Reaction mechanism and rheological properties of polypropylene irradiated under various atmospheres

It is well-known that the melt-strength properties of a polymer increases with molecular weight and with long chain branching due to the increase in the entanglement level. This study is a contribution for the understanding of the following points: — the role of branching, crosslinking and degradation on melt strength properties; — the mechanism and the kinetics of PP irradiation with time of irradiation and the importance of double bond formation.

The results showed that degradation was the major reaction in the initial step of irradiation no matter the atmosphere and or antioxidant. However, double bond formation increased the production of branching and crosslinking reactions. Double bond formation had no effect on the crystallization kinetics, on the other hand, long chain branching had a marked effect on the crystallization temperature.     

High-energy radiation forming chain scission and branching in polypropylene

The degradation of high molecular weight isotactic polypropylene (iPP) subjected to gamma rays irradiation up to 100 kGy in inert atmosphere was analyzed. The investigation relied upon complex viscosity, elastic modulus, gel fraction, morphology of the insoluble fraction and deconvoluted molecular weight distribution (MWD) curves. At low irradiation doses, already at 5 kGy, the MWD curve is strongly shifted to the low molecular weight side showing chain scission, which is confirmed using the calculated chain scission distribution function (CSDF). At high dose levels, the appearance of a shoulder in the high molecular weight side of the MWD curve indicates the formation of chain branching. The presence of a considerable insoluble fraction at these high dose levels indicates also the formation of cross-linking, which has different morphology then the insoluble fraction present in the original iPP. The rheological results show changes in the molecular structure of irradiated samples in agreement with the gel content data. The chromatographic and rheological data has shown that gamma irradiation of iPP produces chain scission, branching and cross-linking.     

Determination of three-dimensional solubility parameters of chlorinated polypropylene by ultrasonic degradation

The degradation reactions of chlorinated polypropylene (CPP) in toluene under ultrasonic irradiation were studied. The Mark–Houwink equation acquired from fractional precipitations was also suitable for estimating the molecular weight of degraded CPP. An objective standard was proposed for judging the solution behaviour of CPP in solvents by the study on the relative solubility of CPP before and after degradation. Then Hansen three-dimensional solubility parameters and the total parameter of CPP were obtained by optimization calculation in terms of the criterion proposed here. It was proved that the total parameter of CPP is creditable by the turbidity method. Compared to other standards, the result obtained from the proposed standard accorded well with the standard of complete miscibility suggested by Flory–Huggins for polymers and solvents as well as the objective reality. This standard may provide a reference for other polymers.     

Comparison of rheological behavior of branched polypropylene prepared by chemical modification and electron beam irradiation under air and N2

Chemical and electron beam irradiation methods were used to introduce a branched structure into polypropylene and propylene–ethylene copolymer. The chemical method was carried out in an internal mixer using initiator and TMPTMA monomer. In irradiation method, the polymer was irradiated by electron beam under air and nitrogen atmosphere. The branched structure in the modified polymer was confirmed by rheological measurements. While degradation was significant in chemical method, branching occurred efficiently by irradiation under air. Small amount of ethylene in the propylene copolymer promoted branching over degradation.     

Processability improvement of polyolefins through radiation-induced branching

Radiation-induced long-chain branching for the purpose of improving melt strength and hence the processability of polypropylene (PP) and polyethylene (PE) is reviewed. Long-chain branching without significant gel content can be created by low dose irradiation of PP or PE under different atmospheres, with or without multifunctional branching promoters. The creation of long-chain branching generally leads to improvement of melt strength, which in turn may be translated into processability improvement for specific applications in which melt strength plays an important role. In this paper, the changes of the melt flow rate and the melt strength of the irradiated polymer and the relationship between long-chain branching and melt strength are reviewed. The effects of the atmosphere and the branching promoter on long-chain branching vs. degradation are discussed. The benefits of improved melt strength on the processability, e.g., sag resistance and strain hardening, are illustrated. The implications on practical polymer processing applications such as foams and films are also discussed.     

