Degradation of polyethylene during extrusion. II. Degradation of low‐density polyethylene,linear low‐density polyethylene,and high‐density polyethylene in film extrusion

The degradation of different polyethylenes—low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and high‐density polyethylene (HDPE)—with and without antioxidants and at different oxygen concentrations in the polymer granulates, have been studied in extrusion coating processing. The degradation was followed by online rheometry, size exclusion chromatography, surface oxidation index measurements, and gas chromatography–mass spectrometry. The degradations start in the extruder where primary radicals are formed, which are subject to the auto‐oxidation when oxygen is present. In the extruder, crosslinking or chain scissions reactions are dominating at low and high melt temperatures, respectively, for LDPE, and chain scission is overall dominating for the more linear LLDPE and HDPE resins. Additives such as antioxidants react with primary radicals formed in the melt. Degradation taking place in the film between the die orifice, and the quenching point is mainly related to the exposure time to air oxygen. Melt temperatures above 280°C give a dominating surface oxidation, which increases with the exposure time to air between die orifice and quenching too. A number of degradation products were identified—for example, aldehydes and organic acids—which were present in homologous series. The total amount of aldehydes and acids for each number of chain carbon atoms were appeared in the order of C5>C4>C6>C7?C2 for LDPE, C5>C6>C4>C7?C2 for LLDPE, and C5>C6>C7>C4?C2 for HDPE. The total amounts of oxidized compounds presented in the films were related to the processing conditions. Polymer melts exposed to oxygen at the highest temperatures and longest times showed the presence dialdehydes, in addition to the aldehydes and acids. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1525–1537, 2004     

Surface oxidation of low‐density polyethylene films to improve their susceptibility toward environmental degradation

Polyethylene wastes, particularly as films, have accumulated over the last several decades resulting in a major visual litter problem. The aim of this study was to investigate the ability of chemical reagents to oxidize the low‐density polyethylene (LDPE) film surface to increase their susceptibility toward photodegradation and thermal degradation. Three chemical agents, namely, potassium permanganate, potassium persulfate, and benzoyl peroxide, were used to oxidize the film surface to generate chromophoric groups, such as carbonyl groups, which are the main reason for the enhanced environmental degradation of photolytic polymers, such as ethylene–carbon monoxide and ethylene–vinyl ketone copolymers. For the chemical treatment, LDPE films of 70 ± 5 μm thickness were prepared by a film‐blowing technique and subsequently reacted with the aforementioned oxidizing agents. To aid the oxidation process, the reaction with potassium persulfate and potassium permanganate was performed under microwave irradiation heating. In the case of benzoyl peroxide aided oxidation, the films were subjected to repeated coating–heating treatments up to a maximum of 10 cycles. The treated films were subjected to accelerated aging, that is, xenon‐arc weathering and air‐oven aging (at 70°C), for extended time periods. The chemical and physical changes induced as a result of aging were followed by the monitoring of changes in the mechanical, structural, and thermal properties. The results indicate that the surface‐oxidized LDPE films exhibited enhanced susceptibility toward degradation; however, the extent was reduced as compared to photolytic or other degradable compositions. The ability of the chemicals to initiate degradation followed the order potassium persulfate < potassium permanganate < benzoyl peroxide. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Additives for ultraviolet‐induced oxidative degradation of low‐density polyethylene

Polyethylene (PE) is one of the most widely produced and widely used plastics in the world. Saturated hydrocarbons cannot absorb the energy of the light reaching earth, so the degradation process is rather slow; this, in return, causes disposal problems. On the other hand, it was observed that in the presence of oxygen and impurities in the polymer matrix, the degradation could be reduced to shorter time intervals. In this study, vanadium(III) acetyl acetonate (VAc), serpentine (SE), and Cloisite 30B (CL) were used as additives, both together and alone, and we followed the photodegradation of PE. The amount of VAc was kept constant at 0.2 wt %, whereas the amounts of SE and CL were varied between 1 and 4 wt %. The samples were irradiated by UV light for up to 500 h. Mechanical and spectroscopic measurements were carried out during certain time intervals to monitor the degradation. VAc containing PE showed the fastest degradation. The elongation at break values of these samples were reduced to half of the initial value of elongation at break within five days. Combinations of the CL and SE additives were also proven to accelerate the degradation of PE; this was followed by an increase in the carbonyl index, which was observed to be at least 10 times greater than that of pure PE. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43354.     

