The thermal stabilization of polyethylene sulfide

It has been shown that the resistance of certain polyethylene sulfides to thermal degradation can be markedly improved by the addition of small amounts of metal dithiocarbamates, thioureas, or diphenyl acetylene together with a suitable carbodiimide. The extent of degradation, as indicated by the concurrent changes in polymer melt viscosity in the temperature range 220°–250°C, was dramatically reduced, in particular, by a stabilizer comprised of 4% hexamethylenebis(tert-butyl)carbodiimide and 2% diphenylacetylene. At 250°C polyethylene sulfide (Dabco initiated) was stable in the melt viscometer for a period of 8–10 min and the resulting extrudate showed little sign of discoloration. The detailed mechanism of stabilizer action cannot be put forward at this stage because of uncertainty regarding the mode of polymer degradation which could occur by radical or ionic processes.     

Mechanism and kinetics of oxidation during the thermal stabilization of polyacrylonitrile fibers

The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N₂ and air were investigated using differential scanning calorimetry and solid‐state ¹³C nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (C?O) groups could occur at 160°C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

硬脂酸铁合成工艺改进研究

搅拌下将硬脂酸加入浓度1%的氢氧化钠溶液中,pH 9~10反应20~25 m in,得到硬脂酸钠溶液。将3%三氯化铁溶液加入硬脂酸钠溶液中,70℃条件下反应至pH 3~4。硬脂酸铁以沉淀析出,纯度95.6%~97.1%,收率达94.6%~95.1%。对硬脂酸铁合成工艺改进后产品收率和纯度均有提高。     

Effects of 6-anilino-1,3,5-triazine-2,4-dithiol,zinc stearate,and barium stearate on the thermal stabilization of PVC

Individual action and synergistic effect in the combination of 6-anilino-1,3,5-triazine-2,4-dithiol (AF), zinc stearate, and barium stearate on the color stabilization of PVC were investigated. In this system, AF selectively reacts with allylic chlorine atoms in PVC. Consequently, unstable allylic chlorine units were converted to thermally stable allylic structures, thus retarding the development of polyene sequences. Zinc stearate accelerated the reaction of AF with allylic chlorine atoms in PVC, forming the zinc salts of AF (AFZ_nSt, St?C₁–H₃₅COO? ) by reacting with AF. Barium stearate reacted with ZnCl₂ which is formed in the above reaction to give St₂Zn and BaCl₂. Consequently, barium stearate led to the selective reaction of AF with allylic chlorine atoms in PVC and the remarkable retarding effect of discoloration of PVC.     

Factors influencing the thermal oxidation of polyethylene

The thermal stability of polyethylene containing conventional antioxidants is adversely affected by contact with copper, by certain pigments, and by the addition of a few percent of polypropylene as a processing aid. Polyethylene inhibited with 0.1% of phenolic antioxidants has approximately the same oxidative stability when in contact with a copper surface as the unprotected polymer. A mechanism is suggested to account for the loss of stability in the presence of copper. Pigments vary in the extent to which they adversely affect the oxidative stability of polyethylene. Since several factors may combine to decrease the stability of protected polyethylene compositions, as determined by accelerated tests, it is essential that their contribution under service conditions be determined. The predicted life at temperatures encountered in service is determined by extrapolating accelerated test data to that constant temperature which is calculated to cause the same degree of degradation as would occur during the daily and seasonal temperature cycles encountered in use.     

多乙烯多胺桥联受阻酚类抗氧剂的合成及其清除DPPH·性能

以二乙烯三胺和三乙烯四胺为桥联基,β-(3,5-二叔丁基-4-羟基苯基)丙酰氯为抗氧化功能基团,通过酰胺化缩合反应合成了两类具有不同对位桥联基团的受阻酚类抗氧剂。采用傅里叶红外光谱和核磁共振氢谱证实了合成的多乙烯多胺桥联受阻酚类抗氧剂的化学结构。DPPH法研究了多乙烯多胺桥联受阻酚类抗氧剂清除自由基的性能,并探索了酚羟基个数和对位桥联基结构对受阻酚类抗氧剂清除自由基性能的影响。结果表明,多乙烯多胺桥联受阻酚类抗氧剂具有良好的清除DPPH·能力,且随着抗氧剂分子中酚羟基个数的增加,清除DPPH·的活性增加,分子中含有4个酚羟基的三乙烯四胺受阻酚类抗氧剂的抗氧化效率（AE）达到2.65×10^-2 L/(mol·s)。对位桥联基结构对受阻酚类抗氧剂清除DPPH·能力有较大影响,季戊四醇为桥联基的受阻酚类抗氧剂1010清除DPPH·能力最强,其抗氧化效率（AE）为3.08×10^-2L/(mol·s);乙二胺为核的1.0代树枝状受阻酚类抗氧剂清除DPPH·能力最弱,其抗氧化效率（AE）为2.60×10^-2 L/(mol·s)。     