The effect of electron beam irradiation on dynamic shear rheological behavior of a poly (propylene‐co‐ethylene) heterophasic copolymer

In this study the effect of electron beam irradiation on rheological properties of a poly (propylene‐co‐ethylene) heterophasic copolymer is evaluated. Using dynamic viscoelastic measurement in the linear viscoelastic range of deformation, it is observed that the complex viscosity and dynamic modulus of polypropylenes were decreased by increasing the irradiation dose. Polypropylene heterophasic copolymers consist of ethylene propylene rubber phase dispersed in polypropylene homopolymer matrix. The high energy electron beams simultaneously affect both isotactic polypropylene (iPP) matrix and ethylene propylene dispersed phase. The molecular chains of polypropylene homopolymer phase breakdown to smaller species, those are prone to degradation and branching as well. Increase in the melt flow rate behavior and shifting the cross‐over point to higher frequencies and increase in melt strength are due to this phenomenon. At the same time, the ethylene propylene phase of the polypropylene copolymer cross‐links due to irradiation, and a significant effect on the rheological behavior of samples are observed. The mathematical modeling of complex viscosity behavior revealed the conformity of experimental data with modified Carreau equation. Copyright © 2010 John Wiley & Sons, Ltd.     

Emission of possible odourous low molecular weight compounds in recycled biofibre/polypropylene composites monitored by head-space SPME-GC–MS

A disadvantage of the use of natural fibres to reinforce polypropylene is their poor thermal stability, which results in their degradation at processing temperatures of the composites. As a result of this, there is a formation of low molecular weight compounds that are responsible for undesirable odours. Head-space-solid phase microextraction (HS-SPME) was used as a sample preparation technique and gas chromatography–mass spectrometry (GC–MS) was used to identify the low molecular weight compounds in natural polypropylene/polypropylene composites after simulating degradation. Among the compounds found in the samples, there are fragments of PP chains as heptadecane, compounds from antioxidants such as 2,4-bis(1,1-dimethylethyl)-phenol, and p-tert-butylphenol, and compounds from biofibres ageing, such as ethylparaben and vanillin. Numerous carboxylic acids were also identified, being these most probably the source of the undesirable odours.     

Gamma polymorph and branching formation as inductors of resistance to electron beam irradiation in metallocene isotactic polypropylene

The influence of a wide dose range of electron beam (EB) irradiation is analyzed on films obtained from metallocene isotactic polypropylene (iPP) under two different thermal processing conditions. A greater irradiation resistance is observed in those films with content in γ crystallites higher than in α monoclinic entities in terms of melting temperature variation with irradiation. This change is on the order of 0.5 °C per 100 kGy in slowly crystallized films while a value of 1.5 °C per 100 kGy is found in quenched specimens. On the other hand, dual studies on films and pellets have proved that branching takes place in this metallocene iPP. The scission processes that occur during irradiation cannot account by themselves for the observed changes in molecular weight distribution as demonstrated by simulation. The observed changes in the rheological behaviour of the irradiated iPP corroborate that increasing degrees of branching are generated by the irradiation process.     

Effect of isotacticity on radiation stability of polypropylene under lower dose irradiation

Relationship between radiation stability and isotacticity of polypropylene was studied. It was found that the higher the isotacticity, the better the radiation stability under lower dose irradiation. Special PP power with higher isotacticity can keep the melting index change less after γ-irradiation in air. Furthermore, the special PP powder with higher isotacticity has lower intention of branching after 1 kGy γ-irradiation in vacuum and has lower intention of degradation after 5 kGy γ-irradiation in vacuum.     

Influence of Maleic Anhydride on the Molecular Weight of Atactic Polypropylene at the Functionalization Reaction

Abstract

The influence of maleic anhydride (MAN) on the molecular mass of atactic polypropylene in the functionalization reaction initiated by organic peroxide was studied. It was found that under certain reaction conditions MAN takes part in the radical degradation of polypropylene after it is grafted. From the results of the study, a mechanism of degradation reaction was suggested.     

A chemiluminescence study on thermal and photostability of ethylene/α-olefin copolymers synthesized with rac-Et(Ind)2ZrCl2/MAO catalyst system

Copolymers of ethylene and α-hexene or α-octadecene were synthesised, and the relationship between their degradation and the type and content of comonomer was investigated by chemiluminescence, FTIR and thermogravimetric analysis. A clear effect of the length of branches on the thermal behaviour of copolymers was found. The insertion of octadecene caused the formation of a higher content of oxidised species in those copolymers. It may be related to the increase of length of branching which favoured the scission of the carbon–carbon bonds to form alkyl radicals in the earlier stages. It was observed that thermal stability decreased as the comonomer was incorporated. The isothermal CL curves under oxygen exhibited double stage, oxygen independent and oxygen diffusion controlled reactions. The intensity of CL of second stage, decreased and shifted to longer time as the comonomer content increased. This effect was attributed to crosslinking processes which are favoured with the increase of branching degree. Otherwise, for copolymers with high comonomer content, the chemiluminescence intensity enhanced and shifted to shorter time. It was associated to the decrease of the molecular weight, and as consequence, more reactive terminal groups which drives to the promotion of the initiation of thermal oxidation.Copolymers were UV-exposured and CL measured at different irradiation period of times. From the beginning, CL intensity slightly increased with time, followed by a drastically enhancement of emission intensity, and enhancement of chemiluminescence decay rate values, as result of degradation of samples and the higher mobility of peroxides to recombine. The results were supported by FTIR and TGA analysis, which revealed the higher degree of degradation for the copolymers as the comonomer content increased.     

