结晶度对高密度聚乙烯光氧老化特性的影响

利用力学试验、衰减全反射红外光谱技术(ATR-FTIR)、热重分析法(TG)、差示扫描量热法(DSC)、凝胶渗透色谱(GPC)、扫描电子显微镜(SEM)比较研究了3种不同结晶度高密度聚乙烯(HDPE)的光氧老化特性,分析了结晶度对HDPE力学性能、化学结构、热稳定性、熔融特性、分子量和表面微观形貌的影响规律。结果表明结晶度越高,HDPE弯曲强度、弯曲模量和冲击强度下降最明显;不饱和度增长越剧烈,支化作用和断链作用更明显,且区别主要集中于老化初期;羰基指数和羟基指数增长越快,热分解特征温度下降最明显,热稳定性下降更强烈,但熔融特征温度和氧化诱导温度下降更不明显,相对分子质量下降更强烈,表面破坏更严重,老化作用更剧烈。结晶度越高,HDPE缺陷也就越多,在热氧环境中越容易发生氧化,老化现象更严重。     

在役埋地聚乙烯给水管道自然老化行为研究

研究了埋地聚乙烯(PE)给水管道在我国台州地区实际服役7～12年过程中的自然老化行为,采用红外光谱仪(FTIR)、差示扫描量热仪(DSC)、扫描电子显微镜(SEM)等分析了管材在服役过程中不饱和基团含量、断链和支化程度、羰基指数、羟基指数等微观结构的变化趋势,考察了不同服役时间管材热稳定性、分子量、氧化诱导时间(OIT...     

顺丁生胶热氧化和光氧化的研究

用红外吸收光谱分析法、动态力学性质测试法和热分析法研究了国产顺丁生胶的热氧化和光氧化。发现在光或热的作用下都会出现顺式—1·4结构转变为反式—1·4结构的异构化现象。检测到伴随断链效应生成的种类繁多的含羟基或羰基的氧化产物,而光、热氧化两种过程中所生成的氧化物的种类一样,但浓度分布不同。顺丁胶热氧化初期断链效应占优势,后期交联效应占主导,因此,它的动态切变模量先下降后陡升。醚式氧桥是其交联键的一种形式,它使顺丁胶的热稳定性下降。以相对切变模量、羟基指数和羰基指数为指标,鉴定了18种稳定剂在顺丁胶中的抗光、热氧化的效能。     

聚乙烯膜降解的红外光谱定量表征方法

本文对聚乙烯光降解膜在老化过程中所产生的羰基等含氧基团,找出一种表征光氧老化程度的红外光谱(IR)分析方法。即以酮式羰基1720cm~(-1)作为定量分析的特征峰,选择CaCO_(3)1794cm~(-1)谱带作为参比峰,计算各谱带在不同降解时间的吸光度比(羰基指数),表征薄膜光氧老化程度。     

低密度聚乙烯热氧老化特性

采用衰减全反射傅里叶红外光谱(ATR-FTIR)、差示扫描量热法(DSC)、连续自成核退火热分级技术(SSA)和力学测试等手段研究了人工加速热氧老化环境条件下低密度聚乙烯(LDPE)微观结构和宏观性能的变化。结果表明:随着老化时间的延长,分子结构产生羰基、羟基等含氧基团,通过ATR-FTIR测得的结晶度在老化期间由于断链的影响,表现出先增加后减小的趋势,熔融特征温度和氧化诱导温度变化不明显;拉伸强度和拉伸模量呈现下降趋势。亚甲基结晶序列长度(MSL)为98的片段先增长后急剧减少,先增长主要是由于链段中烷基自由基的重组,这将导致分支的减少和更长亚甲基结晶序列长度片段的形成,而断链和包括系带分子的分子链的氧化导致了亚甲基结晶序列长度为98的片段随后的减少。     