Ascorbate and phenolic antioxidant interactions in prevention of liposomal oxidation

Efficient prevention of membrane lipid peroxidation by vitamin E (α-tocopherol) may involve its regeneration by vitamin C (ascorbate). Conceivably, the efficacy of antioxidants designed as therapeutic agents could be enhanced if a similar regeneration were favorable; thus, a model membrane system was developed which allowed assessment of interaction of phenolic antioxidants with ascorbate and ascorbyl-6-palmitate. Ascorbate alone (50–200 μM) potentiated oxidation of soybean phosphatidylcholine liposomes by Fe²⁺/histidine-Fe³⁺, an effect which was temporally related to reduction of Fe³⁺ generated during oxidation. Addition of 200 μM ascorbate to α-tocopherol-containing liposomes (0.1 mol%) resulted in marked, synergistic protection. Accordingly, in the presence but not absence of ascorbate, α-tocopherol levels were maintained relatively constant during Fe²⁺/histidine-Fe³⁺ exposure. Probucol (4,4′-[(1-methylethylidine)bis(thio)]bis[2,6-bis(1,1-dimethylethyl)]phenol), and antioxidant which prevents oxidation of low density lipoproteins, and its analogues MDL 27,968 (4,4′-[(1-methylethylidene)bis(thio)]-bis[2,6-dimethyl]phenol) and MDL 28,881 (2,6-bis(1,1-dimethylethyl)-4-[(3,7,11-trimethyldodecyl)thio]phenol) prevented oxidation but exhibited no synergy with ascorbate. Ascorbyl-6-palmitate itself was an effective antioxidant but did not interact synergistically with any of the phenolic antioxidants. Differential scanning calorimetry revealed significant differences among the antioxidants in their effect on the liquid-crystalline phase transition of dipalmitoyl phosphatidylcholine (DPPC) liposomes. Both α-tocopherol and MDL 27,968 significantly reduced the phase transition temperature and the enthalpy of the transition. MDL 28,881 had no effect while probucol was intermediate. The potential for ascorbate or its analogues to interact with phenolic antioxidants to provide a more effective antioxidant system appears to be dictated by structural features and by the location of the antioxidants in the membrane.     

Influence of thermal oxidation on surface and thermo-mechanical properties of polyethylene

Blown low density polyethylene (LDPE) films added with oxo-biodegradable additive containing pro-oxidant were subjected to thermo-oxidation in an oven at 70 °C for varying time periods. The changes in mechanical and surface properties were studied using tensile strength, elongation at break, wettability, surface morphology using SEM, surface topology by AFM, functional groups by FTIR spectroscopy, absorbance spectra by UV-Vis spectroscopy. The thermally aged films with pro-oxidant additive added polyethylene (PE) exhibited a higher level of oxidation as revealed by increase in their carbonyl index than the pristine PE. In addition to it, the DSC melting behavior showed a slight increase in crystallinity (%C) and melting temperature (Tm) of the polymer containing oxo-biodegradable additive. The SEM micrograph reveals the increase in the size of cracks and grooves, according to the increase in the concentration of pro-oxidant additive. The surface roughness also supports the same through AFM image. The increase in the wettability and surface free energy of the thermally aged samples were attributed to the formation of hydrophilic groups on the polymer surface by thermo-oxidation. The pro-oxidant additive offers a new insight into the thermo-oxidation strategy for PE and can be widely used in packaging industry.     

