The Structure-property Relationship And Preparation For Biaxially Stretched Polyethylene Resin

As a kind of film material with excellent performance,biaxially oriented polyethylene(BOPE)film has good puncture resistance,heat shrinkage and optical properties.And it is used in the package of various foods and daily necessities.However,the existing theory cannot effectively guide the development of BOPE resins,which severely limits its application.And due to the characteristics of strong crystallizability and fast crystallization,general polyethylene resin is prone to problems such as uneven film thickness and film breakage during the biaxial stretching process.In those cases,these polyethylenes are difficult to adapt to the biaxial stretching process.Therefore,it is of great significance to study and develop polyethylene products applied to the biaxial stretching technology.This article aims to study the structure-property relationship and preparation for biaxially stretched polyethylene resin.In this paper,by examining the molecular structure,condensed structure,and biaxial stretchability of different LLDPE resins and their blends,the structure-property relationship of biaxially oriented polyethylene was established.And by means of blending and slurry polymerization respectively,polyethylene resin that met the requirements of the biaxial stretching process was synthesized.Here are the main conclusions of this thesis:(1)The effects of molecular weight,MWD,branching distribution,melting behavior and crystallization behavior(including the crystallization rate and its variation,lamellae size and its distribution)of LLDPE resin on its biaxial stretchability were investigated.And the criteria for BOPE resin were established.The experimental results showed that in order to meet the requirements of processing performance,mechanical properties and transparency for BOPE film,the polyethylene resins should have a wider biaxial stretchable temperature range and a lower biaxial stretching temperature to facilitate the biaxial stretching process.It is required for the wider biaxial stretchable temperature range that the molecular weight of LLDPE resin was 7 × 104-12 × 104 g/mol,the crystallization rate C(T)was 27-33,and the variation of crystallization rate E was less than 5.The lower biaxial stretching temperature required polyethylene to have a thinner average lamellae thickness and a lower initial melting temperature.In addition,the biaxial stretching temperature range was closely related to the distribution of components with a molecular weight of 3.16× 104-10 × 104 g/mol.A more uniform lamellae distribution in this area was conducive to expanding the biaxial stretching temperature range.(2)Two types of linear low-density polyethylenes PE-M1 and PE-M2 with different molecular weight and branch distribution produced by gas-phase polymerization process under a condensed-mode operation were blended to prepare the blend suitable for biaxial stretching.The chain structure and condensed structure(mainly including relaxation behavior and crystallization behavior)required for biaxially oriented polyethylene were explored.The experimental results showed that the the biaxial stretching temperature range of blends increased firstly and then decreased with the blending ratio.When the content of PE-M1 was 15 wt%,the blend had the widest biaxial stretching temperature range(8℃)and higher tensile strength(8.40 MPa).The condensed structural parameters of blends did not change monotonously with the blending ratio,and the extremum also appeared when the content of PE-M1 was 15 wt%(λmoorphous=5.2s,λcrystalline=350.3 s,C(T)80%=27.8,E=2.332,FWHM=7.28℃).Therefore,under the premise of meeting above criteria,the special resin used for BOPE must meet the following condensed requirements:longer relaxation time,lower crystallization rate,less variation in crystallization rate and wider melting peak width at half height.By analysis of their chain structure,the appearance of extremum was attributed to that the blending of PE-M1 and PE-M2 made more branches distributed on the shorter chains.And the lamellae distribution formed by various molecular chains with a molecular weight of 3.16×104-10×104 g/mol became more uniform,which was beneficial to widen the biaxial stretchable temperature range.(3)With the goal to synthesize polyethylene meeting the requirements of the biaxial stretching process,a commercial ternary catalyst was used to study the ethylene-butene-hexene ternary copolymerization behavior and its effect on the molecular structure of resin product through slurry polymerization,which could guide the synthesis of special BOPE resin.The experimental results showed that the catalyst had the highest activity when the reaction temperature was 75℃ and Al/Ti was equal to 600.As the increased addition of hydrogen,both the polymerization activity and the molecular weight of polyethylene were significantly reduced,indicating that the catalyst was very sensitive to hydrogen adjustment.But when the hydrogen content was greater than 1.5 bar,the molecular weight of polyethylene had little changes with hydrogen.By increasing the amount of comonomer,the molecular weight of polyethylene could be further reduced to meet the requirements of BOPE special resins.At the same time,the branching content was increased and the crystallinity of copolymer was decreased with the addition of comonomer,which were conducive to the biaxial stretching process.It was found that different comonomers had different effects on molecular structure.The content of butene affected the molecular weight of polyethylene more than that of hexene.When the amount of butene was increased to a certain extent,hexene had a greater influence on the branching content than butene.