Process for separating the constituents of a mutlilayer material

Process for separating the constituents of a multilayer material including at least one layer of a base plastic (A) and a layer of a plastic (B) which are separated by a layer of adhesive plastic (C), in which: (1) the material is heated to a temperature T1 between the crystallization temperature of the plastic B (Tc) and Tc--20.degree. C., (2) the material is next shredded by being subjected to shearing, at approximately the same temperature, so as to produce delamination and thus to convert the material into particles of small dimensions of two types, some (X) consisting essentially of base plastic (A) and others (Y) consisting essentially of plastic B and of adhesive plastic (C), and (3) the particles X and Y are subsequently separated by electrostatic separation.

FIELD OF THE INVENTION 
The present invention relates to the recycling of multilayer materials 
based on plastics. It relates more particularly to the separation of the 
constituents of a multilayer material including at least one layer of a 
base plastic and a layer of a barrier plastic which are separated by a 
layer of adhesive plastic. 
TECHNOLOGY REVIEW 
Such multilayer materials are commonly employed in very diverse industries, 
for example in the manufacture of packaging materials or of fuel tanks. An 
example of a multilayer fuel tank comprises 5 layers: 
PE/adhesive/EVOH/adhesive/PE, where PE denotes high density polyethylene 
and EVOH denotes an ethylene-vinyl alcohol copolymer, it being possible 
for the adhesive to be especially a polyethylene grafted with maleic 
anhydride (PE-g-MA). 
SUMMARY OF THE INVENTION 
During the manufacture of plastics-based articles, especially by blow 
extrusion, a large quantity of scrap is unavoidably produced (often more 
than 40%), the recovery of which is economically advantageous. The same is 
even more the case where whole substandard articles are rejected. In the 
case of multilayer materials the presence of several different plastics 
sometimes presents problems: if it is deemed sufficient to grind the 
multilayer material into particles and to reemploy them in the manufacture 
of a new article, the latter runs the risk of exhibiting mediocre 
mechanical performance and therefore the quantities of material thus 
recycled must be greatly restricted. It is therefore desirable to have 
available a simple and effective method enabling the constituents of 
multilayer materials to be separated with a view to being able to reemploy 
them in a manner which is comparable with virgin constituents, without 
affecting the performance of the new articles manufactured from 
constituents which are thus recovered. 
A number of methods have already been proposed for this purpose. It is thus 
especially known to immerse multilayer materials in appropriate organic 
solvents (for example xylene) at elevated temperature, with a view to 
separating some constituents by selective dissolving. However, such a 
method requires the use of organic solvents, which are costly and 
generally present safety and environmental problems, and a subsequent 
drying which is costly in energy. In addition, even if the temperature of 
the solvent is raised, treatment periods of more than an hour are 
commonplace. These disadvantages are particularly marked when the articles 
to be treated are thick, for example fragments of fuel tanks. This is 
because a preliminary optional grinding generally does not make it 
possible to reduce the thickness of the particles obtained, and this is 
detrimental to the rate at which they dissolve. Such a solution is 
therefore not suited to the treatment of large quantities of material. 
Consequently, the present invention aims to provide a process which is 
simple, fast and efficient, and which does not require large quantities of 
solvents to be employed. 
DETAILED DESCRIPTION OF THE INVENTION 
To this end the present invention relates to a process for separating the 
constituents of a multilayer material including at least one layer of a 
base plastic (A) and a layer of a plastic (B) which are separated by a 
layer of adhesive plastic (C), in which: 
(1) the material is heated to a temperature T1 between the crystallization 
temperature of the plastic B (Tc) and Tc--20.degree. C., 
(2) the material is then shredded by being subjected to shearing, at 
approximately the same temperature, so as to produce delamination and thus 
to convert the material into particles of small dimensions of two types, 
some (X) consisting essentially of base plastic (A) and others (Y) 
consisting essentially of plastic B and of adhesive plastic (C), and 
(3) the particles X and Y are subsequently separated by electrostatic 
separation. 
Plastic is intended to denote any polymer or polymer mixture. The polymers 
in question are preferably thermoplastic. Each of the plastics (A, B, C) 
may in addition optionally contain one or several conventional additives 
such as stabilizers, lubricants, antioxidants, pigments, flame retardants, 
fillers or reinforcing fillers, and the like. 