Degradation of polypropylene impact-copolymer during processing

The effect of processing on molecular structure and properties of polypropylene impact-copolymer (ICPP) was investigated. It was confirmed that multiple extrusions induced changes in molecular weight resulting in increased MFI and decreased long-term thermooxidation stability. In terms of mechanical properties only impact strength well reflected the processing history. Tensile and flexural properties remained almost unchanged. The sizes of rubbery domains observed by SEM exhibited only minimum changes. The results of SSA and TREF techniques provided further data helping to elucidate the phenomena in rubbery phase. Based on indirect indications one could conclude that while typical polypropylene degradation resulting in chain backbone cleavage took place in the PP homopolymer phase, the rubbery phase containing EPR and PE homopolymer underwent a certain extent of crosslinking.     

IR-spectroscopic examination of polytetrafluoroethylene and its modified forms

The results of IR-spectroscopic examinations of the molecular and supramolecular structure of polytetrafluoroethylene and polymer materials thereof were summarized. It was shown that, upon heat, mechanical, and other treatment, as well as under radiation exposure, polytetrafluoroethylene preserves its chain-helical conformation, and the resulting modified forms consist of crystalline and amorphous phases in a ratio depending on the history of the sample. The degree of the structural ordering in the polymer decreases with increasing temperature and pressure. The formation of branched moieties and short macromolecules with double bonds (?CF=CF₂) in the terminal groups is specific for the process caused by thermal and radiation-induced degradation. The oxidation of the polymer macromolecules requires applying high irradiation doses and heating samples in an oxygen or air atmosphere.     

Influence of structural and morphological factors on the mechanical properties of the polyethylenes

Force-length relations at ambient temperature have been determined for a set of polyethylenes which represent a wide range in molecular weight and molecular constitution. Taking advantage of previous work from this laboratory, samples have been prepared in such a manner that the important independent structural variables can be identified and isolated and their influence on the different aspects of the deformation process assessed. Partial melting-recrystallization processes appear to play an important role. For the linear polymers there is a direct influence of molecular weight. The influence of molecular weight manifests itself in the structure of the interlamellar zone which has a major influence on the initial modulus as well as the ultimate properties. Copolymers and branched copolymers display quite different behavior. The most striking difference is the invariance of the ultimate properties with molecular weight, branching, and level of crystallinity. From the set of experimental results that are presented the molecular factors involved in the deformation process can be sorted out. It becomes quite evident that all of the basic structural regions, characteristic of semicrystalline polymers, contribute to the observations. Focus solely on the changes in the crystallite, in analogy to the deformation of small-molecule crystalline systems, is inadequate in the case of crystalline polymers.     

Some aspects of the photo,photothermal and thermal decomposition of polypropylene under vacuum

The thermal degradation of polypropylene is accelerated by 2537 Å radiation (photothermal degradation) and methane and ethylene appear as additional products. These are associated with photoinitiation of the radical process involved, the mechanisms of the thermal and photothermal reactions being otherwise identical. The different overall energies of activation of the two reactions, 50·1 and 11·7 kcal/mole, respectively, may also be associated with the different initiation steps.At 20° irradiation causes an increase in molecular weight while at 200° a decrease is observed. The change-over occurs at approximately 100°. Pre-irradiation causes destabilization of polypropylene, and the sensitivity of polypropylene to 2537 Å radiation is believed to be due to titanium residues from the polymerization catalyst. Crystallinity within the limits 47–70% has no effect on radiation stability.These observations are discussed in terms of a radical mechanism.     

