The use of a polymer in product making generally requires it to be blended with other ingredients, which add certain special characteristics to the final product.
Due to the high cost of plastic as well as the use of certain types of charges and additives, it is advisable to use equipment which optimizes the production process before starting extrusion on a grand scale.
The Plastograph is a blender-kneader machine which allows one to get Polymeric Material Formulations, easily and quickly, on a laboratory scale to make further characterization tests parallel to the production tasks.
The Technological Research Centre for Toys (AIJU), recently checked the efficiency of this machine to obtain reference materials with heavy metals, according to the RoHS Directive, for the verification and validation of the screening technique, which uses X-Ray Fluorescency, and to study the capacity of certain additives to retain volatile substances such as phenol, benzene, xilene, toluene and etilbenzene in order to minimize the presence of toxic pollutants in toys.
So, in collaboration with the New Materials Department of CIDETEC, AIJU has processed different formulations by using a Plastograph to obtain reference materials.
Performance of the Work
The work consisted of, on the one hand, obtaining polyethylene materials with different concentrations of Cd, Pb and Cr to carry out the X-ray Fluorescence Test and compare it with the common test and, on the other hand, in obtaining polyethylene and phenol materials with a certain amount of B20 and B40 fillers in order to study the retention capacity they have for volatile substances as well as their migration values.
Parallel to this, the efficacy of the obtained reference materials will be checked using this preparation method, studying the mechanical properties and the homogeneity of the samples.
To prepare the samples BP Solvay HD5218EA type Polyethylene was used, with a Fluidity Index of 18 g/10 min. and a melting point of 131ºC.
Polyethylene is a versatile, inert, cheap polymer in the plastic sector and chemically speaking the simplest of the polyolefin family.
Figure 1: Polyethylene Chemical Structure.
The metals subjected to the study were used in pure oxides forms (PbO, CdO, CrO3), because they have good thermal stability and are hardly volatile, although as they are relatively toxic, security measures must be taken.
Besides, Phenol and B20 & B40 charges were used for the mechanical properties study.
The following laboratory equipment was used:
• Plastograph, laboratory version by BRABENDER
• Hot plates press.
The Plastograph, is an internal blender built with a high temperature compartment which has two rotors which turn in opposite directions, together they melt and blend the material till an homogeneous mixture is obtained. The resulting mass can be introduced into a Hot Plates Press to get the final samples in a film form.
CIDETEC’s Plastograph with which the samples were obtained.
For each test the Plastograph measures the torque force applied by the rotors when turning, over a time scale for a certain temperature. When the material is introduced in the interior of the compartment, the torque force quickly increases as the rotors change from an empty rotation to a sample-filled rotation. Upon the polymer melting or mixing with fillers takes place, the torque strength turns constant over time.
Figure 4 shows, for a specific temperature, the typical graph obtained which demonstrates the torque force of the rotors when turning based on the time scale for a certain sample.
Illustration of the torque force against the Plastograph Temperature for a determined Polyethylene sample.
Before preparing the reference materials, the process conditions were optimized for the polyethylene used. The most appropriate one for 40 g of polyethylene was 140ºC and 20 minutes of mixing/kneading.
To obtain the materials, polyethylene is introduced from the top side of the chamber which, in this precise case was used in grain form and blended with the corresponding additives. It is closed and secured with a 5 Kg weight to apply pressure which facilitates the melting and mixing and stops the sample leaking out of the top side.
When the mixing time is over, the deposit is opened and the mixture is spread over the rotors and the internal deposit (figure 5) with a copper spatula after letting it cool for a while. The resulting mixture is then pressed in the Hot Plate Press.
Appearance of the mixed sample after the kneading process.
The different resulting reference materials have been split up into two groups. On the one hand, the mechanical properties of the materials have been studied, adding B20 and B40 fillers in different proportions to see the effect they produce. On the other hand, the homogeneity and lineament of the obtained samples with different concentrations of heavy metals in order to verify and validate the screening technique which X-Ray Fluorescence employs, and to verify the efficiency of the resulting materials.
Figure 6: Reference materials prepared for the tests.
With the aim of evaluating the mechanical traction properties of the different materials obtained, test specimens were cut into a special shape (EN ISO 527) and afterwards they were tested in a universal test machine INSTRON 6025.
Figure 7 and 8 compare the Elasticity Range with the breaking point force obtained by adding B20 and B40 fillers in 5% and 10% doses.
When adding 5% of sepiolita B20 or B40 fillers, no change in the elasticity modulus is observed. Nevertheless, 10% of filler in the blend results in an increase in the material stiffness as well as in a higher module.
Breaking point force (MPa).
By adding B40 at 5% and 10% (in weight) a higher stress breaking point value is obtained than in the B20 case. An important decrease in the resistance is also noticed with the adding a 10% amount of B20 with regard to that obtained with only 5%.
Homogeneity: To study the homogeneity, each sample was split up into 10 groups, which were analysed by using X-Ray Fluorescence. The results were generally good, except for lead (Pb), which showed some deviation as can be seen in the following figure 9.
Homogeneity Study for a 1000 ppm sample
Below, as an example, an X-Ray Fluorescence spectrum obtained after the analysis of a sample of 1000 ppm in Pb and Cr and 100 ppm in Cd is included.
X-Ray Fluorescent Spectrum for a 1000 ppm Pb sample.
Regarding the calibration, for lead (Pb) and chrome, acceptable regressions were obtained. However, for cadmium it would be advisable to produce new materials. The following figure shows the lead (Pb) calibration, as an example.
Calibration obtained for Lead
The Plastograph is an efficient way to develop reference materials prior to industrial production by conventional techniques such as injection or extrusion. It allows one to get a varied range of plastic formulations with the aim of studying their mechanical properties, and carrying out characterization studies and chemical tests for project development and process improvement. Moreover, as it uses only a small amount of material, the development cost is minimal.