Good results have been obtained when the base plastic (A) represents more 
than 80% of the total weight of the plastics A, B and C, and in particular 
more than 90%. 
The base plastic (A) is chosen as a function of the mechanical and/or 
chemical properties which the material must have. Conventional polymers 
such as polyolefins or vinyl chloride polymers are generally employed for 
this purpose. The process according to the invention gives good results 
when the base plastic (A) consists essentially of one or more polyolefins 
chosen from the homopolymers and copolymers of ethylene or of propylene, 
and in particular of high density polyethylene (HDPE). 
The process according to the invention is found to be particularly 
advantageous and efficacious when the plastic B is a barrier plastic. A 
barrier plastic is intended to denote any plastic capable of forming a 
layer exhibiting a low permeability to particular fluids such as, for 
example, hydrocarbon-based fuels. Advantageous crystalline polymers which 
have such properties are especially polyamides, fluorine-containing 
polymers and ethylene-vinyl alcohol copolymers. The process according to 
the invention gives very good results when the plastic B consists 
essentially of one or more polymers chosen from polyamides and 
ethylene-vinyl alcohol copolymers, and very particularly of the latter. 
Plastic B often has a low adhesiveness to the conventional polymers of 
which the layer of base plastic generally essentially consists, and this 
often makes it necessary to resort to an adhesive plastic (C). 
The adhesive plastic (C) is chosen as a function of the nature of the 
plastics A and B. A compatibilized polyolefin, and in particular 
compatibilized polyethylene is frequently employed as adhesive plastic 
(C). The compatibilization can be obtained especially by grafting, in 
particular by means of a carboxylic acid anhydride, for example maleic 
anhydride. The adhesive plastic (C) preferably consists essentially of a 
polyolefin grafted with maleic anhydride, in particular polyethylene or 
polypropylene grafted with maleic anhydride. When the base plastic (A) 
consists essentially of polyethylene or of polypropylene the adhesive 
plastic (C) is advantageously a grafted polymer of the same kind (PE or PP 
respectively). 
Besides the abovementioned 3 layers A/C/B the multilayer material subjected 
to the process according to the invention may optionally include one or 
more other layers of identical or different nature. It may thus involve 
especially a material of symmetrical structure A1/C1/B/C2/A2, A1 and A2 
denoting layers of base plastics which are identical or different (in 
nature as in thickness), and C1 and C2 denoting layers of adhesive 
plastics which are identical or different. It may further involve an 
asymmetric structure, for example of the A1/C/B/C/A2/A1 type. Another 
example is that of a material comprising several barrier layers, such as 
A/C/B/C/A/C/B. 
Before the abovementioned stage (1) the process according to the invention 
may optionally include one or more conventional stages such as, for 
example, washing, or else chopping at ambient temperature, intended to 
reduce the material to fragments of medium dimensions (for example of the 
order of a few centimetres). 
The heating (1) can be carried out by any known means, for example by means 
of lamps or resistances emitting infrared radiation. The purpose of this 
heating is to bring the material to a temperature which is suitable with a 
view to shredding (2). 
Stage (2) of the process according to the invention consists in shredding 
the multilayer material at a specific temperature. Surprisingly, it has 
been found that the fact of subjecting the multilayer material to shearing 
forces at a temperature slightly lower than the crystallization 
temperature of the plastic B makes it possible to effect the separation 
of, on the one hand, A and, on the other hand B+C. It will be noted that 
the separation thus produced is original, insofar as the conventional 
methods of recycling multilayer materials comprising a barrier plastic 
have the objective of recovering the latter and otherwise yield a 
generally large quantity of a mixture of base plastic and of adhesive 
plastic, which is awkward to reexploit directly. In other words, an 
important characteristic of the process according to the invention is that 
it is a process which makes it possible to separate the base plastic from 
the other plastics. This is particularly advantageous insofar as, in the 
majority of cases, the base plastic (A) represents the mainvconstituent of 
the material by weight. Its recovery is therefore economically important. 
