Poly vinyl chloride (PVC) has special mechanical and physical properties, with chemical formula (CH2-CHCl)n, it is colorless rigid material. The PVC relatively has high density and low softening point and likewise, it is mostly used as a thermoplastic material . It is commercially produced by a number of techniques but generally emulsion, suspension, solution and bulk polymerization of vinyl chloride. PVC has the advantage of being flame resistant, unlike many other polymers. It is used as rigid pipe in about 50% of cases .
Vinyl chloride polymers have a considerable contribution in the plastic market due to their low production cost. Poly (vinyl chloride) products’ high performance has boosted their use in building, especially in exterior application of buildings side such as windows and covering structures. PVC mostly makes the use of pipes and sidings in North America, while it is primarily applied as pipes and window frames in Asia and Europe. However, PVC subjected to photodegrades when exposed to long periods of sunlight or high heat .
The physical and mechanical properties of PVC may change as a result of this photodegradation problem . PVC chains may form conjugated double bonds (polyene stracture) as a result of dehydrochlorination, resulting in discolouration of PVC . Cross-linking and a lowering in molecular weight likewise happen for PVC chains because of photodegradation operations . The poor stability of PVC hinders its use in areas exposed to direct sunlight, so it should be photo-stabilized in order to be suitable for usage in harsh conditions.
Photostabilization is the process of slowing or stopping a polymer’s photodegradation. Because many commonly used plastics deteriorate when exposed to sunlight, photostabilization of polymers is significant . PVC stabilizers are divided into two types: primary stabilizers that deactivate allylic chlorides produced during photo-degradation of PVC chains, and secondary ones which act as scavengers of chloride radicals and hydrogen chloride .
As PVC additives, a variety of organic compounds have been used. Plasticizers and the other commercial stabilizers can be applied to improve the PVC photostability. A UV stabilizer should have a high solubility and compatibility with the polymer in addition to the ability to successfully protect polymeric materials . Photostabilizers included Schiff base compound [9-11] aromatics material [12-14] and heterocyclic [15-17]. Other materials used as additives stabilizer include complexes metals and inorganic salts [18-25].
In this work, the PVC photostabilizer was done by using heterocyclic and Schiff base compounds. Schiff bases stabilizes PVC by various mechanisms ways, for example screener, UV absorption, and radical scavenger [26,27]. In this research, four compounds [I1-I4] have been synthesized by the method previously described by Mohammed et al.  and their uses as a photostabilizers for poly(vinyl chloride).
Material and methods
These compounds were synthesized by the method previously described by Mohammed et al. , (Table 1).
Method of preparation films
(3 gm) of poly (vinyl chloride) dissolved in (100 ml) tetrahydrofurane (THF) to prepare 3% PVC solution for usage to make the significant thickness of polymer films (40,70, and 100 µm), as calculated by digital micrometer, type (2610, Germany) and as displayed in Figure 1. These films were designed by casting and vaporization techniques in glass trays at 25 ºC for 24 hr. to remove the possible solvent residue. The concentration of additives was (0.1%, 0.3% and 0.5%). After the evaporation of solvent that’s led to the formation of polymer films, the polymeric films are taken from the glass trays and pasted to cardboard sheets that have a dimensional 2×2 cm.
Accelerated testing technique
Q-plate Company produced a quicker weather-meter (QUV, Q-Panel Company, Florida, USA) to irradiation of PVC films. A plate of stainless steel with one hole on the back side and two more on the front is used in the accelerated weathering test. Each side has a 55 Watt lamp (type Fluorescent Ultraviolet Lights) from Q-Panel Company, with wavelengths ranging from (250-380 nm) with the intensity of the light wavelength at (1.87*10-5 Ein dm-3. s-1).
The poly vinyl chloride films were fixed parallel to the lamps to ensure that UV radiation is incident vertically on the films samples. The distance between the polymeric film and the light source was (10 cm) [26,27].
Measuring of photodegradation methods
FT-IR Spectrophotometer was used to track the photodegradation of pure PVC film and PVC films with synthesis compound as additives, in the range of (4000-400) cm-1, the absorption spectra of poly vinyl chloride films were recorded. The position of carbonyl absorption is given at (1724) cm-1. Changes in carbonyl peaks were used to track the photodegradation steps through varying irradiation durations. The carbonyl indexes (Ico) were measured by comparing the FTIR absorption peaks at (1724) cm-1 with reference peaks at (1427) cm-1. This process is known as the band index method, and consists of the following equations (1).
