Mechanisms of antioxidant action. Effect of the polymer medium on the antioxidant efficiency of the dithiolates

Peroxide decomposing antioxidants (e.g. nickel dithiophosphates and thiophosphoryl disulphides) control hydroperoxide formation during processing and on exposure to light. However, these additives are more efficient u.v. stabilizers in polypropylene (PP) than in low density polyethylene (LDPE). It is suggested that this difference results from the more rapid formation of hydroperoxides in the more oxidisable substrate under normal processing conditions. In contrast, nickel xanthates are completely destroyed in PP under the same processing conditions and the transformation products obtained in this case are less effective u.v. stabilizers than the original xanthates. Nickel dialkyl dithiophosphates stabilise both LDPE and PP very effectively, while nickel alkyl xanthates are much less effective u.v. stabilisers in both matrices. However, the difference between the efficiencies of the two dithiolates is much less in the case of LDPE. The nickel dithiophosphates and xanthates effectively synergise with the commercial u.v. absorber Cyasorb u.v. 531 (HOBP) but they show antagonism towards a typical chain breaking antioxidant, Irganox 1076, during u.v. exposure. They are however synergists under thermal oxidative conditions.     

Co-pyrolysis behaviors and kinetics of plastics–biomass blends through thermogravimetric analysis

Co-pyrolysis behaviors of plastics–biomass blends were investigated using a thermogravimetric (TG) analysis from room temperature to 873 K with a heating rate of 5–40 K min^?1 in an inert atmosphere. The selected biomass sample was sawdust of pine wood (WS). Polyvinyl chloride (PVC), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) were selected as plastic samples. The difference of mass loss between experimental and theoretical ones (calculated as arithmetic sums of those from each separated component) was used as a criterion of synergetic effect. The experimental results indicated that a significant synergetic effect existed during the high-temperature region of plastics and WS co-pyrolysis process, specially, the dehydrochlorination reaction of PVC and the degradation of hemicellulose and cellulose in the WS during the co-pyrolysis process showed synergetic effect, as well as the reaction of plastics (LDPE, HDPE, and PP) and WS. Based on the TG data with different heating rates, the kinetics parameters, especially activation energy, were calculated using the Friedman method. The activation energy of plastics, WS, and their blends were from 92.8 to 359.5 kJ mol^?1. The activation energy of the PVC–WS blends was at a range of 180.2–254.5 kJ mol^?1 in the second stages. The activation energies range of LDPE–WS, HDPE–WS, and PP–WS blends were 164.5–229.6, 213.2–234.3, and 198.4–263.6 kJ mol^?1, respectively.     

THE CHEMICAL MODIFICATION OF POLYVINYL CHLORIDE

Abstract

Poly (vinyl chloride) (PVC) is commercially one of the most important thermoplastics in the world today. Its growth rate averaged 7% per annum in the 1970s. In 1980 it was the second largest volume thermoplastic used in the United States (the first being low-density polyethylene (LDPE) and was the lowest priced among the five leading plastics: LDPE, PVC, high-density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS).     

Macromolecular scission and crosslinking rate changes during polyolefin photo-oxidation

Chain scission and crosslinking rates have been derived from molecular mass distributions obtained by gel permeation chromatography at different stages during photodegradation of various thermoplastics exposed to ultraviolet irradiation (UV). Results are given for a high density polyethylene (HDPE); a low density polyethylene (LDPE); a linear low density polyethylene (LLDPE); a polypropylene homopolymer (PPHO); and a polypropylene copolymer (PPCO). As the oxidation progressed, it was observed that the scission rate for HDPE, LLDPE, PPHO and PPCO increased near to the exposed surface whereas for LDPE the rate remained almost unchanged. The crosslink rate fell near to the surface with HDPE and LDPE but increased with PPHO and PPCO. The reaction rates near to the bar centre (∼1.5 mm from the exposed surface) were low for HDPE, PPHO and PPCO; this is attributed to oxygen starvation, caused by consumption of oxygen by rapid reaction near the surface. Reaction was observed in the interior with LDPE and LLDPE, presumably because of a combination of a higher oxygen diffusion rate than for HDPE and a lower rate of consumption of oxygen near the surface than with the polypropylenes.     

Radiation-induced degradation of polyethylene: Role of amorphous region in the formation of oxygenated products and the mechanical properties

Quenched samples of linear low density, medium density and two kinds of high density polyethylene films were irradiated with γ-rays from a ⁶⁰Co source in vacuum and in air at room temperature with irradiation doses ranging from 0 to 100 Mrad. On irradiation in vacuum the extent of crosslinking was about one-and-a-half times greater in the linear low density polyethylene (LLDPE) than in the high density polyethylene (HDPE). On the other hand, irradiation in air produced more crosslinking in high density polyethylene (HDPE). Growth of trans-vinylene unsaturation was found around 10 Mrad in all the samples. Initial increase in elongation and breaking strength (below 5 Mrad) occurred, which was followed by a decrease with increasing dose. LLDPE showed some elongation even at 50 Mrad, while the other samples became brittle and broke at doses far below this value. The mechanism of oxidative degradation is discussed.     