In fact, even though the barrier plastic is often more costly, it often 
represents only a small percentage of the total weight of the material. 
Thus, in the case of the 5-layer fuel tank described in the introduction, 
the weight of EVOH is generally of the order of 3 to 4% relative to the 
total weight of the tank. 
Another advantage of the process according to the invention is that it can 
take place in relatively simple equipment, which must, nevertheless, 
permit the temperature of its contents to be precisely controlled. The 
material is preferably shredded in an impeller mill. This type of 
equipment is well known as such; it generally consists of a rotary drum 
provided with blades at its periphery, rotating in a vessel to which 
blades are also secured. A model capable of producing particles from 
approximately 5 to 12 mm in size is generally chosen, these specific 
dimensions making it possible to perform the separation with a high 
efficiency. It is furthermore advantageous that the knife blades with 
which the mill is provided should not be too cutting, otherwise the 
materials would be cut up without being subjected to significant shearing 
forces. 
The equipment employed for the shredding (2) is preferably used in 
combination with means for thermal conditioning which make it possible to 
maintain the temperature therein within the abovementioned range. In 
general, the shredding produces additional heating of the material, with 
the result that it is often useful to cool the latter, for example with a 
stream of air at ambient temperature, in order to avoid clustering of the 
shredded particles. The same device is advantageously employed for cooling 
the particles and for removing them, by suction, out of the shredder. 
Similarly, before carrying out the electrostatic separation (3) it may be 
useful to remove the fines :(microparticles) produced by the shredding; a 
conventional device such as an air classifier may be employed for this 
purpose. 
The separation (3) of the particles X and Y is performed by electrostatic 
separation. 
Within the electrostatic separation stage a first substage consists in 
charging the particles, for example by corona discharge or else by 
subjecting them to friction. This may involve mutual friction, for example 
in a fluidized bed, or else friction of the particles on a moving member 
(drum, belt or the like) of an appropriate nature (glass, plastic, or the 
like). 
It is advantageous that after the shredding (2) the particles should be 
charged electrically, with a view to electrostatic separation, at a 
temperature (T3) of at least 50.degree. C. In order to bring the particles 
into this temperature range--if they are not yet in it--it is possible in 
particular to employ a conventional heating device such as an infrared 
lamp. It is also possible advantageously to employ a charging equipment 
provided with heating elements such as electrical resistances. It has been 
found that when the charging of the particles was thus carried out at an 
elevated temperature, the selectivity of the subsequent electrostatic 
separation could be surprisingly improved. An advantage of this 
alternative method, in the context of the whole process according to the 
invention, is that at the end of the shredding (2) the particles are 
already at an elevated temperature, and this significantly reduces the 
energy required for the abovementioned heating, as well as the duration of 
this heating. Another advantage of this alternative method is that it 
allows the actual electrostatic separation to be performed with heating, 
and this has been found advantageous in the present context. 
It is furthermore desirable that the substage of charging the particles 
should take place in an atmosphere of the lowest possible humidity. For 
this purpose a gas which has a low moisture content, for example dry air 
or an inert gas such as nitrogen, may in particular be injected into the 
charging equipment employed. 
According to an alternative form which has been found advantageous in this 
context, after the shredding (2) the particles to be separated are charged 
electrically by being brought into contact with a movable member 
consisting essentially, at least superficially, of a plastic similar to 
the adhesive plastic (C). A plastic similar to the adhesive plastic (C) is 
intended to denote a plastic which exhibits similar triboelectric 
characteristics. The surface of the movable member preferably consists of 
a plastic identical with the adhesive plastic (C). 
According to another advantageous alternative form, which may be combined 
with the preceding one, the particles to be separated are charged 
electrically by passing through a hollow rotary drum provided internally 
with components which are approximately perpendicular to its internal 
surface. Such components make it possible to intensify the friction of the 
particles on the internal surface of the drum, and this increases their 
electric charge. Examples of such components which may be employed are one 
or more rods, small plates and/or paddles which are approximately parallel 
to the axis of the drum. The drum is advantageously cylindrical in shape; 
it may, however, have a section other than circular, for example 
polygonal, in particular octagonal. The axis of the drum may be horizontal 
or slightly inclined so as to make it easier for the particles to travel 
forward from one end to the other. The angle of inclination and the speed 
of rotation of the drum allow the residence time of the particles within 
it to be adjusted. 