分子量分布对等规聚丙烯卷绕丝织构形成的影响

本文研究了分子量分布对聚丙烯卷绕丝结构和性能的影响。结果表明,聚丙烯树脂中的高分子量尾端对卷绕丝的结构有明显影响。在通常的纺丝条件下,用控制降解的聚丙烯树脂纺得的卷绕丝具有低取向的次晶结构;而在相同的粘均分子量和纺丝条件下,高分子量尾端的存在使聚丙烯易于在纺丝线上生成结晶性较高的α-晶型,卷绕丝的取向也随高分子量尾端而显著增大。高分子量尾端对卷绕丝结构的影响,导致卷绕丝牵伸性能和成品纤维力学性能变差。     

Ultrasonic Degradation of Hydroxypropyl Cellulose Solutions in Water,Ethanol, and Tetrahydrofuran

Ultrasonic degradations of hydroxypropyl cellulose (HPC) have been carried out in water, ethanol, and tetrahydrofuran (THF) solutions. In the HPC-water system, cavitation intensity did not increase linearly with ultrasound intensity because of a lower threshold of ultrasonic intensity below which cavitation does not occur. At 27°C the rate of degradation in the three solvents followed the order water > ethanol > THF which is not in line with their characteristic impedance values. The rate of degradation for 20 kHz, 70 W ultrasound intensity was found to increase with a decrease in solution volumes, concentration of HPC, and temperature. Increased rate of degradation at lower temperatures supports the concept based on sonoluminescence experiments that it is the cavitation in a polymer solution that is responsible for ultrasonic degradations and the dissolved polymer molecules do not act as cavitation nuclei. Increased surface tension and density of the solvent are thought to be responsible for improved cavitation at low temperatures. Infrared spectroscopy and x-ray analysis of HPC subjected to ultrasonic treatments remained unchanged, suggesting that there were no chemical or structural (e.g., degree of order) changes on irradiation. The decreases in molecular weights on irradiation arise due to random chain scission whereas similar decreases in Huggins coefficients can be attributed to physical changes (decrease in molecular weight or branching) in the degraded HPC samples.     

Review on the production process and uses of controlled rheology polypropylene—Gamma radiation versus electron beam processing

Controlled rheology polypropylene grades are established commodities in the polymer processing market. However, new types, mainly the so-called high melt strength polypropylene (HMSPP) grades, are being introduced in the last two decades and radiation processing has played an important role. The melt strength properties of a polymer increases with molecular weight and with long-chain branching due to the increase in the entanglement level. As polypropylene (PP) is a linear polymer, the way to improve its elongational viscosity is by the production of a bi-modal polymer. Basell's patents claim the production of long-chain branching on PP by irradiating with electrons under oxygen free atmosphere, followed by two heating steps to allow radical recombination and annihilation reaction. Some other companies have issued patents using electron beam processing, but so far there is no actual production other than the Basell one. As a result of a research joint effort, IPEN, BRASKEM (the biggest Brazilian polymer producer) and EMBRARAD (the major Brazilian radiation processing center) developed a new process to produce HMSPP based on gamma processing. This paper will address some characteristics of each technology and the main industrial opportunities.     

A simulation model for long-chain branching in vinyl acetate polymerization: 1. Batch polymerization

A new simulation model for the kinetics of long-chain branching formed via chain transfer to polymer and terminal double-bond polymerization is proposed. This model is based on the branching density distribution of the primary polymer molecules. The theory of branching density distribution is that each primary polymer molecule experiences a different history of branching and provides information on how each primary polymer molecule is connected with other chains that are formed at different conversions, therefore making possible a detailed analysis on the kinetics of the branched structure formation. This model is solved by applying the Monte Carlo method and a computer-generated simulated algorithm is proposed. The present model is applied to a batch polymerization of vinyl acetate, and various interesting structural changes occurring during polymerization (i.e., molecular weight distribution, distribution of branch points, and branching density of the largest polymer molecule) are calculated. The present method gives a direct solution for the Bethe lattice formed under nonequilibrium conditions; therefore, it can be used to examine earlier theories of the branched structure formation. It was found that the method of moments that has been applied successfully to predict various average properties would be considered a good approximation at least for the calculation of not greater than the second-order moment in a batch polymerization. © 1994 John Wiley & Sons, Inc.     

Reactions of polyolefins with strong lewis acids

Treatment of a cyclohexane solution of low density polyethylene and polystyrene with anhydrous aluminum chloride causes chemical reaction between the two polymers which results in the formation of a graft copolymer. The initial copolymer-forming reaction is very rapid, and prolonged contact of the polymers with aluminum chloride causes subsequent degradation in molecular weight. Treatment of separate solutions of polyethylene, isotactic polypropylene, and ethylene–propylene copolymers with aluminum chloride was studied as a function of time. The intrinsic viscosities of the polymers dropped from initial values of 2.4–6.5 to 0.55–0.85 in 5 min, followed by a slower decline over the next 2 hr. In the case of polypropylene, the low molecular weight fragments largely retained the isotactic structure, which demonstrates that stereochemical isomerization is not a major reaction.