by Marcelo Augusto Gonçalves Bard1, Rafael Auras2 and Luci Diva Brocardo3 Machado, 1Grupo de Pesquisas em Meio Ambiente e Sustentabilidade (GPMAS), Universidade São Francisco (USF); 2School of Packaging, Michigan State University (MSU); 3Comissão Nacional de Energia Nuclear (CNEN), Instituto de Pesquisas Energéticas e Nucleares (IPEN)
Editor’s Note: A member of the UV+EB Technology Editorial Board heard Dr. Bardi speak at Abrafati 2017, a three-day event hosted by the Brazilian Coating Manufacturers Association, and felt the topic was of interest to readers of the magazine. Dr. Bardi and his co-authors submitted the following paper, which is being printed in its entirety. The Issue Champions noted a heavy academic bent not typical of articles in this magazine, but believe its important to increase industry awareness of what is being done on this topic in the academic world. As recommended reading, the article “Interaction of UV/EB-Cured Print Inks Applied to a Compostable Polymer Blend,” found in Rad Tech Report V4 2008 (www.radtech.org), provides insight into understanding the background of this paper and the referenced coating formulations.
Polymers are widely used in the manufacturing industry, with a range of applications ranging from low life materials, such as packaging of plastic and conventional products, to special applications, such as aircraft components and spacecraft. One of the main advantages in the use of plastics is their extreme resistance to degrading physical factors, such as electromagnetic radiation.
Degrading processes have a general reaction path of a radical nature. Similar to the curing process, degradation reactions also are autocatalytic, usually associated with the oxidation of hydrocarbons, either during processing or in the use of the material. On the other hand, the stabilization of polymers to a certain degrading process also follows the same reaction, however with different radicals6.
Biotic degradation is directly associated with the intrinsic characteristics of each polymer material. There may be materials which are biodegradable by nature and thus directly attacked by enzymes, such as amylase or cellulase, or those in need of physical or chemical abiotic agent to then become bio-accessible. As examples of the latter group, there may be mentioned hydro-biodegradable polymers, such as poly lactic acid and aliphatic-aromatic polyesters, which have hydrolyzable groups and therefore undergo hydrolysis prior to the biodegradation process2.
Abiotic degradation is usually analyzed by exposing the material to some degradation-inducing agent. It is important to note, however, that the rate of abiotic degradation is entirely dependent on the geographic and environmental conditions of the film’s exposure site. Cracking and fragmentation are expected when the abiotic degradation process is satisfactory after a certain exposure time, which is directly related to the step of reducing molar mass, increasing crystallinity, reducing mechanical properties and increasing the concentration of carbonyl groups2.
In the specific case of polyolefins containing additive catalysts of abiotic degradation, the rapid formation of peroxidase chains was observed, whose molecules are more hydrophilic and, therefore, able to increase the capacity of biological degradation in the later stage. According to the authors, UVA and UVB radiation represent the total radiant energy in the ultraviolet spectrum emitted by the sun that reaches the Earth’s surface, since the wavelengths related to the UVC region are absorbed by the atmosphere. In addition, diffusion of oxygen in the interior of the polymer chain is relevant for oxidative degradation reactions, although variations in molecular oxygen concentration do not interfere in the total process.
The biotic degradation process is usually accompanied by the evolution of the production of carbon dioxide, with the mineralization of the carbon atoms present in the polymer structure. In this type of process, it is important to highlight that the surface area of the sample exposed to the microorganisms plays a fundamental role, since the more fragmented the sample is, the better the diffusion of degradation by-products.
For the material to be mineralized, specific enzymes must be produced by specific microorganisms. In general, when there is compatibility between the polymer film and the microbiological being, biofilm formation occurs on the surface of the polymer that acts as a substrate, providing the necessary nutrients for the development of the colony. Biofilms can be seen as blackheads in films and are related to the presence of fungi or bacteria. However, it is necessary to provide appropriate environmental conditions for the development of these microbial colonies, such as temperature and humidity (2).
This work aims to study the natural compostability of printing inks, applied to polymer substrates, biodegradable or not, and cured by ultraviolet radiation, in order to reduce the shelf life under natural composting environments.
Methodology
Radiation curable polymer coatings were prepared as described in Table 1. Details on the formulation can be found in BARDI, AURAS & MACHADO (2014).
