Chemo-mechanical stripping of paint from plastic parts

A process for stripping of used parts or reject parts, in particular of polycarbonate (PC) or of a polycarbonate blend, preferably with polybutylene terephthalate (PBT/PC), for the purpose of recycling the plastic. In this process the plastics parts are first mechanically comminuted to a pourable size and then are circulated together with anhydrous alkalified diethylene glycol or propylene glycol in a mass with mutual abrasion of the plastics chips at elevated temperatures of at least 50.degree. C., preferably from 80.degree. to 100.degree. C., until complete removal of the paint. Thereafter the stripped plastics chips are separated from the stripping agent, flushed and then dried. The quantity of glycol added corresponds to from 1 to 10% of the volume of the mass to be treated, so that the circulated parts are only wetted by the glycol, but with stripping agent being circulated continuously through the treatment space of the mass from a relatively large reservoir. The circulation of the mass of parts is preferably carried out in a closable free-fall or positive mixer or in a rotary tube mixer. During the abrasive circulation the plastics parts are preferably admixed with abrasive particles whose specific apparent density corresponds roughly to the specific apparent density of the plastics parts.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to a process for the stripping of paint from plastics 
parts such as used parts or reject parts with the aim of recycling the 
stripped plastic, in which the plastics parts are first mechanically 
comminuted to a pourable size, in which the plastic chips are circulated 
at elevated temperature together with an alkalified glycol, and in which 
after a certain treatment time the stripped plastics chips are separated 
from the stripping agent, flushed and then dried. British Patent Document 
GB-B 1 134 967 describes a process of this general type. 
In recent years there has been a continual increase in the use of plastics 
in the automotive industry. In order to adapt the plastics, for example 
polycarbonates, even better to their use in the automotive sector, their 
surfaces are coated. Coatings are used to afford protection, for example 
against environmental effects, to bumpers and other plastics parts which 
form an integrated component in the body, and to integrate these parts 
visually into the color scheme of the motor vehicle. The recycling of the 
materials mentioned gives rise to quite specific problems. The colored 
layer of two-component paints breaks down mechanically when the plastics 
are recycled into small tinsel-like particles of paint which lead to an 
impairment of the recovered raw materials, and consequently this material 
can then only be processed into low-grade products. The particles of paint 
which remain behind cause, for example, a mechanical weakening of articles 
produced from recycled material, since again and again they interrupt an 
integral material bond and constitute sites where cracks can start, in 
other words micronotches. Articles made from recycled plastic which has 
not been stripped beforehand have only a very low tensile strength and 
notched impact strength and a likewise very low tear strength and 
elongation at break in comparison to virgin plastic of the same type. For 
this reason and, on the other hand, because stripping is a comparatively 
expensive process, the stripping of used plastics has generally been 
abandoned and used parts of this kind have been disposed of in other ways. 
In this context there has been no lack of attempts to develop different 
processes for the stripping of plastics. 
In particle blasting processes, the consumption of abrasive agent for the 
stripping operation is a concomitant phenomenon which on the one hand 
causes an unfavorable rise in the consumption of the material for 
reprocessing operation and on the other hand, by the incorporation or 
diffusion of fragments of abrasive agent into the base material, leads to 
impairment of the recycled material. Moreover, uniform or complete paint 
removal cannot be achieved. Furthermore, in cases in which each workpiece 
is treated as a whole, then these processes have to be carried out 
individually and locally, and therefore manually, so that it is not 
possible to employ these processes in the stripping of used plastics at an 
acceptable level of cost. 
Water blasting processes are unsuitable for plastics parts because of the 
low density and weight of the latter, since the plastics parts move away 
under the pressure of the blasting jet. In addition, the angle of the jet 
is an important parameter in these processes and cannot always be 
maintained when the surfaces are sharply curved or structured. 
In the so-called melt filtering process in which the non-plastifiable 
constituents such as, for example, the particles of paint, are filtered 
off from the plastified polymer melt during extrusion, the degree of 
filter fineness required for adequate removal of the fine particles of 
paint and the considerable filter area required for this and the resulting 
high filter-area load are disadvantageous. According to investigations by 
the applicant, the high extrusion temperature which is required for 
satisfactory flowability through the filter and the marked differential 
pressure both degrade the polymer matrix of the plastic. In an expanded 
process, i.e., with combined abrasive treatment and filtering of the 
polymer melt, a large proportion of the paint is first removed by way of a 
cone-type press and a pin mill, in order to reduce the amount of residual 
paint before filtering, the required filter area in the subsequent melt 
filtration and the filter load during this filtration is large. An 
objectionable feature of this process is again the damage to the plastic, 
both by the severe abrasive processes and by the melt filtration. 
In a process for stripping the paint from complete bumper panels, which 
process is recommended by one vehicle manufacturer, these panels are 
softened in an oven and pressed flat before the paint is abrasively 
removed by means of brushes. A disadvantage of this process for the 
treatment of whole plastics parts is that noncomminuted and therefore 
bulky plastics parts have to be collected and treated centrally. Such 
bulky parts have a very low transportation density, which has an adverse 
effect on the transport costs and on the gross weight to be transported in 
comparison with the ecological benefits. 