Melt strength of linear low‐density polyethylene/low‐density polyethylene blends

Linear low‐density polyethylenes and low‐density polyethylenes of various compositions were melt‐blended with a batch mixer. The blends were characterized by their melt strengths and other rheological properties. A simple method for measuring melt strength is presented. The melt strength of a blend may vary according to the additive rule or deviate from the additive rule by showing a synergistic or antagonistic effect. This article reports our investigation of the parameters controlling variations of the melt strength of a blend. The reciprocal of the melt strength of a blend correlates well with the reciprocal of the zero‐shear viscosity and the reciprocal of the relaxation time of the melt. An empirical equation relating the maximum increment (or decrement) of the melt strength to the melt indices of the blend components is proposed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1408–1418, 2002     

Weathering of low‐density polyethylene grafted with itaconic acid in laboratory tests

Polyethylene grafted with itaconic acid was subjected to weathering under laboratory accelerated conditions. The course of the photo‐oxidative degradation process of that material was studied by FTIR spectroscopy both through quantitative measurements of changes in absorbance values at selected wave numbers and through measurements of surface area values for absorption bands which were separated by means of deconvolution. The use of both those procedures of quantitative determinations resulted in a general conclusion that the oxidation process was initiated from the very first moment of irradiation, and it produced ketones, acids, esters (intramolecular and of acetate type), peracids, peresters, hydroperoxides, and alcohols. The molecular weight values and gel number values, which were established as well, pointed out that oxidation was accompanied by cracking of polymer macromolecules, and also by polymer crosslinking to a limited degree. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of nitrogen and carbon dioxide gases on the degradation of low‐density polyethylene during extrusion and origin of the color

The effect of inert gases, nitrogen and carbon dioxide, on the oxidative degradation of low‐density polyethylene appearing as colored spots has been studied during an extrusion process in competition with an antioxidant. Extrusion under inert gases significantly decreases the degradation level in the critical region of the process in comparison with classical extrusion under air. The effect of antioxidants on degradation during extrusion at a high temperature is weak. The main processes acting on this reduction of polymer oxidation and the origin of the color of degraded domains have been investigated. Energy‐dispersive spectra of particles have confirmed that degradation is caused by thermooxidation. The nature of chromophore groups in degraded areas has been identified by IR microscopy. We found that β‐conjugated ketoenols are present inside colored spots and seem to be responsible for the color of degraded parts. Quantum calculations have confirmed that such chemical structures absorb visible light and create reddish and brown colors. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation and characterization of linear low‐density polyethylene/low‐density polyethylene/TiO2 membranes

Because of their special functions, the application of nanoscale powders has recently attracted both industrial and theoretical interest. In this study, nanoscale TiO₂, which exhibited a special UV absorption and consequent antibacterial function, was added to a low‐density polyethylene/linear low‐density polyethylene hybrid by melt compounding to yield functional composite membranes. TiO₂ exhibited an apparent induced nucleation effect on the crystallization of polyethylene, and the size of the crystallites decreased while the number increaed with the introduction of TiO₂; however, the crystallinity of polyethylene changed little. Also, TiO₂ exhibited an ideal dispersion in the membrane with an average size less than 100 nm, and this excellent dispersion provided the membranes extra UV absorption; moreover, the transparency of the membranes was maintained to satisfy common requirements. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 216–221, 2005     

Blend of high‐density polyethylene and a linear low‐density polyethylene with compositional‐invariant mechanical properties

This article presents the tensile properties and morphological characteristics of binary blends of the high‐density polyethylene (HDPE) and a linear low‐density polyethylene (LLDPE). Two constituents were melt blended in a single‐screw extruder. Injection‐molded specimens were evaluated for their mechanical properties by employing a Universal tensile tester and the morphological characteristics evaluated by using a differential scanning calorimeter and X‐ray diffractometer. It is interesting to observe that the mechanical properties remained invariant in the 10–90% LLDPE content. More specifically, the yield and breaking stresses of these blends are around 80% of the corresponding values of HDPE. The yield elongation and elongation‐at‐break are around 65% to corresponding values of HDPE and the modulus is 50% away. Furthermore, the melting endotherms and the crystallization exotherms of these blends are singlet in nature. They cluster around the corresponding thermal traces of HDPE. This singlet characteristic in thermal traces entails cocrystallization between these two constituting components. The clustering of thermal traces of blends near HDPE meant HDPE‐type of crystallites were formed. Being nearly similar crystallites of blends to that of HDPE indicates nearness in mechanical properties are observed. The X‐ray diffraction data also corroborate these observations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2604–2608, 2002     

