-based foams generally have lower resistance to environmental degradation than synthetic polyurethane [42
]. In addition, almost no natural products are available as a substitute for synthetic UV stabilizers. In this respect, tannin can be employed as a reactive polyhydroxyl filler to prepare polyurethane composite foams; this is thanks to its chemical structure, alongside the aforementioned UV stabilizing properties. In fact, Ge et al. [43
] reported that the phenolic hydroxyl groups present on the B-ring of tannin (Figure 1
) are able to react with the isocyanatic group due to the higher electron densities on the oxygen atoms present in the B-ring compared to those in the A-ring.
In this work, the synthesis of polyurethane-based composite foams by using methylene diphenyl isocyanate (isocyanate source), ethoxylated cocoalkyl amine (polyol), and condensed tannins (as a UV stabilizer and additional –OH source) in the presence of a suitable amount of water (as a blowing agent), catalysts, and silicone surfactant was investigated. Different foam formulations were prepared to evaluate the effects of tannin and water contents on the foaming kinetics, mechanical properties, and foam morphology. The foams were exposed to accelerated degradation cycles, simulated by exposure to UV radiation, and the effects on the chemical and mechanical properties were evaluated.
2. Materials and Methods
2.1. Raw Materials
Methylene diphenylisocyanate (MDI) (Voranate M229 with isocyanate group (NCO) content equal to 31.1 wt % and functionality of 2.7) and ethoxylated cocoalkyl amine polyol (EtCO) (Lutensol® FA 12 with ethoxylated chains between 8 and 20 carbon atoms, with an average of 12) were purchased from Dow and BASF s.r.l. (Italy), respectively. CH3COOK and Niax PM40, chemicals used to regulate both the polymerization and blowing reactions, and L6164, used as a surfactant, were kindly provided by Momentive (Italy). Distilled water (H2O) was used as a blowing agent. Profisetinidin/prorobinetinidin condensed tannin (CT) was supplied by Silvateam S.p.a. (Italy). The moisture content of CT, assessed by a thermogravimetric method, was 2.5 wt % and was considered as part of the H2O blowing agent amount in the formulation of the composite polyurethane foams.
2.2. Preparations of PU and TaPU Foams and UV Treatment
The polyurethane foams were synthesized by using an OH(EtCO)/NCO(pMDI) ratio equal to 1. Different tannin based foam samples (TaPU) were obtained by properly changing the CT and distilled water contents with respect to the reference PU formulation. For sample preparation, CT, CH3
COOK, Niax PM40, and L6164 were first mixed with EtCO at 200 rpm for 10 min using a magnetic stirring plate. Subsequently, distilled water was added to the mixture and mixed at 200 rpm for 1 min. Finally, MDI was added and stirred for 15 s. The resulting mixture was left to rise in a closed rectangular mold (10 cm × 10 cm × 3 cm) and, subsequently, the produced foams were cured at 40 °C for 5 h before any characterization. In order to minimize boundary effects and assure that the cellular structure was homogenous, chemical–physical and mechanical characterizations were performed on samples cut from the center of the plates. Foam samples were cut into cubic shapes with different dimensions. In Table 1
, the analyzed formulations along with their sample codes are reported.
UV degradation tests were carried out by exposing samples to a 300 W UV lamp (Ultra Vitalux from OSRAM, Italy) in a closed box for selected times (3, 6, 12, or 24 h). Cubic samples (15 mm × 15 mm × 15 mm in size) were exposed at a distance of 20 cm from the lamp. The measured temperature and relative humidity in the box were 50 °C and 30%, respectively. In order to irradiate all surfaces, the sample was manually turned every 30 min.
2.3. Physical Properties Evaluaton
The foaming process was analyzed in detail by means of FOAMAT equipment (model 281 from Format, Messtchnik GmbH, Karlsruhe, Germany) and the software “FOAM” version 3.8 was used to analyze the recorded parameters. This device records changes in height, temperature, and dielectric polarization of the foam during its growth. The software also calculates several parameters such as induction, rise, gel, and curing times. Three samples for each formulation were tested.
The sample density was calculated as the ratio between the weight and volume of cubic specimens of about 30 mm × 30 mm × 30 mm. The weight was measured using an analytical balance (model AB265-S from Mettler Toledo L.L.C., Columbus, OH, USA), and sample dimensions were evaluated using a high-resolution caliper (model 500-181-30 from Mitutoyo, Japan). The calculated density values were averaged among four samples.
The mechanical behavior of foams in compression was measured by means of a universal testing machine (model 4304 from SANS, Shenzhen, China) with a calibrated 1 KN load cell. Parallelepiped samples were cut from foamed slabs 50 mm × 50 mm × 30 mm in size. Compression tests were carried out at room temperature and Young’s modulus and compression strength were calculated from compressive curves. The change in mechanical behavior during the UV treatment was evaluated on smaller samples by using a dynamic mechanical analyzer (DMA 2980, TA Instruments Inc., New Castle, DE, USA) in order to have a higher sensitivity with regard to the performance change. Specimens 10 mm × 10 mm × 10 mm in size were carefully cut to ensure a cubic shape with parallel surfaces. They were compressed at 25 °C and at a constant strain rate of 0.1 min−1 up to the maximum compressive stress allowed by the instrument (190 KPa).
2.4. Cellular Morphology and Spectroscopy
The morphology of the foams was analyzed with a scanning electron microscope (model S8000, Tescan Brno s.r.o., Brno, Czech Republic). Specimens were cut from the middle of the foam in the direction of growth. They were coated with gold with a sputter coater (model SC500, emScope-now Quorum Technologies Ltd, Laughton, UK) before observation. Low-magnification pictures of foam sections were taken with an optical microscope (model Z16 APO, Leica Microsystems GmbH, Wetzlar, Germany) and used to evaluate the mean cell size and the cell number per unit volume. Sections with at least 50 entire cells were chosen to assure a statistically representative evaluation.
FTIR spectra were recorded at room temperature by using a FT-IR spectrometer (model Frontier Dual Ranger, PerkinElmer Inc., Waltham, MA, USA) in attenuated total reflectance (ATR) mode from 400 to 4000 cm−1
. ATR spectra were collected on the surface of the cubic foam sample before and after UV treatment. Spectra were recorded at 4 cm−1
resolution, and are the average of 64 scans. The spectral region ranging from 900 to 1800 cm−1
was normalized for an invariant peak at 1070 cm−1
and deconvoluted with OriginPro 8.0 software (OriginLab Corp., Northampton, MA, USA) by using Gauss–Lorentzian functions. The positions of absorption bands, corresponding to specific vibrational mode assignments of urethane linkages (related to coupled peaks of νs
C–N and δ
N–H, and νs
C=O free and H
-bonded) and urea linkage (related to the νs
], were determined by the automatic peak finding feature.