RECYCLING OF REINFORCED ACRYLIC PLASTIC WASTES BY HIGH ENERGY MILLS

Transcription

1 RECYCLING OF REINFORCED ACRYLIC PLASTIC WASTES BY HIGH ENERGY MILLS Jaan Kers *, Dimitri Goljandin, Priit Kulu, Department of Materials Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia Abstract Acrylic (PMMA) sheet material is most commonly used in the manufacture of bathroom equipment. Preheated PMMA sheet material has good technological properties for vacuum forming of acrylic cells in bathtubs and mini-pools. Laminated vacuum formed acrylic cells combined with a thermosetting synthetic resin, such as polyester or epoxy resin, and a reinforcing material, like glass or carbon fibre, produce reinforced acrylic plastics. Generation of wastes takes place after cutting off the technological edges of vacuum forming. Reinforced acrylic plastic wastes have low volume weight and thus have to be precrushed to save transportation and land filling costs. Land filled reinforced acrylic plastic wastes are not subjected to decomposition, thus they remain there for centuries. PMMA is a brittle material with low toughness properties. Thus it is retreatable by direct milling. This study offers a solution for recycling of reinforced acrylic plastic wastes by help of mechanical methods. The method of collision was selected for the treatment of acrylic plastic wastes and a disintegrator mill was used. Reinforced acrylic plastic wastes have different mechanical properties because of plasticity of glass fibre, but they are retreatable by the selective milling system. In our experiments, the particle size of acrylic plastic was reduced and glass fibres intact were separated. To estimate grindability, the specific energy of treatment was used and the particle size distribution of the ground product was described. Our first attempts to use recycled plastics in the casting technology as filler and as a reinforcement of acrylic cells were promising. Keywords: reinforced acrylic plastic wastes, collision method, disintegrator, selective milling, reuse of plastic powder. 1. Introduction Our planet is running out of oil that was clear already in the mid 1970s. With ever increasing oil prices, the reuse of plastics as valuable materials has become topical and more attention is being paid to the retreatment of used plastics so as to reuse recycled plastics. In fact, applied separately, polymers are rarely useful. They are most often modified or compounded with additives to form useful materials. However the retreatment of a compounded product is more complicated. Processes of mechanical recycling of plastics made from semicrystalline polymers, like polyolefins (LDPE, HDPE, PP, etc) are quite similar. Size reduction of plastic wastes takes place in shaft shredders or granulators, followed by their extrusion through a strand die face. Whether a virgin or a reclaimed material, once the resin is extruded and cooled, the pelletizer cuts the strands at high speed, yielding pellets of a prescribed size for optimal processing in manufacturing environments. Polyolefines are most commonly used plastic materials in injection, blow and rotational moulding of containers, toys, plastic bags, bubble and waterproof wrap, refrigerated containers, carpets, etc. The PMMA is an amorphous material and thus cannot be reproduced like semi-crystalline materials. After granulation of acrylic plastic sheet wastes it cannot be melted and extruded again as a sheet material. Other sheet materials like ABS can be recycled and reproduced as recycled ABS sheets to form products with lower quality requirements. During remelting, their colour changes and plastic loses its glossy finish. Reinforced acrylic plastic is a composite material, consisting of thermoplastic PMMA and thermoset polyester resin with glass fibre reinforcement. One of the methods of size reduction of plastics is milling by collision. Theoretical studies of milling by the collision method conducted at Tallinn University of Technology were followed by the development of * Author/s to whom correspondence should be addressed: jaan.kers@mail.ee, Phone , Fax