Metallocene polyolefins and their blends#

Commercial copolymers of 1‐octene and ethylene: metallocene catalyzed (mLLDPE) and Ziegler‐Natta catalyzed (znLLDPE), a low density polyethylene (LDPE), and high density polyethylene (HDPE), were characterized with respect to branching, crystallization behaviour and dynamic‐mechanical properties. It was found that the crystallinity of the polymers is more influenced by the homogeneity of the short‐chain branching than by its content. The study of blends of mLLDPE and znLLDPE with LDPE and HDPE showed that the interaction between mLLDPE and LDPE is stronger than between znLLDPE and LDPE. Blends containing mLLDPE showed a composition depending improvement of the storage modulus G' which was not observed in znLLDPE/LDPE blends. The HPDE blends followed a linear mixing rule. Co‐crystallization was found mLLDPE/LDPE and partially in znLLDPE/LDPE and znLLDPE/HDPE blends, respectively.     

Adhesive effect of linear low-density polyethylene gels on polyethylene moldings

Adhesive effect of linear low density polyethylene (LLDPE) gels in organic solvents such as decalin, tetralin, and o-dichlorobenzene on high density polyethylene (HDPE) moldings has been investigated by shearing tests, and DSC measurements. For all of the gels the temperature at which the heated gel starts to exhibit the adhesive effect was about 70 °C, which is similar to the result of LDPE gel. In particular, when heated at 110 °C, LLDPE gel in tetralin showed such a strong bond strength that polyethylene plates of 3 mm in thickness and 20 mm in width gave rise to necking. It was found that LLDPE gel behaved as though it added LDPE gel to HDPE gel namely LDPE-like components in LLDPE resin exerted the adhesive effect at lower heating temperature, HDPE-like components exerted the strong adhesive effect at higher heating temperature.     

Additive‐like amounts of HDPE prevent creep of molten LDPE: Phase‐behavior and thermo‐mechanical properties of a melt‐miscible blend

Low‐density polyethylene (LDPE) is the preferred type of polyolefin for many medical and electrical applications because of its superior purity and cleanliness. However, the inferior thermo‐mechanical properties as compared to, for example, high‐density polyethylene (HDPE), which arise because of the lower melting temperature of LDPE, constitute a significant drawback. Here, we demonstrate that the addition of minute amounts of HDPE to a LDPE resin considerably improves the mechanical integrity above the melting temperature of LDPE. A combination of dynamic mechanical analysis and creep experiments reveals that the addition of as little as 1 to 2 wt% HDPE leads to complete form stability above the melting temperature of LDPE. The investigated LDPE/HDPE blend is found to be miscible in the melt, which facilitates the formation of a solid‐state microstructure that features a fine distribution of HDPE‐rich lamellae. The absence of creep above the melting temperature of LDPE is rationalized with the presence of tie chains and trapped entanglements that connect the few remaining crystallites. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 146–156     

Tear anisotropy in films blown from polyethylenes of different macromolecular architectures

Blown films of different types of polyethylenes, such as branched low‐density polyethylene (LDPE) and linear high‐density polyethylene (HDPE), are well known to tear easily along particular directions: along the film bubble's transverse direction for LDPE and along the machine direction (MD) for HDPE. Depending on the resin characteristics and processing conditions, different structures can form within the film; it is therefore difficult to separate the effects of the crystal structure and orientation on the film tear behavior from the effects of the macromolecular architecture, such as the molecular weight distribution and long‐chain branching. Here we examine LDPE, HDPE, and linear low‐density polyethylene (LLDPE) blown films with similar crystal orientations, as verified by through‐film X‐ray scattering measurements. With these common orientations, LDPE and HDPE films still follow the usual preferred tear directions, whereas LLDPE tears isotropically despite an oriented crystal structure. These differences are attributed to the number densities of the tie molecules, especially along MD, which are considerably greater for linear‐architecture polymers with a substantial fraction of long chains, capable of significant extension in flow. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 413–420, 2005     

HDPE,LLDPE或LDPE高取向薄膜微结构的电子显微学研究

木工作用透射电子显微术及电子衍射技术研究3种PE(HDPE,LLDPE或LDPE)均聚物高取向薄膜的微结构。定量测定了它们的结晶尺寸。通过倾斜样品电子显微学研究确定了不同种PE纤维结构的对称性。     

Polymer blends—I. Mechanical and morphological behaviour of polyethylene/polyvinyl chloride blends

As part of a fundamental study of the behaviour of mixed plastics during reprocessing and in service, blends of low density polyethylene (LDPE) and polyvinyl chloride (PVC) have been investigated. It was found that Young's modulus increased steadily from pure LDPE to pure PVC whereas both tensile strength at break and elongation at break passed through a minimum at about 5% PVC. Optical and scanning electron microscope studies have related this mechanical behaviour to morphological changes in the two phase system under stress.     

