INJECTION MOULDING PROCESS USING POLYMER BLENDS CONTAINING HIGH MOLECULAR POLYETHYLENE OF HIGH DENSITY

Abstract: Process for producing injection moulded parts having markedly improved mechanical strength properties wherein a blend of high molecular polyethylene having high density with a matrix material which consists of a polymeric material with which the high molecular polyethylene has a limited compatibility is injection moulded at high injection moulding pressures exceeding 250 MPa, whereby a fibre reinforced injection moulded article having improved mechanical properties is produced.

Description: INJECTION MOULDING. PROCESS USING POLYIIER ELΞLIDS CONTAINING HIGH MOLECULAR POLYETHYLENE OF HIGH DENSITY The present invention relates to a process for injection moulding at high pressures exceeding 250 MPa (300 MP.a) of blends of high molecular polyethylene having high density (HMWHDPE) with a-polymeric material with which HMWHDPE has 5 a limited compatibility (solubility) for producing an injection moulded article, wherein HMWHDPE forms a fibre phase in a matrix of the other polymeric material. From the Swedish Patent Specification 401,129 it is previously known that an essential increase of the pressure 10 in the mould cavity in the processing of thermoplastic resins by means of injection moulding results in a number of property improvements of the moulded parts and at the same time certain technical process advantages can be obtained. Thus, said Swedish Patent Specification discloses 15 an injection moulding process wherein the pressure in connection with the injection and subsequent solidification of the plastic melt exceeds 250 MPa. Said pressure range shall be compared with the pressure range 50-150 MPa which is the normal pressure range in injection moulding. 20 Among the improvements of the properties of the moulded part which are provided with process pressures exceeding 250 MPa there is to be noted a general increase of the level as regards the stiffness (modulus of elasticity) and strength parameters of the - - product as has been-obtained .in experim.en.ts.. ith .polyethylene 25 of varying density and with polypropylene. - Furthermore, in measurements on both crystalline as amorphous polymers a substantial decrease in. the.mould shrinkage has been observed. In certain cases said change was so large that the mould shrinkage assumed negative values. An additional property improvement which has been observed in experiments of this type has been a substantial decrease of the internal stress level. As is well known internal stresses are a defect of normal injection mouldings; they i.a. can result in warpage, undesirable formation of crazing and cracking etc. Their reduction, in certain cases complete elimination, can be traced back to an increase of the melting and softening point.respectively of the plastic with the pressure. Thus, a pressure within the range 250 - 1000 MPa has been found to increase these critical temperatures so that a practically momentary solidification of the melted plastic material injected in the mould cavity is obtained. This prevents the formation of an internal stress structure, which, as is well known, is due to a gradually solidification of the melt and to the forces which the already solidified and the still liquid portions of the moulded part exert on each other. As regards the improvement of the property profile of the moulded part,especially as regards the mechanical parameters, the effects obtained when increasing the process pressure are dependent on the structure of the plastic used. Thus, the greatest changes have been observed with crystalline plastics, especially polyethylene,wherein "they have been traced back to changes in the structure of the crystalline phase. In the- crystallization of a polyethylene melt under pressure exceeding about 300 MPa a crystal modification is obtained wherein the folded lamellar structure present at normal pressures is converted to a structure consisting of straight chains (extended chain-structure) to an extent increasing with the pressure. The improved property profile can be explained by the presence of said new structure. The new modification with straight chains i.a. gives a more organized package of the molecules in the space. Especially remarkable results are obtained with HD-poly- ethylene having high molecular weight, which refers to polyethylene having a molecular weight of from 200.000 up to 1.5 millions and higher (corresponding -to a melt index of less than 1 g/10 minutes at 190 C/2 kg load) " hereinafter designated HMWHDPE. As compared to the improvements of modulus and strength obtained with this and similar qualities the corresponding values- obtained with HOPE- of .normal, molecular weight were relatively small. As an illustration of the extremely striking increase of the property level with an increase of the pressure reference is made to the values shown in the following table L TABLE I E GP a ° MPa HMWHDPE 3,4 130 HDPE 0,6 25 Process conditions: Injection pressure 500 MPa Mould temperature 30°C E = modulus of elasticity On = tensile strength at rupture According to the present invention it has been found that in the injection moulding at high injection moulding pressures (injection and holding pressures) exceeding 250 MPa of blends of HD-polyethylene having very high molecular weight (HMWHDPE) , which refers to a molecular weight of 200.000 and up to 1.5 millions and more, with polymeric materials with which HMWHDPE has a limited compatibility (solubility) HMWHDPE forms a fibre phase in a matrix of the other polymeric material. Thus, by using said blend of HMWHDPE and said other polymeric material in injection moulding with high pressures fibres of extended chain-type of " HMWHDPE with extremely good mechanical properties are obtained which form a reinforcement in the matrix material. The generation of extended chain-fibres is facilitated by the large shear forces which prevail between the two differen melts of material during the filling