Stretch-blow molding of PET copolymers - Influence of molecular architecture

International audienceThe purpose of the paper is to study the influence of molecular architecture of poly(ethylene terephthalate) (PET) on its ability to be processed by stretch-blow molding, which is not well documented in the literature. To evaluate this process ability, it proposes an original strategy combining laboratory analyses and experiments on a prototype machine. PET copolymers were prepared from three types of comonomers: diethylene glycol (DEG), isophthalic acid (IPA) and trimethylolpropane (TMP). It is first shown, through laboratory experiments, that the nature of the polymer in terms of chain constitution (copolymerization), chain length (intrinsic viscosity) and purity (catalytic residues) greatly affects many properties: melt crystallization, thermal properties, polymer rigidity and drawability. These different properties obviously induce very different behaviours at the different steps of the stretch-blow molding process: injection-molding of the preform (quenchability), heating (IR absorption), stretch-blow (rigidity and drawability). The stretch-blow step has been simulated on a prototype apparatus designed in our laboratory. It has been shown that free blowing can be used to characterize the process ability of the polymer. A statistical analysis has confirmed the great differences between the materials investigated and pointed out the complexity of the material response during blowing

Influence of chemical architecture of PET on ability to stretch blow moulding

International audiencePET specific properties make this material a good candidate for stretch blow moulding. However chemical architecture of the resin can be varied by the supplier. These changes induce different draw abilities or different behaviours whilst blowing. In this study some well controlled PET resins are characterized both from the point of view of their intrinsic properties and from the point of view of their process abilities. Results enlighten clearly the tight correlation between the chemical structure and the behaviour of the polymer. Coupling effects between the structure, the laboratory properties and the process ability exist making this process a highly not intuitive processing. This study allows, in parallel, proposing a global approach to estimate blowing ability from laboratory analysis

Propriétés rhéologiques de polyéthylènes basse densité de structures différentes

National audienceQuatre polyéthylènes obtenus par voie radicalaire, deux par procédé tubulaire et deux par procédé autoclave, ont été caractérisés en cisaillement. Ils présentent des distributions de masse molaire différentes, mesurées par chromatographie d'exclusion stérique. Les mesures en rhéométrie rotative en fréquence fournissent le comportement dans le domaine linéaire. Différents temps de relaxation, correspondant aux différentes chaînes des macromolécules, sont déduits des mesures rhéologiques. Si ces temps ne sont pas directement reliés à la structure des différents matériaux, leur distribution semble l'être : on montre qu'elle dépend directement de la structure des ramifications, qui sont plus nombreuses pour les matériaux autoclave que pour les matériaux tubulaires

Cristallisation du polyéthylène basse densité : mesures rhéologiques et calorimétriques

International audienceDeux polyéthylènes basse densité, obtenus par procédé tubulaire et procédé autoclave, ont été caractérisés par chromatographie d'exclusion stérique. Leur masse molaire moyenne en masse est sensiblement identique, mais le matériau autoclave présente une queue de plus fortes masses. Ceci induit des différences notables sur les conditions de cristallisation. Les mesures de spectrométrie mécanique effectuées en fréquence et température mettent en évidence que le facteur de glissement suit une loi d'Arrhenius dans le domaine fluide et un changement de régime au début de la cristallisation, dû à l'apparition des premiers sphérolites. Les mesures calorimétriques et rhéologiques en condition anisotherme corroborent ces résultats en terme de température de transition

Conditions d'apparition du phénomène de désenchevêtrement dans le cas de polyéthylènes branchés

International audienceLe phénomène de désenchevêtrement s'observe sur les polyéthylènes branchés obtenus par procédés radicalaires lorsqu'on leur applique un écoulement de cisaillement. Il se manifeste par la modification des propriétés viscoélastiques et par la diminution de la viscosité en écoulement permanent. Selon le dispositif utilisé, les taux de cisaillement appliqués peuvent varier dans une large gamme, de 0,01 s-1 à 1 s-1 en écoulement entre cône et plan à quelques s-1 dans une géométrie de Couette. Nous ne présentons ici que le cas de la géométrie cône plan. Le protocole de mesure consiste à faire subir au matériau, préalablement recuit et stabilisé, un saut de vitesse de déformation pendant un temps très long, pour observer la décroissance du couple qui suit la phase d'établissement de l'écoulement permanent. Notre travail va consister à déterminer le taux de cisaillement minimum à appliquer pour observer la diminution du couple, sur quatre matériaux de différente architecture moléculaire. Une interprétation de l'existence de ce taux de cisaillement minimum sera proposée à partir des travaux publiés sur l'influence des conditions de mise en œuvre sur la cristallinité