As = Absorbance for band at study.
Ar = Absorbance for reference band.
Is = Group index at study.
The absorbance (A) at wavenumber specific for PVC carbonyl is measured by the relation explained in equation (2).
A = 2-log %T (2)
%T = the transmittance percentage.
The baseline technique is used to calculate the actual or real absorbance that differences between the highest peak’s absorbance and the baseline (A Top beak-base line) .
Viscosity– average molecular weight determination(Mv)
The intrinsic viscosity η as role of average molecular weight, it is calculated through equation (3) .
Where K and α are constant quantities for PVC at temperature and solvent system.
Intrinsic viscosity was examined by using the U-tube Ostwald viscometer which allows reading the times of flowing of polymer solution (t) and the THF solvent (t0). The viscosities below were calculated by the following equations.
Relative viscosity ηrel = t/t0 (4)
Specific viscosity ηsp = (t/t0) – 1 (5)
Intrinsic viscosity η = [(√2/ c)( ηsp – ηre )]1/2 (6)
Using the following relation, the quantum yields of main chain scission (фcs) were derived from viscosity measurements .
In which, C=concentrations (3gm/100 mL), A = Avogadro’s numbers, (Mvo) = the initial viscosity averages molecular weights, [ηo] = Intrinsic viscosity of PVC polymer before irradiation, [η] = Intrinsic viscosity of polymer after irradiation, Io=Incident intensity (1.87*10-5 Ein. dm-3. s-1) and t = Irradiation time in second.
The number of average chain scission (S) was determined by using the equation (8) .
Mvo And Mvt are the viscosity average molecular weights before and after irradiation time, respectively.
According to the following equation (9), the degree of deterioration (α) changes over time.
Where m and Mvo are the molecular weights of the monomers and polymer before irradiation time, respectively .
Results and discussion
Compounds [I1-I4] were used as the additives for photostabilization of PVC films. The irradiation of PVC films with UV light at wavelength 313 nm, led to appearance of bands at 1772 cm-1 and 1724 cm-1 was attributed to the formation of carbonyl groups related to carbonyl compounds such as (Chloroketone and aliphatic ketone) . Using an FTIR spectrophotometer, the carbonyl index was monitored as a function of irradiation times. When the PVC films were irradiated, the FTIR spectrum changed dramatically. Figures 2 and 3 display the difference in FT-IR spectra of PVC film without additives and whit 0.5 %additives, respectively.
Because the intensity of this band is proportional directly to the degradation bands, changes in the intensity of the carbonyl group extended absorption bands were used to track the range of polymer breakdown through irradiation .
The results from Figures 4-7 illustrate that when the weight ratio of additives increases, the carbonyl index (ICO) decreases and demonstrates that additive concentrations of 0.5% by weight gave the best results. The following figures are displayed with the 40μm films thickness.
The photodegradation rate of PVC films was studied in different thickness of (40, 70, and 100 μm) without and with fixed concentration of (0.5%) of additive. Figures 8-10 displays the relationships between the carbonyl index and irradiation time, it explains that the rate of photo-oxidation (Ico) at fixed irradiation time decreases when the film thickness of the additive utilized increases, as does the rate of photo-oxidation (carbonyl index) for the PVC control. Figure 11 depicts the correlations between the carbonyl index (ICO) and thickness film, indicating that as the film thickness increases, photodegradation decreases.
The average molecular weight of PVC degradation was calculated by viscosity measurement using equation (3) [K=1.5×10-4 g/ml, α'=0.77] and THF solvent at 30 ºC. When PVC films (with or without additive) are irradiated, the intrinsic viscosity and viscosity averages molecular weight decrease as the irradiation period increases. The change in the Mv of PVC films with and without additions after irradiation time, with (0.5 wt%) additives is shown in Figure 12.
The diagram in Figure 12 indicate a reduction in (Mv) suggesting that the reduction in (Mv) is due to the split of the main chain at numerous sites which are spread over the polymer chain.
By using the equation 8, the average number of chain scissions (S) was computed. Figure 13 displays the plot of (S) against time. The relationship suggests that crosslinking may result in a rise in the degree of branching.
The plot illustrate that a rise in the branching degree and this results from occurrence crosslinking. It has been observed that some films after irradiation are not completely soluble in THF solvent, and this indicates on the idea of the crosslinking occurrence in these films during irradiation.