无规共聚聚丙烯DSC原位加速老化研究

对无规共聚聚丙烯(PPR)进行差示扫描量热(DSC)原位热氧加速老化特性研究,分析了老化温度对PPR熔点和热稳定性能的影响,通过傅里叶变换红外光谱法(FTIR)探究了老化温度与PPR分子结构变化之间的关系。结果表明,随着DSC热氧老化温度的上升,样品的老化作用逐渐加剧,对结晶结构的破坏程度增大。当热氧老化处理温度高于200℃时,样品的老化以氧化作用为主,且随热氧老化温度的升高,样品分子链的支化和断链程度加剧,同时伴随着交联现象,总体氧化稳定性变差。     

分子量分布对低密度聚乙烯光氧老化特性的影响

采用衰减全反射红外光谱技术（ATR-FTIR）、热重分析法（TG）、凝胶渗透色谱（GPC）、扫描电子显微镜（SEM）和力学试验比较研究了不同分子量分布指数低密度聚乙烯（LDPE）的光氧老化特性,分析了分子量分布对LDPE化学结构、热稳定性、平均分子量、表面微观形貌和力学性能的影响规律。结果表明分子量分布越宽,LDPE不饱和度增长越剧烈,支化作用增长越显著;分子量分布越窄,羰基指数增长越快;分子量分布对于分子结构的断链行为并无影响。分子量分布指数越大,LDPE起始热分解温度和失重5%对应温度下降更快,热稳定性更容易变差,平均分子量下降更多,表面微观形貌老化现象越严重;弯曲强度和冲击强度受影响更显著,指数为6.0的LDPE老化24 d冲击强度就已丧失。分析认为,分子量越大、分布越窄表明分子链越长、短分子链越少,与氧接触而产生自由基的概率也越小,因此聚乙烯分子量分布越宽,材料越容易老化。     

分子量分布对低密度聚乙烯光氧老化特性的影响

采用衰减全反射红外光谱技术(ATR-FTIR)、热重分析法(TG)、凝胶渗透色谱(GPC)、扫描电子显微镜(SEM)和力学试验比较研究了不同分子量分布指数低密度聚乙烯(LDPE)的光氧老化特性,分析了分子量分布对LDPE化学结构、热稳定性、平均分子量、表面微观形貌和力学性能的影响规律。结果表明分子量分布越宽,LDPE不饱和度增长越剧烈,支化作用增长越显著;分子量分布越窄,羰基指数增长越快;分子量分布对于分子结构的断链行为并无影响。分子量分布指数越大,LDPE起始热分解温度和失重5%对应温度下降更快,热稳定性更容易变差,平均分子量下降更多,表面微观形貌老化现象越严重;弯曲强度和冲击强度受影响更显著,指数为6.0的LDPE老化24 d冲击强度就已丧失。分析认为,分子量越大、分布越窄表明分子链越长、短分子链越少,与氧接触而产生自由基的概率也越小,因此聚乙烯分子量分布越宽,材料越容易老化。     

聚合物材料表层的老化过程

本文用不稳态动力学理论并考虑到老化过程是从材料表面向内部扩展,对材料在紫外光和扩散氧同时作用下而产生的聚合物老化过程进行了理论分析。假设聚合物吸收了其特征吸收带附近的紫外光而使聚合物分子活化,产生解聚和光氧化反应而导致老化并以光化学方式进行下去。研究表明,在一定条件下,老化深度的增长约与曝露时间的平方根成正比。这种t~1/2(抛物线)规律在以前研究混凝土的中性化和金属的氧化过程时也观察到。该理论研究指出,抛物线规律应该也适用于聚合物材料的老化。许多聚合物材料在老化过程表现出与曝露时间成指数规律(t~n,n=0.5～1.0),这种与理论的差别在于复杂的老化机理,包括了解聚和光氧化反应。聚合物材料抗弯强度随时间而降低可解释为受破坏的表面层使材料强度降低。     

XPS研究不饱和聚酯人工加速老化行为

对不饱和聚酯试样进行实验室人工加速老化,利用X射线光电子能谱(XPS)探讨其表面的化学结构变化,分析其老化机理.结果表明,材料经热氧老化,主链受氧气攻击生成烷氧自由基,烷氧自由基进一步形成C-O和CO,而CO经过再氧化生成OCO,热氧老化后不饱和聚酯的表面基团主要为C-O和OCO;材料经氙灯老化,前期的老化基团变化与热氧老化相似,后期变化主要以OCO基团的降解为主,并生成更多的自由基,引发大分子链的进一步降解;光是不饱和聚酯材料老化降解的主要因素.     