Structure development during polymer processing. Morphological and crystallographic textures in polyethylene blown films

La publication originale est disponible sur le site http://www.revue-metallurgie.orgInternational audienceThe crystallographic texture of a great number of polyethylene films manufactured by the film blowing process has been investigated by X-ray diffraction. Some films present a classical texture (c-axis in the machine direction), which can be interpreted using existing morphological models. Other exhibit an original texture (c-axis in the film thickness) and no satisfactory morphological model has been proposed until now

Stability of cast-film extrusion of various metallocene polyethylenes

International audienceThe Cast Film process is one of the most widely used polymer processing technology to produce polymer films. A polymer melt is extruded through a flat die and then stretched in air by a chill roll to generate a thin film as illustrated in Figure 1. The molten film is quenched on the surface of the chill roll (typically water-cooled and chrome-plated) by an air knife or a vacuum box

Crystallization kinetics of polypropylenes. Effect of nucleating agents?

International audienceThermal conditions and formulation affect the crystallization mechanisms of polymers and the associated kinetics in a coupled manner. In that field, the objective of this investigation is to compare overall crystallization kinetics and structural organization of one clarified polypropylene (specifically designed for stretch-blow molding) and a homopolypropylene. Liquid/solid transitions are investigated in- and ex-situ under isothermal and non-isothermal conditions combining crossed-polarized optical microscopy, differential scanning calorimetry and X-ray diffraction. Clarified polypropylene has a very 'singular behavior' compared to homopolymer since no spherulites can be observed. However, it exhibits a semi-crystalline structure. The major α-phase coexists with some γ-phase even under quiescent conditions. Overall crystallization kinetics is rapid suggesting the existence of very efficient nucleating agent(s) and resulting in an increase of crystallization temperature. In parallel, the melting temperature of copolymer decreases by 20 °C compared to homopolymer, suggesting a drastic change in lamellae thickness. It is concluded that this unusual structure results from nucleation, which enforces high temperature crystallization, and copolymerization, which constraints the crystalline organization

An Analysis of Transcrystallinity in Polymers

International audiencePolymer crystallization often occurs in the presence of foreign bodies, such as walls of processing tools. In such cases, there is a competition between nucleation in the bulk polymer and nucleation on well-identified surfaces. If many nuclei are activated at the surfaces, their proximity imposes that entities emanating from these nuclei grow preferentially normal to the surfaces, leading to transcrystalline zones. The competition between surface and bulk nucleation can be studied through crystallizations of thin polymer films in contact with pan surfaces in a DSC apparatus. These experiments show that in thin samples transcrystallinity is limited by sample thickness. When thickness increases, the transcrystalline zones can grow, but up to a limiting value, because at a certain stage their development is stopped by the growth of bulk spherulites. A specific analysis of these DSC experiments gives access to crystallization parameters such as the number of nuclei per unit surface or the growth rate, and makes it possible to determine the crystallization kinetics of the polymer not disturbed by transcrystallinity

Contribution of Thermal Modelisation to Understand adhesion Mechanisms in Extrusion Coating: Case of Polypropylene on Aluminium

International audienceA multilayered structure (polypropylene/grafted polypropylene/aluminium) manufactured by extrusion coating has been studied. The polypropylene film is extruded through a slit die, slightly stretched in air, coated on an aluminium foil in a laminator consisting of a chill roll and a flexible pressure roll, and finally cooled on successive chill rolls. Adhesive properties of polypropylene can be improved by grafting maleic anhydride on the polymer chain which may react with the aluminium surface. The reaction kinetic is more important for high temperature conditions