Once they have been electrically charged, the particles can be easily 
separated, in a manner known per se, by falling between two deflecting 
electrodes exhibiting a continuous and high potential difference. 
According to a preferred alternative form the actual separation is 
performed by depositing the precharged particles on the external surface 
of a rotary drum whose axis is horizontal and which is connected to a 
terminal of a DC voltage source, the other terminal thereof being 
connected to a deflecting electrode consisting, for example, of a planar 
plate placed on the side of the said drum, parallel to its axis, generally 
at a distance of a few centimetres. In this way the particles carrying an 
electric charge of opposite sign to the deflecting electrode will be 
diverted towards the latter on falling from the drum, and this allows the 
particles to be separated according to their charge and hence according to 
their nature. It is preferred that the drum in question should at least 
superficially consist of an insulating material, for example of PVC, its 
internal surface being provided with electrically conductive means or 
coating ensuring a homogeneous distribution of the electric charges. The 
use of an insulating drum allows high electric fields to be obtained, of 
the order of 3 to 15 kV/cm, in contrast to the known processes employing a 
conductive drum, with which the electric field cannot exceed 2 or 3 kV/cm 
without leading to a risk of breakdown. In the process according to the 
invention, when employing an insulated separating drum, fields of 6 to 8 
kV/cm have given very good results. 
The electrostatic separation is preferably performed with heating, the 
particles being at a temperature of at least 50.degree. C. 
After separation of the particles X and Y the particles Y are 
advantageously subjected to a subsequent stage (4) of separation making it 
possible to collect, on the one hand, the plastic B and, on the other 
hand, the adhesive plastic (C). To give an example, if the plastic B 
consists of EVOH, it can be recovered by dissolving the Y particles in a 
water/alcohol solution, preferably with heating (for example a 
water/methanol mixture at 70.degree. C.). 
The present invention also relates to a process for recycling fuel tanks 
including at least one layer of a base plastic (A) and a layer of a 
barrier plastic (B) which are separated by a layer of adhesive plastic 
(C), or fragments of such tanks, in which the tanks or tank fragments are 
subjected to the process of separation defined above. As already 
indicated, this process can be applied particularly well to tanks in which 
the layer(s) of barrier plastic (B) consists (consist) essentially of 
EVOH, the layer(s) of adhesive plastic (C) consisting essentially of 
PE-g-MA.

EXAMPLE 
Fragments of fuel tanks with 5 layers of the HDPE/PE-g-MA/EVOH/PE-g-MA/HDPE 
type were subjected to the process according to the invention. These 
fragments, the mean length of which was of the order of 10 to 20 cm, were 
first heated to approximately 150.degree. C. and then shredded in an 
impeller mill (of Rapid.RTM.trademark, model 3026), at the same 
temperature. The particles thus obtained, of a mean size of approximately 
8 mm, were next charged electrically by passing through a hollow rotary 
drum (length: 100 cm, internal diameter: 40 cm, speed of rotation: 30 to 
70 rev/min) arranged horizontally, the internal wall of which was coated 
with PE-g-MA. The mean temperature within the drum was approximately 
80.degree. C. On leaving this first drum the particles were deposited on 
the upper generatrix of a second rotary drum, made of PVC (diameter: 40 
cm, length: 40 cm, thickness: 10 mm, speed of rotation: 15 to 50 rev/min), 
the axis of which was arranged horizontally and perpendicularly to that of 
the first drum. A source of DC voltage (70 kV) was connected, on the one 
hand, to the metal coating applied to the internal surface of the second 
drum and, on the other hand, to a metal plate placed laterally facing the 
second drum, at a distance of 3 to 8 cm. 
A number of trials were carried out, with mean particle throughputs of 30 
to 120 kg/h. 
The separation thus produced made it possible to collect, on the one hand, 
HDPE particles and, on the other hand, particles of EVOH/PE-g-MA mixture, 
and to do this with an excellent selectivity, since the HDPE thus 
recovered exhibited a purity of more than 99.5% by weight.