2 appropriate devices, called disintegrators and different disintegrator milling DS-series systems (Tamm and Tymanok, 1996). Size reduction of a material takes place as a result of fracturing the treated material. At the collision of the particle with a grinding element, an intensive pressure wave spreads from the point of contact. Stresses are approximately an order higher than the strength of the material. The particle as a whole remains intact until the spreading of the compression wave reaches the opposite side of the particle. It reflects as a tension wave of the same intensity. Behind the tension wave, with a certain delay, the particle falls into pieces. After particle break-up, in each piece stress waves run many times back and forth in a very complicated manner. The main parameters of collision are: the velocity of loading (150 m/s) and duration in the active zone (0.01 s). Selective milling is suitable for the treatment of multi-component materials, such as the components of industrial and domestic wastes etc. With traditional grinding methods, selectivity is weak, if weak and strong particles are in an active milling zone at the same time, then both strong and weak particles will be broken. In this process the breaking force is unevenly distributed between the strong and the weak particles. When treating materials by collision, stresses that appear in the particles depend on the velocity of collision and are independent of particle size and material. It is possible to use a velocity low enough to keep particles intact, and a velocity high enough that would cause break-up of both particles. Between the two extreme velocities there is a limit velocity, for which the strong particle remains intact and the weak particle breaks up. Treatment of materials by collision provides the highest range of selectivity, with other methods of grinding, it is unattainable (Tamm and Tymanok, 1996). In our previous studies related to the mechanical recycling of acrylic plastic wastes, it was established that one of the ways to reuse the plastic material is to produce powdered materials from the technological waste of production and old products (Kers and Kulu, 2004). Thus, high-energy milling is suitable for producing powder from acrylic wastes. The milling method produces a powder of a certain quality, particle size and morphology. In addition to size, morphology is a very important parameter. Technological properties of powders, like bulk density, flowability and surface area as well as the properties of products from recycled materials, will depend on the characteristics of the milled product. It was demonstrated that the size and shape of the produced powder particles depend on the milling parameters (Kulu, 1999). This study describes mechanical retreatment of composite plastic wastes in high-energy mills in the direct and selective milling systems. The results of the separation of two materials acrylic plastic and glass fibre were analysed and the main characteristics of powders: the particle size (granularity) and particle shape (morphology) are discussed. 2. Experimentals 2.1 Plastics to be treated PMMA produced by radical chain polymerization in mass (MW 1 million) as a sheet material was used for the extrusion of sheet materials by vacuum forming. PMMA sheet material vacuum formed and reinforced with glass fibre in the matrix of polyester resin was used as the technological waste. Physical properties of PMMA were at normal temperature (23 C): tensile strength 78 N/mm 2, tensile modulus 3.33 MN/mm 2, impact strength 12 kj/m 2, and density 1200 kg/m 3. The physical properties of hardened polyester resin with % glass fibre reinforcement were: tensile strength 75 N/mm 2, tensile modulus 7.70 MN/mm 2, flexural modulus 6.70 MN/mm 2 and density 1700 kg/m Disintegrator milling systems Reinforced acrylic plastic waste plates with dimensions 100 mm length, 100 mm width, and thickness 5 mm, were retreated by the mechanical method milling by collision. The retreatment technology consisted of three steps: preliminary milling of reinforced acrylic stripes by the DSL-158 disintegrator in a direct milling system; sieves for separating glass fibre from the milled material were used; intermediate milling by the DSA-2 disintegrator in the conditions of multi-stage milling; powder samples for sieve analyses were taken and the percentage of the glass fibre separated was determined; final fine milling by the DSL-115 disintegrator, using the direct and selective milling system to remove the glass fibre from the milled material. Main characteristics of the equipment used and the milling systems are shown in Table 1.

3 Table 1. Characteristics of the disintegrators used Parameter Rotor system Diameter of rotors, mm Number of pins/blades roads, Rotation velocity of rotors, rpm Impact velocity, m/s Specific energy of treatment E s, kwh/t Possible operating system Input (maximum particle size), mm Milling environment Industrial disintegrator DSA-158 Mono-rotor system up to 1500 up to direct 100 centrifugal Semi-industrial disintegrator DSA-2 Mono-rotor system up to 3000 up to direct 50 centrifugal Laboratory disintegrator DSL-115 Two-rotor system up to 3000 up to / 50 direct / selective 10 inertial or centrifugal 2.3 Adhesion test The aim of the retreatment of PMMA composite plastic wastes was to reduce the size of PMMA plastic and separate glass fibre. The solution of emerging problems depends on the strength of the composite and on adhesion between acrylic plastic and glass fibre. Mechanical properties of vacuum formed acrylic cells improved by adding reinforcements depend both on the adhesion between the resin matrix and glass fibre, and on the adhesion between acrylic plastic and the reinforcement layer. To measure adhesion between the PMMA sheet material and the composite, a special tensile test was performed (see Fig. 1). The aim of the test was to define the adhesion strength between the PMMA sheet and reinforcement and to determine the parameters of selective milling. F F 4 Figure 1. Reinforcement adhesion test for PMMA sheet: 1- PMMA sheet plate 150 x 150 mm 2, 2 - glass fibre reinforcement layers, 3 50 mm spacer rings with welded M8 bolts, 4 75 mm spacer ring, 5 M8 nuts for fixing the joint. 2.4 Granularity and morphology study The granularity of powders can be determined by different methods (sieve analysis, image analysis, laser analysis, etc.). The question is which method will provide an adequate description of powder granularity. To evaluate coarse powder granularity (particle size more than 50 µm), the sieve analysis (SA) that ensures sufficiently good results was used. Particle size distribution is adequately described by the modified Rosin- Rammler distribution function, and the method may be used to characterize powders produced by collision. The particle size data obtained by the image analysis method was primarily described through the arithmetical mean diameter d m of the measured values. The values of d m depend on the number of particles. To characterize particle shape, different shape factors were calculated. a) The ellipticity parameter. To characterize ellipticity, aspect ratio AS (similar to elongation in literature) was calculated by AS = a/b, (1) where a and b are the axes of the Legendre ellipse (ellipse is an ellipse with the centre in the object s centroid and with the same geometrical moments up to the second order as with the original object area).