Blending of low‐density polyethylene with vanillin for improved barrier and aroma‐releasing properties in food packaging

Modification of low‐density polyethylene (LDPE) with vanillin to obtain flavored packaging film with improved gas barrier and flavor‐releasing properties has been studied. The modification of LDPE with vanillin was monitored by Fourier transform infrared spectroscopy, wherein the appearance of new peaks at 1704.7, 1673.6, and 1597.2 cm^?1 indicates the incorporation of vanillin into LDPE matrix. Films of uniform thickness were obtained by the extrusion of modified LDPE. Modified LDPE was found to have significantly higher gas barrier properties and grease resistance. Sensory quality of food products viz, doodhpeda (milk‐based solid soft sweet), biscuit, and skimmed milk powder packed in LDPE‐vanillin film showed that the doodhpeda sample had clearly perceptible vanilla aroma, whereas biscuit had marginal aroma and skimmed milk powder did not have noticeable aroma. When viewed in the light of imparting desirable vanilla aroma, results of the study indicated that LDPE‐vanillin film has better prospects as a packaging material for solid sweets with considerable fat content when stored under ambient conditions. The release of vanilla aroma was further confirmed by gas chromatography–mass spectrometery analysis. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Oxidation and crosslinking processes during thermal aging of low‐density polyethylene films

This work analyzes the influence of thermal degradation on the microstructure and the mechanical properties of low‐density polyethylene subjected to aging at 70°C in the dark for times up to 21 months. It is found that the polymer shows a gradual increase of its elastic modulus and a dramatic reduction of its ductility, due to secondary crystallization. Infrared spectroscopy (FTIR) reveals the autoaccelerated oxidation of the polymer after 5 months aging. It is observed that the unsaturated vinylidene groups initially present in the material are gradually overridden by vinyl groups and, eventually, by t‐vinylene groups. Nuclear magnetic resonance (¹³C NMR) shows that the initial butyl chain branches are progressively completed by shorter ramifications, namely ethyl branches. These results are discussed in term of macromolecular mechanisms: (i) oxidation, (ii) chain scission, and (iii) crosslinking. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Influence of climatic ageing on the mechanical properties and the microstructure of low‐density polyethylene films

Blown extruded films of low‐density polyethylene (LDPE) have been subjected to climatic ageing in a sub‐Saharan facility at Laghouat (Algeria) with direct exposure to sun. Samples were characterized by complementary techniques after prescribed amounts of time up to 8 months. It was shown by tensile testing that the mechanical properties are quite sensitive to ageing: (i) the elastic modulus increases and saturates, (ii) the tensile stress increases slightly, and (iii) the rupture energy decreases dramatically after 4 months weathering. Fourier‐transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (¹³C NMR) were performed to identify the evolution of the polymer microstructure. The FTIR spectra reveal the initial presence of vinylidene groups that exhaust rapidly after 4 months ageing. Also, it detects the progressive multiplication of vinyl groups and oxidation products of many kinds. The NMR technique revealed specifically the carbon–carbon configurations in the polymer chains. By contrast to the original film that contained almost exclusively butyl chain branches, the aged specimens presented shorter ramifications, namely ethyl branches. Also, the presence of quaternary atoms was detected after long ageing times. The discussion of these complementary results in the light of current literature makes possible to identify the leading mechanisms that control the decay of LDPE film properties. Although these mechanisms are numerous and complex, they can be schematically summarized within three main classes: oxidation, scission, and crosslinking. Each class is discussed in details. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal and mechanical properties of linear low‐density polyethylene/low‐density polyethylene/wax ternary blends

The thermal and mechanical properties of uncrosslinked three‐component blends of linear low‐density polyethylene (LLDPE), low‐density polyethylene (LDPE), and a hard, paraffinic Fischer–Tropsch wax were investigated. A decrease in the total crystallinity with an increase in both LDPE and wax contents was observed. It was also observed that experimental enthalpy values of LLDPE in the blends were generally higher than the theoretically expected values, whereas in the case of LDPE the theoretically expected values were higher than the experimental values. In the presence of higher wax content there was a good correlation between experimental and theoretically expected enthalpy values. The DSC results showed changes in peak temperature of melting, as well as peak width, with changing blend composition. Most of these changes are explained in terms of the preferred cocrystallization of wax with LLDPE. Young's modulus, yield stress, and stress at break decreased with increasing LDPE content, whereas elongation at yield increased. This is in line with the decreasing crystallinity and increasing amorphous content expected with increasing LDPE content. Deviations from this behavior for samples containing 10% wax and relatively low LDPE contents are explained in terms of lower tie chain fractions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1748–1755, 2005     