等离子体处理云母对填充聚烯烃结构影响的研究

<正> 近年出现的用冷等离子体处理填料的方法被认为是偶联剂技术的一个新发展。它是通过等离子体作用,使填料表面产生有利于与基体树脂相容的结构变化,从而实现改善填充体系使用和加工性能的目的。这种处理方法具有作用强度大而穿透力小、效率高,无公害等特点。与采用硅烷、钛酸酯偶联剂等较成熟的处理方法不同,等离子体处理的研究尚在起步阶段,基本局限于对处理效果的探讨,有关处理对填充体系结构影响的研究尚不多见。本文以经乙烯等离子体处理的云母为填料,初步研究了填料与HDPE、IDPE及PS界面的水扩散特性、界面极化特性,以及基体的松弛转变行为。     

Study of pyrolysis and incineration of disposable plastics using combined TG/FT-IR technique

Thermogravimetric analysis (TG) provides information regarding mass changes in the sample resulting from heat treatment under controlled environment. However, it does not provide any chemical information regarding the gases evolved during the thermal degradation. Using FT-IR spectrometry in combination with TG, it is often possible to identify the evolved gases, and also monitor their evolution profiles during thermal degradation. In this study, we present the TG/FT-IR combined analysis of incineration and pyrolysis of some common plastics such as high density polyethylene (HDPE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polystyrene (PS). This study demonstrates the utility of such combined analysis in providing useful information regarding the use of thermal treatment for recycling or incineration.     

Quadruple‐shape‐memory effect of TPI/LDPE/HDPE composites

Shape‐memory polymers are important smart materials with potential applications in smart textiles, medical devices, and sensors. We prepared trans‐1,4‐polyisoprene, low‐density polyethylene (LDPE), and high‐density polyethylene (HDPE) shape‐memory composites using a simple mechanical blend method. The mechanical, thermal, and shape‐memory properties of the composites were studied. Our results showed that the shape‐memory composites could memorize 3 temporary shapes, as revealed by the presence of broad melting transition peaks in the differential scanning calorimetry curves. In the trans‐1,4‐polyisoprene/LDPE/HDPE composites, the cross‐linked network and the crystallization of the LDPE and HDPE portions can serve as fixed domains, and all crystallizations can act as reversible domains. We proposed a schematic diagram to explain the vital role of the cross‐linked network and the crystallization in the shape‐memory process.     

Resistivity-Temperature Behavior of CB-Filled HDPE Foaming Composites

High-density polyethylene/carbon black foaming conductive composites were prepared from acetylene black（ACEY） and super conductive carbon black（HG-1P） as conductive filler, low-density polyethylene（LDPE） as the second component, ethylene-vinyl acetate（EVA） and ethylene propylene rubber（EPR） as the third component, azobisformamide（AC） as foamer, and dicumyl peroxide（DCP） as cross-linker. The structure and resistivity-temperature behavior of high-density polyethylene（HDPE）/CB foaming conductive composites were investigated. Influences of carbon black, LDPE, EVA, EPR, AC, and DCP on the foaming performance and resistivity-temperature behavior of HDPE/CB foaming conductive composites were also studied. The results reveal that HDPE/CB foaming conductive composite exhibits better switching characteristic; ACET-filled HDPE foaming conductive composite displays better positive temperature coefficient（PYC） effect; whereas super conductive carbon black（HG-1P）-filled HDPE foaming conductive composite shows better negative temperature coefficient（NTC） effect.     

Surface ozonation and photooxidation of polyethylene film

High-density polyethylene (HDPE) and low-density polyethylene (LDPE) films were oxidized by treatment with ozone and by photooxidation with a low-pressure mercury lamp. The changes that resulted in the surfaces of the films were followed by ESCA. On ozonation, the surface of LDPE initially is oxidized more rapidly than that of HDPE; however, extended ozonation produces a surface composition that corresponds to C₈O for HDPE and to C₁₈O for LDPE. The surface oxidation products are mainly carboxyl groups, with lower levels of carbonyl and C? O groups. For both polymers photooxidation provides more extensively oxidized surfaces than ozonation, although the surface of HDPE oxidizes slightly faster than that of LDPE treated under identical conditions. In both cases the surface stoichiometry after extensive photoxidation is C₆O. The functional groups formed are mainly carboxyl and C? O. The effects of ozonation and photooxidation on the polyethylene surfaces are compared with those produced by several other means of surface oxidation.     

