THE EFFECT OF ADDITION OF NANOTUBES TO THE THERMAL PROPERTIES AND PROCESSING
2. Experimental 1 Test stand
Experiments were performed using an injection moulding machine Arburg Allrounder 320 C 500 – 170 (Fig. 2). The injection moulding machine has a cylindrical screw with a diameter of 30 mm and the length to diameter ratio of 20. The highest peripheral screw speed equals 42 m/min and its torque is up to 250 Nm. The screw is made as one-piece and single-flight, finished with a ring preventing the polymer from retreating to the plasticizing system during the injection of the polymer to the injection mould cavity. The barrel is also uniform and finished with an exchangeable injection nozzle.
The plasticizing system of the Arburg injection moulding machine has five heating zones of the same length and a cooled feed opening zone. It makes it
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possible to inject into the forming cavity of the injection mould up to 80 cm3 of polymer plasticized with maximum injection pressure of up to 200 MPa.
The ttransmission of the injection moulding machine is powered with three-phase alternating current with voltage of 400V and frequency of 50 Hz. The injection moulding machine ensures the clamping force of the mould of 500 kN.
The area between the columns of the opening-closing tool with dimensions of 320 mm x 320 mm, and the size of tables of the movable and immovable subassembly of the injection mould is 550 x 550 mm. A two-part injection mould was mounted on the platens of the injection moulding machine. In one of its parts, the flow channels of the mould were created, that is sprue, runners and gates. The injection moulding machine has two mould cavities, with the shape and size in accordance with standardized specimens for strength testing. The second part of the mould is a flat plate that constitutes the closing of the first part. The mould is thermostatized with oil of the temperature in the range from 12 to 50 °C, flowing through the channels in the mould.
The temperature of the heating zones of the plasticizing system of the injection moulding machine, starting with the hopper, equalled 100, 140, 160, 170 and 180
oC while the temperature in the feed opening zone was set at 30 oC. The total time of the injection process cycle equalled 34,16 s, including the cooling time of 20 s. The composite polymer was injected into the mould at the pressure of 100 MPa and the holding pressure equalled 85 MPa. The temperature of the injection mould was set at 18 oC.
Fig. 2. Injection moulding machine Arburg Allrounder 320 C used for testing
Research stand for testing melt flow index (MFR) is the extrusion plastometer Zwick 4105.100 with additional equipments and analitical scale PRL TA14. The mass of samples obtained using the extrusion plastometer was determined by
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means of an analytical balance of the type PRL T A14 and symbol WA 34, with the measuring range from 0 to 100g and measuring accuracy 0.00001 g.
The measurements of heat distortion temperature (HDT) and Vicat softening temperature (VST) were made using the Instron-manufactured CEAST HV3 provided with three work stations.
2.2 Materials
The polymer used in the experimental tests was low density polyethylene LDPE marketed under the trade name Malen E and symbol FGAN 18-D003, produced by Basell Orlen Polyolefins company (tab. 1). The PE used in the experiment is manufactured for extrusion. The recommended processing temperature for this product ranges from 170oC to 220oC.
Table 1. Basic properties of the tested polymer as given by the manufacturer
Property Unit Value
Density
Melt flow rate (190 oC; 2.16 kg) Tensile modulus
Tensile strain at break Shore hardness
Vicat softening temperature B50 (50°C/h, 50N)
kg/m3
Polyethylene grafted with maleic anhydride (PE-graft-MA), by Sigma-Aldrich company, as a compatibilizer was used. Its melt temperature was 105 oC and density 920 kg/m2.
2.3 Tested parameters
Given the objective of the study, we devised a set of key parameters describing the processing, thermal and granulometric properties of the tested material. The parameters were divided into four groups.
Resulting parameters:
- melt flow rate MFR, g/10min.
- heat distortion temperature HDT, oC;
- Vicat softening temperature, oC;
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- nanofiller type:halloysite nanotubes;
- weight in weight concentration of the nanofiller in the test piece: ζ = 2;
4; and 6 %.