Additive interactions in the stabilization of film grade high-density polyethylene. Part I: Stabilization and influence of zinc stearate during melt processing

The melt stabilization activity of some of the most commercially significant phenolic antioxidants and phosphites (alone and in combination), without and with zinc stearate, was studied in high-density polyethylene (HDPE) produced by Phillips catalyst technology. Multiple pass extrusion experiments were used to degrade the polymer melt progressively. The effect of stabilizers was assessed via melt flow rate (MFR) and yellowness index (YI) measurements conducted as a function of the number of passes. The level of the phenolic antioxidant remaining after each extrusion was determined by high-performance liquid chromatography (HPLC). Phenolic antioxidants and phosphites both improved the melt stability of the polymer in terms of elt viscosity retention; the influence of zinc stearate was found to be almost insignificant. However, phosphites and zinc stearate decreased the discoloration caused by the phenolic antioxidants. A correlation was found between the melt stabilization performance of phosphites and their hydroperoxide decomposition efficiency determind via a model hydroperoxide compound. Steric and electronic effects associated with the phosphorus atom influenced the reactivity towards hydroperoxides. Furthermore, high hydrolytic stability did not automatically result in lower efficiency. Besides phosphite molecular structure, stabilization activity was also influenced by the structure of the primary phenolic antioxidant and the presence of zinc stearate.     

Stability of carotenoids, phenolic compounds, ascorbic acid and antioxidant capacity of tomatoes during thermal processing

In the present investigation we ascertained the stability of lycopene, beta-carotene, ascorbic acid, polyphenolic compounds and total antioxidant capacity (AC) during the process of concentrating tomatoes into two tomato pastes (10 and 15 degrees Brix). Thermal processing increased the content of lycopene, total phenolic compounds, total flavonoids, and the individual phenolic compounds quercetin, rutin, chlorogenic and cafeic acids, whereas it decreased the other analysed compounds. However, lycopene in the 15 degrees Brix-tomato paste decreased due to the extension of thermal processing, which led to degradation. The AC of aqueous and organic extracts was measured and different AC values were observed depending on the antioxidant profile of the extract and assay used (TEAC and FRAP). AC expressed in dry matter decreased as result of ascorbic acid losses. Overall, thermal processing enhanced the nutritional value of tomatoes, mainly by increasing the lycopene and phenolic antioxidants, but the extension of treatment must be controlled to prevent lycopene degradation.     

Synergistic thermal stabilization effect of polyamide/melamine on polyoxymethylene

The synergistic thermal stabilization effect of polyamide (PA) and melamine (MA) on polyoxymethylene (POM) was studied by isothermal weight loss analysis and nonisothermal thermogravimetric analysis, which showed that the complex stabilizer PA/MA was more efficient than the single‐use formaldehyde absorbents MA or PA in improving the thermal stability of POM. The nonisothermal degradation kinetics study further demonstrated the synergistic thermal stabilization effect of PA/MA on POM. The mechanical property investigation showed that the addition of a proper amount of PA or the proper reduction of MA content improved the impact strength of POM. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2387–2391, 2005     

Effect of metal stearate stabilizers on the thermal degradation of PVC in solution: The reversible blocking mechanism of stabilization

Systematic thermal degradation studies of PVC solutions have been carried out in the presence of different metal carboxylates (Pb-, Cd-, Ba-, Ca- and Zn- stearates). ZnSt₂ markedly accelerates degradation of the polymer. Significant induction periods (t_i) for the appearance of free HCI are obtained in the presence of the other salts. CaSt₂ acts mainly as an HCI-scavenger and has no direct influence on the elimination process. Mainly short polyenes are formed during the induction period when PbSt₂, CdSt₂, and BaSt₂ are used. After consumption of stabilizers rapid HCI loss occurs, and suddenly longer polyenes are formed, i.e., the amount of double bonds and the average length of polyenes sharply increases. At the same time, the rate of initiation (i.e., the rate of the polyene sequence formation) does not change at all around t_i. None of the investigated stabilizers retard effectively the random initiation of HCI loss. In the presence of PbSt₂, BaSt₂, and CaSt₂, and CaSt₂, the initiation rate is nearly identical to that of the unstabilized PVC. The concentration of labile sites (h₀) in PVC at the end of the induction period has been estimated by kinetic analysis. It has been found that in some cases h₀ is higher than the concentration of labile structures in the virgin PVC. These results indicate that the main role of metal soap stabilizers is the blocking of the fast zip-elimination of HCI from the PVC chain. It is likely that blocking occurs by attachment of a carboxylate group at the end of a propagating zip. This is a reversible process: the blocked structures become active again mainly after the consumption of stabilizers presumably by HCI-catalyzed hydrolysis. In contrast to other stabilization mechanisms by Frye and Horst, Minsker and coworkers, and Michell the reversible blocking mechanism of PVC stabilization is able to explain the experimental findings presented in this study. It also explains such facts of practical importance as color stability of the resin during the induction period and fast blackening after the consumption of stabilizers.     