镍系顺丁硫化胶氧化链断裂行为的解析

在热氧老化过程中,镍系顺丁硫化胶的链断裂特征随硫化体系的不同而异。连续化学松弛不能用单一的Maxwell松弛方程描述。比较连续与间歇化学松弛曲线,发现链断裂过程伴随着交联反应产生,且交联反应速度是逐渐增加的。     

Competition between chain scission and branching formation in the processing of high‐density polyethylene: Effect of processing parameters and of stabilizers

Two samples of high‐density polyethylene with different molecular weight were processed in a batch mixer and the rheological and structural properties were investigated. In particular, the effect of different processing parameters and the eventual presence of different stabilizers were evaluated. Actually, two reactions may occur during processing: branching/crosslinking or chain scission. The results indicate that when the processing conditions promote a scarce mobility of the macromolecular chains (lower temperatures, lower mixing speed, and higher molecular weight), branching is more favored than chain scission. On increasing the mobility of the chain (higher temperature, higher mixing speed, and lower initial molecular weight), branching appears still the predominant reaction, but the chain scission becomes progressively more important. In both cases, no crosslinking occurs. The presence of stabilizers has different effects on the relative extent of branching or chain scission reactions depending on the kind and of the amount of stabilizer and on the processing condition adopted. The correct choice of a stabilizer system will, therefore, depend on the desired final structure. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Modeling of branching density and branching distribution in low-density polyethylene polymerization

Low-density polyethylene (ldPE) is a general purpose polymer with various applications. By this reason, many publications can be found on the ldPE polymerization modeling. However, scission reaction and branching distribution are only recently considered in the modeling studies due to difficulties in measurement and computation of scission effect and branchings of polymer. Our previous papers [Kim, D.M., et al., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of CSTR. Chemical Engineering Science 59, 699-718; Kim, D.M., Iedema, P.D., 2004. Molecular weight distribution modeling in low-density polyethylene polymerization; impact of scission mechanisms in the case of a tubular reactor. Chemical Engineering Science, submitted for publication] are concerned with the scission reaction during ldPE polymerization and its effect on molecular weight distribution (MWD) of ldPE for various reactor types. Here we consider branching distributions as a function of chain length for CSTR and tubular reactor processes. To simultaneously deal with chain length and branching distributions, the concept of pseudo-distributions is used, meaning that branching distributions are described by their main moments. The computation results are compared with properties of ldPE samples from a CSTR and a tubular reactor. Number and weight average branchings and branching density increase as chain length increases until the longest chain length. The concentrations of long chain branching (LCB) are close to those of first branching moment in both CSTR and tubular reactor systems. The branching dispersity, a measure for the width of the branching distribution at a certain chain length, has the highest value at shorter chain length and then monotonously decreases approaching to 1.0 as chain length increases. Excellent agreements in branching dispersities between calculation with branching moments and prediction with assumption of binomial distribution for a tubular reactor and CSTR processes show that the branching distribution follows a binomial distribution for both processes.     

Sensitized photodegradation of polyisobutylene film by addition of tris(α-thiopicolinanilide)–cobalt(III)

The photodegradation of polyisobutylene (PIB) film in air at a temperature where volatile formation is negligible was studied by means of light scattering, chemical actiometry, and spectrophotometric techniques. The degradation is accelerated by addition of tris(α-thiopicolinanilide)—cobalt(III) (TPAC). The sensitization and the course of the degradation were determined by weight-average molecular weight, energy of activation, and quantum yield of the photolysis of the polymer film with 254-nm light. The plots of molecular weight, weight-average chain scission, and degree of degradation vs. irradiation time are linear and confirm the random nature of chain scission of the polymer. The unsaturation produced is proportional to the time of irradiation. Ultraviolet and infrared absorption spectra have been employed to substantiate a mechanism of the degradation process which does not involve hydrogen abstraction from the polymer, but direct cleage of the polymer backbone and addition of initiating radicals of TPAC at the sites of scission.     