Constant parameters:
- geometry of the plasticizing unit, tools and injection mold:
- temperature of the injection mold 18oC:
- temperature along the plasticizing unit of the injection molding machine:
100, 140, 160, 170 and 180 oC;
- injection pressure: 100 MPa; holding pressure: 85 MPa; holding time: 6s, injection cycle time: 34,16s; cooling time: 20s;
- Vicat load 10N;
- oil bath heating rate 120 K/h.
Disturbing parameters:
- electric voltage: 219 – 241 V, - relative air humidity: 55 – 65 %, - ambient temperature: 20 – 24oC.
The results of the investigated direct twin-screw extrusion and injection molding processes revealed that the disturbing parameters have a negligible effect on the results of the measurements.
2.4 Methods
Measurements of melt flow rate (MFR) of a low-density polyethylene modified with a nanofiller in the form of nanotubes of the content from 2% to 6%
by mass in relation to the matrix with the addition of 5% of compatibilizer and without it were made in accordance with ISO 1133-1:2011. Before measurements are started, the cylinder and the piston of the extrusion plastometer are heated to a set temperature (190 °C), which must be maintained for 15 minutes. Pellets of the nanocomposite are introduced into the cylinder and a weighted piston (2.16 kg) is placed in the cylinder; after about 4 min, the test material reaches the measurement temperature. Measurement (which involves cutting off pieces of the test material) starts when the bottom ring mark on the piston rod reaches the upper edge of the cylinder and ends when the top ring mark reaches the upper edge of the cylinder. During this time, extruded sections of the test material are cut off every 60s. When the cutting is completed and the polymeric composite extrudate has solidified, the sections are weighed, using an analytical balance, to an accuracy of 0.001g, and then the average mass of the extruded sections and the melt flow rate (MFR) are calculated from the formula:
MFR (T, mnom) = 600 m/t (1)
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where: T – means test temperature in oC; mnom – loading mass in kg; m – average mass of extruded polymer sections in grams; 600 is calculation coefficient (600s
= 10 min) meaning cutting period of time in seconds, matched in relation to the tested polymer, in such a way that polymer section length is within the range from 10 to 20 mm.
The Vicat softening temperature (VST) test was performed according to ISO 306. Before being placed in measuring stations specimens were measured with a precision caliper to an accuracy of 0.01 mm. The specimen dimensions obtained were entered into the computer program which calculated the dedicated load for the given each specimen. VST was measured using the A50 method, in which a specimen is loaded with a force of 10 N and the temperature increase rate is 50 degrees C/h. After placing the specimens in the measuring stations, they were immersed in an oil bath, and the gauge needle's sensors were calibrated. The starting temperature of the process was 30 oC, and the maximum oil temperature was set at 120 oC. According to the norm, VST is a temperature in °C, at which the specimen is penetrated to a depth of 1 mm by a hardened steel needle with a circular cross-section of 1 mm2 under a specific load at a specific temperature increase.
The heat deflection temperature (HDT) test was performed according to ISO 75. Before being placed in measuring stations specimens were measured with a precision caliper to an accuracy of 0.01 mm. The specimen dimensions obtained were entered into the computer program which calculated the dedicated load for the given each specimen. The load causes constant bending stress which equals 0,45 MPa (defined by standard). After placing the specimens in the measuring stations, they were immersed in an oil bath, and the gauge deflection sensors were calibrated. The starting temperature of the process was 30 oC, and the maximum oil temperature was set at 120 oC. According to the standard the heat deflection temperature is the temperature at which the specimen deflects by a defined value (0,32 mm) because of constant bendig stress. After the deflection computer software called Visual Therm connected to the test station automatically stops the device, reports the results and draws the curve of penetration as a function of temperature growth. Heat Deflection Temperature results were used to calculate the arithmetic mean and standard deviation.
3. Results and discussion