Effect of metal stearate antacid on the melt stabilization performance of phenolic/phosphite antioxidants in metallocene LLDPE. Part 2: Discoloration

The first part of this series of papers indicated that the melt stabilization performance of phenol/phosphite/metal stearate combinations in metallocene LLDPE (mLLDPE) was influenced by the purity of the fatty acid salt. In this second part, it is shown that the quality of the metal stearate has a direct influence on the discoloration of the aforementioned formulations in the melt state. On the basis of the peroxide and oxide impurity levels in the metal stearates (Part 1), it is suggested that discoloration is directly related to the formation of oxidative transformation products of the phenolic antioxidant. Furthermore, it was observed that the calcium stearates discolored upon aging in air at processing temperatures, while zinc stearates did not. Such discoloration is believed to be partly due to the combination of high levels of conjugated systems consisting of unsaturation and carbonyl groups formed during aging. J. Vinyl Addit. Technol. 10:144–148, 2004. © 2004 Society of Plastics Engineers.     

Identification of volatile compounds resulting from the thermal oxidation of polyethylene

The migration of compounds from polymer-based packaging may impart undesirable odors to foods. We, therefore, undertook a study of the volatile compounds produced during the heating of polyethylene (PE) in the presence of excess O₂ at temperatures of 150–350°C and for heating times of 5–15 min. Eightyfour volatile compounds in the range of C₅—C₂₃ were identified by gas chromatography mass spectrometry. The major products were aliphatic hydrocarbons, aldehydes, ketones, and olefins. Changes in temperature and heating times affected the amount and type of compounds produced, with hexanal being found in the largest amount and 300°C resulting in the greatest quantity of volatile compounds. At 350°C, greater amounts and numbers of low-boiling and fewer high-boiling compounds were formed. Only small amounts of volatiles were produced at 150°C. Many of the compounds identified have been reported to have odor and/or toxicological significance. © 1993 John Wiley & Sons, Inc.     

抗氧剂MBP在ABS接枝共聚物中的稳定作用

在采用乳液聚合方法制备丙烯腈-丁二烯-苯乙烯（ABS）接枝共聚物时,将受阻酚类抗氧剂2,2′-亚甲基双（4-乙基-6-叔丁基苯酚）（MBP）以乳液的形式加到ABS胶乳中。通过对粉料进行动态和静态DSC分析和TG分析,测定粉料的拉伸性能和老化后的ABS接枝共聚物的黄色指数的变化等对不同含量的抗氧剂MBP在ABS中的稳定作用进行了研究。结果表明：当w（MBP）=0.1%时,即能对ABS接枝共聚物起到较好的保护作用;w（MBP）=1.5%的ABS接枝共聚物的断裂伸长率下降速度最快;其中当w（MBP）=1%时,ABS接枝共聚物的黄色指数在整个实验范围内最高。     

酚醛树脂热降解过程中的结构变化

采用综合热分析对比研究了自制酚醛树脂和商业酚醛树脂的热降解过程,利用固体核磁共振和红外光谱技术研究了热降解过程中树脂结构的变化规律以期指导成炭率高、热稳定性高的新型酚醛树脂的合成。结果表明:酚醛树脂的热降解过程可分为3个阶段:首先是醚键以及未反应的羟甲基等端基的热降解;其次是亚甲基热解断裂为甲基;然后是酚羟基发生脱水环化成炭。其中亚甲基的热解对酚醛树脂的热稳定性及高温下的成炭性能有着至关重要的作用。固体核磁共振比红外光谱更直接,更清晰地反应出酚醛树脂在热降解过程中的结构变化。     

硬脂酸盐热稳定剂对胶乳法氯化橡胶热稳定性的影响

用多种分析方法探讨硬脂酸盐热稳定剂对胶乳法氯化橡胶(CNRA)热稳定性的影响。结果表明．硬脂酸盐改善CNRA热空气老化变色性能的优劣顺序为：硬脂酸铅、硬脂酸钙、硬脂酸钡、硬脂酸镉、硬脂酸锌；在一定范围内．硬脂酸盐质量分数增大，CNRA的变色程度降低；硬脂酸铅盐并用体系抑制氯化氢生成能力的大小顺序为：硬脂酸铅／硬脂酸镉／硬脂酸钡(并用比l：0．5：1．5)并用体系、硬脂酸镉／硬脂酸钡(并用比l：3)并用体系、硬脂酸铅／硬脂酸锌／硬脂酸钙(并用比l：0．4：1．6)并用体系、硬脂酸锌/硬脂酸钙(并用比1：4)并用体系。