Effects of ultraviolet absorbers on the ultraviolet degradation of rice‐hull/high‐density polyethylene composites

In China, rice‐hull powder is widely used as a fiber component to reinforce polymers because of its ready availability and lower cost compared to wood fibers. However, an issue concerning these composites is their weathering durability. In this study, the effects of two ultraviolet absorbers (UVAs), UV‐326 and UV‐531, on the durability of rice‐hull/high‐density polyethylene (HDPE) composites were evaluated after the samples were exposed to UV‐accelerated weathering tests for up to 2000 h. All of the samples showed significant fading and color changes in exposed areas. X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were used to detect surface chemical changes. The results indicate that surface oxidation commenced immediately within the first 500 h of exposure for all of the samples. However, the control rice‐hull/HDPE composites underwent a greater degree of oxidation than those with the UVAs. Scanning electron microscopy revealed that the rice‐hull/HDPE composites degraded significantly upon accelerated UV aging, with dense cracking on the exposed surface. The UVAs provided effective protection for the rice‐hull/HDPE composites, and UV‐326 had a more positive effect on the color stability than UV‐531. The results reported herein serve to enhance our understanding of the efficiency of UV stabilizers in the protection of rice‐hull/HDPE composites against UV radiation, with a view toward improving their formulation. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Processing characteristics of low‐density polyethylene filled with calcium carbonate of different size distributions

Low‐density polyethylene (LDPE) was filled with blends of different proportions of two sizes of calcium carbonate (CaCO₃; 600 and 2500 mesh). The torque of the LDPE/CaCO₃ samples was measured with a torque rheometer. The results showed that the process torque values of the LDPE/CaCO₃ samples obviously decreased when LDPE was filled with a blend of two sizes of CaCO₃ (600‐ and 2500‐mesh CaCO₃ blend) in comparison with samples filled with CaCO₃ of a single size (600 or 2500 mesh). When the ratio of 600‐mesh CaCO₃ to the total CaCO₃ was in the range of 40–60 wt %, the lowest torque value of the LDPE/CaCO₃ samples was achieved. When the content of CaCO₃ in a sample was 30 wt %, LDPE filled with CaCO₃ of different size distributions showed the largest decrease in the torque ratio in comparison with the samples filled with CaCO₃ of a single size. The torques of LDPE samples filled with CaCO₃ of a single size and those filled with CaCO₃ of different size distributions at different temperatures were also studied. The results showed that the flow activation energy and flow activation entropy of LDPE samples filled with CaCO₃ of different size distributions increased obviously. The increase in the flow activation entropy was used to explain the phenomenon of the process torque decreasing for LDPE samples filled with CaCO₃ of different size distributions. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Controlled gas‐phase sulfonation of low‐density polyethylene films

In the current research, a highly controllable system operating at low temperatures and for short reaction times is employed for the surface sulfonation of low‐density polyethylene. This system provides the advantages of short reaction times and low reaction temperatures, as compared with previous methods of surface sulfonation. Low‐density polyethylene films were sulfonated at 40°C for time periods ranging from 5 to 30 min. Subsequently, all films were analyzed by SEM, EDX, horizontal ATR–FTIR, surface roughness, and dynamic contact‐angle measurements. Sulfonation was effected at all reaction times. The degree of surface sulfonation increased through 10 min and reached a maximum between 10‐ and 30‐min reaction times with concomitant changes in the physicochemical properties of the material. At 30 min, the film topography changed substantially, indicating that sulfonation was no longer limited to a strictly surface reaction. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1865–1869, 2000     

Modeling the oxidative degradation of ultra‐high‐molecular‐weight polyethylene

Understanding the sequence of reactions that occur in ultra‐high‐molecular‐weight polyethylene (UHMWPE) following ⁶⁰Co γ irradiation has been the focus of numerous experimental studies. In the study reported here, we have incorporated recent experimental findings into a mathematical model for UHMWPE oxidation. Simulation results for shelf aging and accelerated aging are presented. It is shown that very reasonable simulations of shelf‐aging and accelerated‐aging data can be obtained. It is also shown that simulations of shelf aging in reduced oxygen environments predict that the subsurface peaks of ketones will be shifted to the exterior surface. In vivo aging can be simulated if we assume that the oxygen level in the synovial fluid is about one‐eighth that of atmospheric levels. Some reduced irradiation doses are predicted to significantly reduce the ketone formation for shelf‐aging periods of up to 10 years. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 814–826, 2003     