Mechanochemical process in cotton cellulose fiber

Mechanical processing of cotton cellulose by means of a fiber cutter resulted in the disaggregation and defiberation of fiber bundles, shortening of fiber length, and loss of degree of polymerization. It is evident that the mechanical energy supplied by shear forces is sufficient to cause homolytic scission of cellulose main chains. Mechanoradicals formed in the interim were verified by ESR studies. The crystallinity of cellulose was not influenced by mechanical treatments, but accessibility of the polymer was substantially increased due to the creation of new surfaces. The mechanically treated fiber inclined to proceeded oxidative chain reaction during aging. Regardless of their treatment conditions, cellulose fiber reached its limiting lower molecular weight after 100 days of aging.     

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen

The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.     

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. III. Structural changes occuring in low-density polyethylene at oxygen contents below 1.2%

Samples of low-density polyethylene, free from additives, were kept at temperatures between 284° and 355°C under nitrogen containing 1.16% oxygen or less. Changes in molecular weight distribution (MWD) and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). Both chain scission and molecular enlargement occur simultaneously. Chain scission accounts for the formation of low molecular weight material and volatiles. Molecular enlargement reactions cause an increase in LCB and ultimately the formation of insoluble material. At lower temperatures (284°C) an increase in the high molecular weight end of the MWD is observed. The amount of olefinic unsaturation, carbonyl, and ether groups increase with degradation. Conjugated systems are formed. The formation of thin discolored and insoluble surface layers indicate that the attack of oxygen is diffusion controlled. The DSC thermograms undergo large changes at 333° and 355°C, increasing with time and oxygen content. A reaction scheme for the thermo-oxidative degradation of polyethylene is discussed. Both inter- and intramolecular hydrogen abstractions by peroxy radicals are suggested to occur. Thus, the formation of trans-vinylene and ether groups results from intramolecular abstraction, while internal carbonyl groups are formed by intermolecular abstraction. Chain scission will be accomplished by both routes and together with “back-biting” is suggested to accoun for the formation of volatiles. The formation of conjugated sequences causing discoloration is correlated with the formation of trans-vinylene groups. Because of the restricted accessability of oxygen under our conditions, the reactions discussed previously for pure thermal degradation¹ are also considered to be important. The molecular enlargement observed is thus proposed to be mainly due to the combination of alkyl radicals even when oxygen is present.     

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. III. Structural changes occurring in low-density polyethylene at oxygen contents below 1.2%

Samples of low-density polyethylene, free from additives, were kept at temperatures between 284° and 355°C under nitrogen containing 1.16% oxygen or less. Changes in molecular weight distribution (MWD) and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). Both chain scission and molecular enlargement occur simultaneously. Chain scission accounts for the formation of low molecular weight material and volatiles. Molecular enlargement reactions cause an increase in LCB and ultimately the formation of insoluble material. At lower temperatures (284°C) an increase in the high molecular weight end of the MWD is observed. The amount of olefinic unsaturation, carbonyl, and ether groups increase with degradation. Conjugated systems are formed. The formation of thin discolored and insoluble surface layers indicate that the attack of oxygen is diffusion controlled. The DSC thermograms undergo large changes at 3333° and 355°C, increasing with time and oxygen content. A reaction scheme for the thermo-oxidative degradation of polyethylene is discussed. Both inter- and intramolecular hydrogen abstractions by peroxy radicals are suggested to occur. Thus, the formation of trans-vinylene and ether groups results from intramolecular abstraction, while internal carbonyl groups are formed by intermolecular abstraction. Chain scission will be accomplished by both routes and together with “back-biting” is suggested to account for the formation of volatiles. The formation of conjugated sequences causing discoloration is correlated with the formation of trans-vinylene groups. Because of the restricted accessability of oxygen under our conditions, the reactions discussed previously for pure thermal degradation¹ are also considered to be important. The molecular enlargement observed is thus proposed to be mainly due to the combination of alkyl radicals even when oxygen is present.