According to the equation 9, the degree of deterioration (α) changes with time. In Figure 14, the diagram of (α) as a function of irradiation periods illustrates that α values of the irradiated films are high for free PVC (PVC control) and lower when the additives are present. The value of α rises quickly with time in the early steps of photodegradation of PVC these signs mention a breaking of bonds randomly occurrence in PVC chain.
The quantum yields of chain scission ( Фcs) were obtained using equation (7) and summarized as indicated in Table 2 fore PVC films without additive and PVC films with (0.5% wt/wt) of additives.
PVC films without additives have higher (Фcs) values than those with additives, which grow in the following trend:
PVC > I2 > I1 > I3 > I4
The reason for the low (Фcs) values in the presence of additives is that during the irradiation the energy absorbed by the PVC is from one site and the electronic excitation are distributed over all the existing bonds, and this reduces the probability of bonds’ breaking, or that the absorbed energy is disposed of or distributed through non-reactive processes.
Some mechanics were proposed based on the findings of the experiment [31,32] Schemes 1-3.
From the Figures 8-10 that explain the relationship between the carbonyl index and irradiation time for PVC films (40, 70, and 100 μm thickness) with different concentration of I2 compound additive, we demonstrate that the compound I2 initially act as photo degradation for PVC polymer. And from FT-IR tests for just I2 compound before and after irradiation Figure 15, we notice that with the increase the irradiation time, the growth of the carbonyl group increase and the intensity of the amine group decreases, this indicate that the compound I2 initially act as a photo degradation for PVC polymer, and after irradiation the polymeric films by UV light for a long time, it is dissociated into the other compounds which acted as photo-stabilizers for PVC polymer.
Surface morphological study of poly (vinyl Chloride) films by scanning electron microscope and light microscope as a criterion for the photostabilizing efficiency
The morphology of surface films of blank PVC (nonirradiated PVC), PVC irradiated for 250 hr., and PVC in the presence of I4 as organic additives (irradiated for 250 hr.) was studied with scanning electron microscope (SEM) (Figure 16). The surface of PVC film (blank) was smooth and free of any white spots indicating degradation, while the surface of PVC film irradiated for 250 hour was filled with white spots indicating to HCl had evolved resulting the holes or grooves. While in the case of PVC in the presence of additive (I4) irradiated for 250 hour, the surface was almost smooth, and less white spots appeared and there is no holes, this indicated the high stabilizing efficiency of the examined organic stabilizer and the extent to which the polymer surface is protected from degradation by dehydrochlorination .
Figure 17 displays a light microscope images tests of polymer films surfaces using a high-power field (400X magnifications) before and after irradiation, before irradiation we note that the film (PVC + 0.5% I2) comparing it with pure PVC film (PVC control), it is the beginning degradation and the change in the crystalline regularity of the surface, becomes rough, and the formation of spots, holes and bubbles due to the breakage of the polymeric chains, and by comparing it again with its image after irradiation, we notice that there is no significant change and the presence the same spots, holes, and bubbles. This is another indication for I2 compound work as a photodegradation for PVC before irradiation, and after subjected it to UV light irradiation for a period time, it is dissociated into other compounds (fragments) which acted as photostabilizers for PVC polymer .
The suggested mechanism of dissociated of I2 compound by UV light explained in Scheme 4.
The photostabilization of (PVC) films containing the additives (0.5 percent by weight) was following the trend I4 > I3 > I1 > I2 > PVC as the additives’ efficiency increased. In comparison to PVC control, the carbonyl index growth for the additive (I4) was lowered. These additives stabilize poly vinyl chloride films through UV absorption, peroxide decomposer, and radical scavenger mechanisms. According to the photostability and mechanisms discussed above, the additives were to be the most effective in photostabilization processes. The compound I2 initially act as the photo degradation for PVC polymer, and after irradiation polymeric films by UV light for a long time, it is dissociated into the other compounds (fragments) that acted as photostabilizers for PVC polymer.
The authors like to thank all their professors in the Department of Chemistry, College of Science, Mustansiriya University, for providing facilities in the Polymer Research Unit.
Mohammed Alwan Farhan: https://www.orcid.org/0000-0003-0795-2058
How to cite this article: Mohammed Alwan Farhan*, Olfat Abed Nief, Wassan Baqir Ali. New photostabilizers for poly (vinyl chloride) derived from heterocyclic compounds. Eurasian Chemical Communications, 2022, 4(6), 525-543. Link: http://www.echemcom.com/article_147155.html
Copyright © 2022 by SPC (Sami Publishing Company) + is an open access article distributed under the Creative Commons Attribution License(CC BY) license (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.