Degradation behavior of linear low‐density polyethylene films containing prooxidants under accelerated test conditions

This article reports the results of studies on the photooxidative and thermooxidative degradation of linear low‐density polyethylene (LLDPE) in the presence of cobalt stearate. Various amounts of cobalt stearate (0.1–0.9% w/w) blended with LLDPE and films of 70 ± 5 μ thickness were prepared by a film‐blowing technique. The films were subjected to xenon arc weathering and air‐oven aging tests (at 70°C) for extended time periods. We followed the chemical and physical changes induced as a result of aging by monitoring changes in the mechanical properties (tensile strength and elongation at break), carbonyl index, morphology (scanning electron microscopy), melt flow index, and differential scanning calorimetry crystallinity. Cobalt stearate was highly effective in accelerating the photodegradation of LLDPE films at concentrations greater than 0.2% w/w. The kinetic parameters of degradation, as determined by nonisothermal thermogravimetric analysis, were estimated with the Flynn–Wall–Ozawa isoconversion technique, which was subsequently used to determine the effect of cobalt stearate on the theoretical lifetime of LLDPE. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermal analysis of high‐density polyethylene and low‐density polyethylene with enhanced biodegradability

The thermal properties of high‐density polyethylene (HDPE) and low‐density polyethylene (LDPE) filled with different biodegradable additives (Mater‐Bi AF05H, Cornplast, and Bioefect 72000) were investigated with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The DSC traces of the additives indicated that they did not undergo any significant phase change or transition in the temperature region typically encountered by a commercial composting system. The TGA results showed that the presence of the additive led to a thermally less stable matrix and higher residue percentages. The products obtained during the thermodegradation of these degradable polyolefins were similar to those from pure polyethylenes. The LDPE blends were thermally less stable than the HDPE blends. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 764–772, 2002     

Recycled milk pouch and virgin low‐density polyethylene/linear low‐density polyethylene based coir fiber composites

The viability of the thermomechanical recycling of postconsumer milk pouches [a 50 : 50 low‐density polyethylene/linear low‐density polyethylene (LDPE–LLDPE) blend] and their use as polymeric matrices for coir‐fiber‐reinforced composites were investigated. The mechanical, thermal, morphological, and water absorption properties of recycled milk pouch polymer/coir fiber composites with different treated and untreated fiber contents were evaluated and compared with those of virgin LDPE–LLDPE/coir fiber composites. The water absorption of the composites measured at three different temperatures (25, 45, and 75°C) was found to follow Fickian diffusion. The mechanical properties of the composites significantly deteriorated after water absorption. The recycled polymer/coir fiber composites showed inferior mechanical performances and thermooxidative stability (oxidation induction time and oxidation temperature) in comparison with those observed for virgin polymer/fiber composites. However, a small quantity of a coupling agent (2 wt %) significantly improved all the mechanical, thermal, and moisture‐resistance properties of both types of composites. The overall mechanical performances of the composites containing recycled and virgin polymer matrices were correlated by the phase morphology, as observed with scanning electron microscopy. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Photobiodegradation of low‐density polyethylene/banana starch films

The effects of the starch content, photosensitizer content, and compatibilizer on the photobiodegradability of low‐density polyethylene (LDPE) and banana starch polymer blend films were investigated. The compatibilizer and photosensitizer used in the films were PE‐graft‐maleic anhydride (PE‐g‐MA) and benzophenone, respectively. Dried banana starch at 0–20% (w/w) of LDPE, benzophenone at 0–1% (w/w) of LDPE, and PE‐g‐MA at 10% (w/w) of banana starch were added to LDPE. The photodegradation of the blend films was performed with outdoor exposure. The progress of the photodegradation was followed by determining the carbonyl index derived from Fourier transform IR measurements and the changes in tensile properties. Biodegradation of the blend films was investigated by a soil burial test. The biodegradation process was followed by measuring the changes in the physical appearance, weight loss, and tensile properties of the films. The results showed that both photo‐ and biodegradation rates increased with increasing amounts of banana starch, whereas the tensile properties of the films decreased. The blends with higher amounts of benzophenone showed higher rates of photodegradation, although their biodegradation rates were reduced with an increase in benzophenone content. The addition of PE‐g‐MA into polymer blends led to an increase in the tensile properties whereas the photobiodegradation was slightly decreased compared to the films without PE‐g‐MA. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2725–2736, 2006