Fuels obtained by thermal cracking of individual and mixed polymers

Utilization of oils/waxes obtained from thermal cracking of individual LDPE (low density polyethylene), HDPE (high density polyethylene), LLDPE (linear low density polyethylene), PP (polypropylene), or cracking of mixed polymers PP/LDPE (1: 1 mass ratio), HDPE/LDPE/PP (1: 1: 1 mass ratio), HDPE/LDPE/LLDPE/PP (1: 1: 1: 1 mass ratio) for the production of automotive gasolines and diesel fuels is overviewed. Thermal cracking was carried out in a batch reactor at 450°C in the presence of nitrogen. The principal process products, gaseous and liquid hydrocarbon fractions, are similar to the refinery cracking products. Liquid cracking products are unstable due to the olefins content and their chemical composition and their properties strongly depend on the feed composition. Naphtha and diesel fractions were hydrogenated over a Pd/C catalyst. Bromine numbers of hydrogenated fractions decreased to values from 0.02 g to 6.9 g of Br₂ per 100 g of the sample. Research octane numbers (RON) before the hydrogenation of naphtha fractions were in the range from 80.5 to 93.4. After the hydrogenation of naphtha fractions, RON decreased to values from 61.0 to 93.6. Diesel indexes (DI) for diesel fractions were in the range from 73.7 to 75.6. After the hydrogenation of diesel fractions, DI increased up to 104.9.     

Biofouling and biodegradation of polyolefins in ocean waters

High density polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP) coupons were immersed for a period of 6 months in Bay of Bengal near Chennai Port (Port) and Fisheries Survey of India (FSI). Samples were retrieved every month and the extent of biofouling and biodegradation were monitored by measuring biological and physicochemical parameters. Dissolved oxygen and oxidation reduction potential were higher at Port than at FSI. Total suspended solids and organic matter were more on PP, followed by HDPE and LDPE indicating hydrophobic surfaces favour more biofouling. Pseudomonas sp., anaerobic, heterotrophic and iron-reducing bacteria were observed on polymer surface. Biofouling was found to depend on the season, loading being highest in the month of August. Chlorophyll was higher at FSI than at Port due to higher pollution levels and also being closer to the shores. Maximum weight loss was seen in LDPE (1.5-2.5%), followed by that in HDPE (0.5-0.8%) and finally in PP (0.5-0.6%) samples deployed at Port in the six month time period.     

Degradability of linear polyolefins under natural weathering

High density polyethylene (HDPE), linear low density polyethylene (LLDPE), and isotactic polypropylene (PP) containing antioxidant additives at low or zero levels were extruded and blown moulded as films. An HDPE/LLDPE commercial blend containing a pro-oxidant additive (i.e., an oxo-biodegradable blend) was taken from the market as supermarket bag. These four polyolefin samples were exposed to natural weathering for one year during which their structure and thermal and mechanical properties were monitored. This study shows that the real durability of olefin polymers may be much shorter than centuries, as in less than one year the mechanical properties of all samples decreased virtually to zero, as a consequence of severe oxidative degradation, that resulted in substantial reduction in molar mass accompanied by a significant increase in content of carbonyl groups. PP and the oxo-bio HDPE/LLDPE blend degraded very rapidly, whereas HDPE and LLDPE degraded more slowly, but significantly in a few months. The main factors influencing the degradability were the frequency of tertiary carbon atoms in the chain and the presence of a pro-oxidant additive. The primary (sterically hindered phenol) and secondary (phosphite) antioxidant additives added to PP slowed but did not prevent rapid photo-oxidative degradation, and in HDPE and LLDPE the secondary antioxidant additive had little influence on the rate of abiotic degradation at the concentrations used here.     

Adhesive effect of low-density polyethylene gels on polyethylene moldings

Adhesive effect of low density polyethylene (LDPE) gels in organic solvents such as decalin, tetralin, ando-dichlorobenzene on high density polyethylene (HDPE) moldings has been investigated by shearing tests, electron microscopy, and DSC measurements. When heated at 110°C for 2 h, all of the gels showed strong adhesive strengths around 30 kg/cm², which is sufficiently strong for practical uses. It has been found that the adhesive strength increases with the heating temperature and that the temperature at which the heated gel begins to exhibit the adhesive effect depends upon solvents and is about 30° lower than that of the HDPE gels.