228_jaan kers_05_02_07

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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 semi-
crystalline 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 +372 56690118, Fax +372 620 3196 
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/mm2, tensile modulus 
3.33 MN/mm2, impact strength 12 kJ/m2, and density 1200 kg/m3. 
The physical properties of hardened polyester resin with 25-30 % glass fibre reinforcement were: tensile strength 
75 N/mm2, tensile modulus 7.70 MN/mm2, flexural modulus6.70 MN/mm2 and density 1700 kg/m3. 
2.2 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. 
Table 1. Characteristics of the disintegrators used 
Parameter Industrial 
disintegrator 
DSA-158 
Semi-industrial 
disintegrator 
DSA-2 
Laboratory 
disintegrator 
DSL-115 
Rotor system 
Diameter of rotors, mm 
Number of pins/blades roads, 
Rotation velocity of rotors, rpm 
Impact velocity, m/s 
Specific energy of treatment Es, kWh/T 
Possible operating system 
Input (maximum particle size), mm 
Milling environment 
Mono-rotor system 
600 
1 
up to 1500 
up to 40 
0.2 
direct 
100 
centrifugal 
Mono-rotor system 
480 
3 
up to 3000 
up to 75 
2.4 
direct 
50 
centrifugal 
Two-rotor system 
480 
5 
up to 3000 
up to 150 
6.7 / 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. 
 
 
 
 
 
 
 
 
 
Figure 1. Reinforcement adhesion test for PMMA sheet: 1- PMMA sheet plate 150 x 150 mm2 , 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 dm of the measured values. The values of dm 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). 
F
F 
1
2
4 
5 
3 
b) The elongation EL is defined as 
EL=log2(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=P2/4πA (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 13-25 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. 
0,1
1
10
100
0,0 0,2 2,4 4,8 7,2 9,6 12,0 50,0
Specific energy Es , kWh/T
M
ed
iu
m
 s
iz
e 
d5
0
, m
m
 
Figure 2. Dependence of particle size d50 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). 
 
0,40
0,45
0,50
0,55
0,60
0,65
0,70
0,75
0,80
0,85
0,2 2,4 4,8 7,2 9,6 12
Specific energy of treatment Es , kWh/T
D
en
si
ty
, k
g/
m
3
tap density
apparent density 
 
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) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 0.355 mm – 25 %; the main fraction of glass fibre was 0.180 mm (more 
than 85 %). 
0
10
20
30
40
50
60
70
80
90
0.0100.1001.00010.000100.000
Size d , (mm)
f(m), %
Aggregate of acrylic 
plastic (AP) and glass 
fibre plastics(GFP) 
before separation separated GFP 
separated AP
 
Figure 5. Separation of glass fibre plastics (GFP) and acrylic plastic (AP) 
 
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. % 
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 
Granularity Morphology 
Specific energy 
of treatment Es, 
kWh/T 
Main fraction 
µm (70%) 
Mean 
diameter dm 
µm 
Mean 
aspect 
AS 
Mean 
roundness 
RN 
12 +700 
-355 
547 1.39 1.32 
55 +355 
-180 
303 1.37 1.31 
 
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 matrixwas 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 0.5-1 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-115. 
 
4. 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 1.31-
1.32, 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, 2795-2797, Madrid, 
Kulu, P. and A. Tymanok, 1999, Treatment of different materials by disintegrator systems, Proc. Estonian Acad. Sci. Eng., 5, 
3, 222-242, Tallinn 
Tamm, B. and A. Tymanok, 1996, Impact grinding and disintegrators. Proc. Estonian Acad. Sci. Eng., 2.2. 209-223, Tallinn 
Wojnar, L., 1999, Image analysis: applications in materials engineering. CRC Press LLC, Boca Raton