Low density polyethylene EB-853/72 (batch RSAB2A096E) (Braskem S.A., São Paulo, SP, Brazil) and Ecobras® (BASF S.A., São Paulo, SP, Brazil) were used as substrates. LDPE films were prepared by blow type extrusion, Chillroll Lab 16 model, with an L / D ratio of 26 (AX Plásticos Ltda., Diadema, SP, Brazil), available at Universidade Presbiteriana Mackenzie. The temperatures used for zones 1, 2 and 3 were 178, 185 and 190ºC, respectively, and the thread speed was 80 rpm. The TMI 549M micrometer (Testing Machines, Inc., Amityville, NY, USA) was used to measure the thickness of the films obtained, averaging 21 ± 0.5 µm.
Pro-degrading additives based on cerium (Ce), cobalt (Co) and manganese (Mn) were incorporated into the blend, with a nominal concentration of 2%, by mass of the final film. The materials obtained were named according to the present additive, ie, LDPE, LDPECeSt, LDPECoSt and LDPEMnSt.
Ecobras®, a commercial blend of poly(butylene adipate-co-terephthalate)/thermoplastic starch (PBAT/TPS), was used as supplied.
The pasty films were applied to the substrates and converted into dry films by curing reactions induced by ultraviolet (UV) radiation. In the first case, a model UVC tunnel (Germetec Ultraviolet and Infrared Technology Ltda., Rio de Janeiro, RJ, Brazil) was used, consisting of a medium pressure mercury lamp, elliptical reflectors and a conveyor belt, with a velocity of 9 m s-1, UV lamp at the power of 300 W pol-1, and approximate exposure of 519 mJ cm-2, with irradiance of 1 035 mW cm-2, by passing the sample under the beam.
The accelerated aging of samples of polymeric substrates, coated or uncoated, was carried out using a weathering chamber model EQUV (Equilam Ind. And Com. Ltda., Diadema, SP, Brazil). According to ASTM D5208-09, fluorescence lamps with main emission in the UVA region were used at 340 nm, with irradiance of 0.89 W (m2 nm)-1.
The samples were conditioned in the chamber so that the bundles of UV rays were incident at 90º on the coated surface. The irradiation was kept constant at 50 ± 3°C for 250 h.
An organic compost pile consisting of cow dung, wood chips and bovine feed residues was prepared at the Composting Facility facility of Michigan State University (East Lansing, MI, USA) and used to evaluate the biodegradability of the substrate samples coated with printing inks for six months. The pile had maximum dimensions of 3.0 × 12 × 3.7 m (Figure 1), which was positioned on asphalt pavement.
The temperature profile of the pile at 2.1 m above the ground is shown in Figure 2.
Compound aliquots were removed, and their physicochemical parameters were determined by thermogravimetry. The assays were performed on a Q50 apparatus (TA Instruments – Waters L.L.C., New Castle, DE, USA). The measured values were:
- Ash content: 9.5 ± 0.5%;
- Total volatile solids: 11.5 ± 1.2%;
- Moisture content: 66.8 ± 0.5%;
- pH: 7.9 ± 1.0.
Samples were placed in wooden boxes, with dimensions of 0.6 × 0.3 m, the lower part made with glass fiber screen, with 0.3 mm aperture, in order to retain and easily identify the samples and the surrounding compound for further analysis. The boxes were placed about 2 m above the ground and 1 m inside the compost pile (FIG. 3), where the maximum composting temperatures were obtained.
Samples were cut into 2 × 2 cm squares, in duplicate. In order to facilitate the withdrawal of the samples for the characterization tests, they were surrounded by the same glass mesh used as the casing coating. Then, the sample is in direct contact with the compound at the same time as it is protected when the box is removed after the exposure period.
Four boxes were placed, with four extra boxes as replicas, each set of two boxes being withdrawn each month of analysis. The boxes were removed after 1, 2, 4 and 6 months of exposure to the composting environment. Only the samples previously exposed to the accelerated aging chamber were submitted to the composting test in real environment. Thus, the individual mass of each sample is measured before and after the treatment, represented by m0 and m, respectively. The retained mass index Δm can be defined according to EQ.1.