4 b) The elongation EL is defined as EL=log 2 (a/b) (2) Elongation of the circle is 0 and of an ellipse with the ratio of axes 1:2, is equal to 1. c) Irregularity or surface smoothness. The roundness RN value was calculated by RN=P 2 /4πA (3) 3. Results and discussion 3.1 Results of the adhesion test between PMMA and the composite layer The test showed that the adhesion strength between the PMMA sheet plate and the glass fibre layer is stronger than inside the glass fibre component. After loading 3.5 kn, breakage inside glass fibre started and with an increment of the load to 6 kn, the lateral crack spread inside the glass fibre composite layer. The breakage inside the glass fibre layer showed a considerable adhesion between the PMMA sheet plate and the composite layer. 3.2 Size reduction by milling of PMMA composite plastic wastes Particle size of the output at the DSA-158 disintegrator was approximately mm. The material preliminarily crushed, was suitable for direct milling in the DSA-2 disintegrator. To estimate grindability, specific energy of treatment was used. Particle size distribution was described by the modified Rosin-Rammler distribution function. 100 Medium size d50, mm ,1 0,0 0,2 2,4 4,8 7,2 9,6 12,0 50,0 Specific energy Es, kwh/t Figure 2. Dependence of particle size d 50 distribution of the milled composite material on the specific energy of treatment As it follows from Fig. 2, an intensive size reduction (from 100 mm to 13 mm) takes place by the preliminary milling in the DSA-158 disintegrator and by milling in the DSA-2 disintegrator (in the first two stages, a substantial size reduction takes place - from 13 to 1 mm). Final milling in the DSL-115 disintegrator in the direct milling conditions at double collision velocity reduced the size by 50 percent (0.71 mm to 0.35 mm). The volumetric density of the milled powder was measured by weighing cylinder volume at vibration (tap density) and with no vibration (apparent density). At vibration, of volume density was about 20 percent higher (see Fig. 3).

5 Density, kg/m3 0,85 0,80 0,75 0,70 0,65 0,60 0,55 0,50 tap density apparent density 0,45 0,40 0,2 2,4 4,8 7,2 9,6 12 Specific energy of treatment Es, kwh/t Figure 3. Density of PMMA milled powder The results of separation of glass fibre and acrylic plastic are presented in the following charts (see Figs. 4 and 5) I stage PMMA Composite 100 wt. % II stage PMMA Composite 83.7 wt. % III stage PMMA Composite 71.5 wt. % Final product PMMA powder 55 wt. % Separated glass fibre in wt. % I stage II stage III Stage Total 16.3 wt.% 12.2 wt.% 16.5 wt. % 45 wt. % Figure 4. Results of separation in the three-stage milling Before separation, the curve of the mixed plastics had two modes. After separation, acrylic plastic had two main fractions: particles of 1.4 mm - 46 % and mm 25 %; the main fraction of glass fibre was mm (more than 85 %). f(m), % Aggregate of acrylic plastic (AP) and glass fibre plastics(gfp) before separation Size d, (mm) separated AP separated GFP Figure 5. Separation of glass fibre plastics (GFP) and acrylic plastic (AP)

6 3.3 Plastic powder particle shape The results of morphology study are given in Table 2. As it follows from the table, the mechanism of the fracture of the PMMA material was the same in direct and selective milling (the roundness parameter of the particle changed a little). Table 2. Shape factors of milled PMMA composite material Specific energy of treatment E s, kwh/t Granularity Main fraction µm (70%) Morphology Mean Mean Mean diameter d m aspect roundness µm AS RN Conclusions 1. As a result of preliminary milling of reinforced acrylic plastic wastes in DSA-158, about 36-wt. % of glass fibre intact in the resin matrix was removed after precrushing of the PMMA composite by sieves. 2. During the precrushing in DSA-158 and as a result of intermediate milling in the multi-stage direct milling in DSA-2, a remarkable size reduction of acrylic plastic takes place. After two-stage milling the mean particle size was about mm. 27-wt. % of glass fibre intact was removed after multi-stage milling of the PMMA composite by sieves. 3. Suitable for the treatment of dry PMMA composite materials, selective grinding reduces the size of acrylic plastic and leaves the glass fibre content as intact as possible. 37-wt. % of glass fibre intact was removed after selective milling by DL Comparative analysis of the granularity of the milled powder of PMMA and that of the PMMA composite showed similar particle distribution. 5. Morphology studies of acrylic plastic powder particles revealed the roundness parameter in the range , independent of the number of collisions. References Kers, J. and P. Kulu, 2004, Retreatment of industrial plastic wastes by high energy disintegrator mills, Global Symposium on Recycling, Waste Treatment and Clean Technology. Vol. 3, , Madrid, Kulu, P. and A. Tymanok, 1999, Treatment of different materials by disintegrator systems, Proc. Estonian Acad. Sci. Eng., 5, 3, , Tallinn Tamm, B. and A. Tymanok, 1996, Impact grinding and disintegrators. Proc. Estonian Acad. Sci. Eng., , Tallinn Wojnar, L., 1999, Image analysis: applications in materials engineering. CRC Press LLC, Boca Raton