and packing stage during the injection cycle. By means of this fibre reinforcement an injection moulded part with superior mechanical properties, such as tensile strength and modulus of elasticity, is obtained in one single step. Said properties are highly improved as com¬ pared to the corresponding properties for the matrix material and exceed when using injection moulding pressur exceeding 400 MPa using certain materials, such as HD-PE, as matrix material even the corresponding properties of pure HMWHDPE. Suitable matrix materials together with which HMWHDPE can be injection moulded in the process according to the present invention are such polymeric materials with which HMWHDPE has a limited compatibility. These polymeric materials includes polyethylene of HD-, MD- and LD-types of normal molecular weight,which refers to such materials which normally are used in injection moulding or extrusio Other suitable materials are e.g. other polyolefins, such as polypropylene and polystyrene, polyvinylchloride, poly acetal, polyamides, co olymers of these and others. In the enclosed drawings Figure la shows the modulus of elasticity plotted against injection pressure for pure HMWHDPE and for blends of HMWHDPE with HDPE; Figure lb shows the tensile strength at rupture plotted against the injection pressure for the same materials as figure la. OMPI Figure 2 shows the modulus of elasticity and the tensile -strength plotted against the- pressure for a blend of LDPE and ' HMWHDPE. Figure 3 shows the elongation at rupture-for a blend of 5 LDPE and HMWHDPE and for pure HMWHDPE plotted against the pressure. The invention is illustrated by means of the following specific examples which describe embodiments of the invention but which are not intended to limit the invention 10 in any respect. Example 1 A blend of 40 % by weight of HMWHDPE and 60 % by weight of HDPE of normal molecular weight was injection moulded with different pressures from 100 MPa and upwards. The 15 conditions used during the experiments can be seen from the following disposition. Material: hz^gh. density polyethylene having a very high molecular weight (HMWHDPE) , melt index 0.1 g/10 minutes (MFI 190/2), density 0.953 cr/cm 2 . -20 High density polyethylene having normal molecular weight (HDPE), melt index 7 g/10 minutes (MFI 190/2), density 0.960 g/cm 3 . Injection moulding conditions Injection pressure: 100 - 500 MPa 25 Mould temperature: 30°C Cylinder temperature: 250 - 280°C. The results obtained in the experiments are shown in figures la and lb which show the modulus of elasticity, E, for blends of HMWHDPE and HDPE as compared to the corres- 30 ponding property of pure HMWHDPE (la) and the strength at rupture, 0" B / for the same materials (lb) respectively plotted against the pressure used. From the figure it can be seen that both said parameters rapidly increase with increasing injection pressure and attains 5 very high values at a pressure level of 500 MPa. From figure la ^ t can ^ e seen that in s p ite of the fact that E „ is smaller for the composi^ematerial than for pure HMWHDPE at 200 MPa the situation is reversed at highe pressures. Thus, at the higher pressures the fibre 10 generation described (of extended chains) of HMWHDPE in the ' matrix material took place and a fiber reinforced moulde part having excellent mechanical strength properties was obtained. The fibre character of the moulded parts and the composite material was confirmed by means of scanning ] _5 electron measurements. Example 2 Exactly the same procedure as in example 1 was used except that 60 % by weight of HDPE was replaced with 60 % by weight of low density polyethylene (LDPE) having the 20 following properties: melt index 7 g/10 minutes, density 0.917 g/cm 3 . The materials were blended in an extruder in the propor¬ tions 60 % by weight of LDPE and 40 % by weight of HMWHDPE. The injection moulding conditions were exactly the same as 25 those used in example 1. Figure 2 shows the results obtained for the composite material. From the figure it can be seen that the modulus of elasticity, E, and tensile strength increase with increasing pressures. At high pressures 30 (400 - 500 MPa) these parameters assume high values, especially bearing in mind that the major part of the composite materials consisted of soft LDPE. Figure 3 shows the elongation at rupture plotted against the injection pressure for the blend of 60 % LDPE/40 % HMWHDPE and for pure HMWHDPE. From the figure it can be seen that in spite of the substantial increase in stiff- ness and strength with increasing pressure for the composite material this material is rather ductile also at high pressures. From this point of view the material is superior to pure HMWHDPE injection moulded at e.g. 100 MPa which both has lower tensile strengths at rupture and elongation at rupture. OMPI

Claims: CLAIMS 1. A process for producing injection moulded articles having markedly improved mechanical strength properties characterized by injection moulding at high injection moulding pressures exceeding 250 MPa a blend of high molecular polyethylene with a molecular weight of between 200,000 and 1,5 millions and higher and having hig density with a matrix material which consists of a polymer material with which the high molecular polyethylene has a limited compatibility which produces a fibre reinforced injection moulded article having improved mechanical properties. 2. A process according to claim 1, characterized in that the matrix material ' consists of high density polyethylene of normal molecular weight. 3. A process according to claim 1, characterized in that the matrix material consists of polyethylene of. low density. 4. A process according to any of the preceding claims, characterized in that the process is carried out using an injection moulding pressure which is between 300 and 800 MPa.

Inventors: DJURNER J, KUBAT J, RIGDAHL H
Applicants: DJURNER J, KUBAT J, RIGDAHL H
Application year: 1979
Main classification: B29F1
IPC codes: B29C45, B29K23, B29K105