Results and Discussion
The composting methodologies reproduce a real situation in which the materials can be exposed after their useful life, culminating in the reinsertion of their organic components to the natural biogeochemical cycles. It is common to find these situations in cities that have landfills or composting reactors. In this case, conditions of elevated temperature (greater than 50ºC), possibility of anaerobic reactions and high moisture content are preserved, which will allow the development of thermophilic microorganisms, highly active in carbohydrate rich media7.
Figures 4 through 9 present the percentage change data of the samples cured by UV, aged and submitted to the composting test in real environment. The data are grouped according to the color of each composition.
Figures 4 through 9 represent the mean mass variation data associated with the composting process in real environment during the six-month period. It can be observed that, among the variables considered in the experiment – pigment, substrate and pro-degrading additive – only the last two directly affect the mass loss of the material previously aged by UV. This is probably associated with the poor interaction that the coating layer has with the substrate, in addition to the ability to absorb photons during aging.
As the substrate layer is much larger than that of paint and only one face of the substrate is coated, it is expected that there will be no variation of mass loss between the different compositions with different pigments. Thus, the substrate must be the main actor in the degrading process of the formulations studied. Note that UV-color-LDPE compositions were not significantly affected by the composting process, as opposed to those based on PBAT/TPS, which were completely fragmented during the test period.
The LDPE molecules have extremely high molar mass and are formed simply by -CH2- type monomer units7. Thus, there are no active centers that allow nucleophilic or electrophilic attacks, which limits the number of radical chemical reactions required for an efficient degradation process. Furthermore, in solid LDPE, the molecules are densely aligned, forming semi-crystalline structures, which provides a hydrophobic surface, reducing water diffusion capacity, and thereby inhibiting hydrolytic reactions.
In cases where there is a reduction of mass in the studied samples, “defects” in the structure of the polymer sre observed due to the inclusion of stabilizing additives or even pro-degrading additives. If we look at the data of Figures 4 through 9, it can be clearly noticed that the presence of pro-degrading additives to the polymer substrate LDPE provided a greater mass variation, either positive, indicating mass gain by absorption of water from the environment or by oxidative reactions, or negative, by mineralization and bio-conversion of the polymer components during composting. This factor may be associated with changes in the structure of the material due to accelerated aging or the simple presence of the pro-degrading additive, causing reduction of the molar mass of the polymer. In this case, if the additive is based on transition metals, the following sequential reactions are observed: thermal peroxidation and biodegradation of low molar mass products7.
The most efficient additives to catalyze the biodegradation of polyolefinic materials are capable of releasing two metal ions with similar ionic stability, but with different oxidation numbers, such as Co, Ce and Mn3. Thus, the rate of degradation will depend on the concentration of the additive, the class of polymer employed and the form of reaction catalysis. Specifically to Figures 4 through 9, it is observed that samples containing cobalt stearate (CoSt) are those that have the greatest negative mass variations, suggesting that the reactions induced by UV aging were able to leave the polymer substrate more susceptible to the degradation process in the environment of composting. Interestingly, only UV-〖color〗 formulations showed a significant mass variation, suggesting that the concentration of the CoSt additive influences and accelerates the degradation process of the previously oxidized polymer substrate.
Regarding the PBAT/TPS films, it was observed that all the samples studied reached total fragmentation after the six-month composting period. The biodegradation of PBAT/TPS blends has already been studied by several researchers, but always in controlled laboratory environments. The biodegradation rate of PBAT / TPS blends is higher than that of PBAT itself5.
Conclusions
Regarding the composting process, it was observed a reduction in the mineralization of the coated samples, mainly for the biodegradable substrates, in comparison to the uncoated films. This fact confirms the initial premise of this study that the coating layer acts as a protector of the polymeric substrate, increasing its durability when exposed to various weather conditions. With respect to the substrate, it is noted that the pro-degrading additives were more compatible with the polyolefin material. It was observed that the presence of the cobalt salt accelerated superficial changes in the studied films, facilitating the adhesion of microbial colonies.
Acknowledgements
The authors are grateful to FAPESP (Grant 2010/02631-0) for financial support, CNPq for scholarship and Flint Ink do Brasil for the preparation of ink compositions. The authors are grateful for the collaboration of Edgar Castro Aguirre, Rodolfo Lopez-Gonzalez, Rijosh John Cheruvathur and Tanatorn Tongsumrith during the composting test.
References
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