Although conceivable, chemical stripping with halogenated hydrocarbons 
(chlorofluorocarbons, chlorocarbons) is prohibited by law because of the 
environmental damage caused by these substances. 
Another vehicle manufacturer specifics a process for the chemical stripping 
of plastics parts which uses an organic sulphide solution at from 
80.degree. to 100.degree. C. This solution is not environmentally 
compatible. In addition, because of the ether and alcohol components there 
is the danger of autoignition or explosion, and therefore the possibility 
of simultaneous mechanical treatment can also be abandoned. 
English Patent 1 134 967 describes a process for the recovery of film 
wastes in the photographic and film industry, which wastes consist of 
apparently expensive polyester, specifically and preferably 
poly-1,4-cyclohexanedimethylene terephthalate, thus making recovery 
worthwhile. The photosensitive layer can, for example, be softened in hot 
water and removed. However, before recovery of the base material of the 
film it is also necessary for an approximately 25 .mu.m, thin layer of 
adhesion promoter to be removed. And the layer of adhesion promoter 
consists in turn of a copolymeric vinylidene halide, preferably a 
copolymeric vinylidene chloride, which contains;: 
from 30 to 98% by weight, preferably at least 35% by weight, of vinylidene 
halides, 
from 0.5 to 40% by weight, preferably from 10 to 15% by weight, of 
acrylonitriles and/or preferably an additional 1 to 12% by weight of alkyl 
esters of an acrylic, methacrylic or itaconic acid, and 
from 1 to 25% by weight of an acrylic, methacrylic or itaconic acid. 
In order to remove this layer of adhesion promoter, the film material, 
which has been comminuted to chips having a lateral length of about 1 cm, 
is brought into contact with at least six times its weight, preferably 
from 10 to 50 times the quantity, of glycol which has been alkalified with 
sodium hydroxide or potassium hydroxide solution, at least 90.degree. C. 
and preferably from 145.degree. to 155.degree. C.; in other words, it is 
suspended in the alkalified glycol and this suspension is stirred or 
circulated for a length of time, preferably about 2 minutes, until at 
least the majority of the chips have been stripped. Subsequently the 
stripped chips and the glycol are separated from one another. According to 
the proposal, the glycol to be used is preferably ethylene glycol and 
specifically 1,4-cyclohexanedimethanol, in other words precisely that 
glycol which in its chemical structure corresponds to the specific 
polyester of the base material to be stripped. By means of the glycol 
which is recommended for the chemical stripping and is chemically related 
to the base material, this base material, at least at elevated 
temperature, is broken up or dissolved and in this way the bond with the 
layer to be removed is destroyed or broken, the layer to be removed coming 
off in sheets. In this process a partial breaking-up or dissolution of the 
base material is not merely tolerated but indeed is a specific aim. And 
indeed the upper limit recommended for the treatment temperature is a 
temperature at which approximately 5% by weight of the base material may 
be dissolved, but in any case less than about 200.degree. C. The proposed 
stripping process hails from a time (1966) when ecological considerations 
did not as yet play any decisive role in process design, but instead 
considerations of cost were the priority. Indeed, the use of ethylene 
glycol in such large amounts is highly objectionable from an ecological 
point of view, since this glycol is classified as "slightly toxic" on the 
toxicity scale. 
An object of the invention is to specify a process which achieves complete 
removal of a coating from plastics parts with minimal damage to the 
plastics material while using environmentally compatible means and with a 
low consumption of material and energy, and which, at least in the case of 
plastics parts which are pourable, or have been precomminuted so as to 
render them pourable, can be carried out in a process which allows 
automation. 
Starting from the process on which the generic art is based this object is 
achieved in accordance with the invention by means of a process wherein, 
in order to strip the coated plastics parts, they are circulated together 
with a stripping agent of an anhydrous, alkalified diethylene glycol or 
propylene glycol, in a mass with mutual abrasion of the plastics parts at 
at least 50.degree. C., the quantity of stripping agent added being from 1 
to 10% of the volume of mass to be treated, so that the circulated parts 
are only wetted by the stripping agent. In especially preferred 
embodiments, the circulation is carried out at a temperature between 
80.degree. C. and 100.degree. C. The chemical dissolution effect is 
brought about by means of an environmentally compatible two-component 
mixture of, on the one hand, diethylene glycol and/or propylene glycol 
and, on the other hand, potassium hydroxide and/or sodium hydroxide. 
Diethylene glycol is in water hazard class zero. After neutralization it 
is acceptable to pass on potassium hydroxide, as a dietary salt solution, 
for effluent treatment. In the course of stripping the polycarbonate is 
hydrolysed, a reaction which does not liberate any by-products classified 
as objectionable. The mechano-abrasive effect is designed so that no 
damage occurs to the plastics material but at the same time so that the 
paint is removed on all sides of the particle, irrespective of the 
specific particle form. Treatment in the reactor is carried out 
automatically and permits the stripping of various particle forms after 
appropriate precomminution. The reactor is closed during the treatment in 
order to save heat energy and to minimize the loss of chemicals by 
evaporation. 
The advantages of the inventive embodiment of the stripping process are as 
follows: 
complete removal of the paint from the plastics parts can be achieved; 
minimal damage to the plastics parts; 
minimal consumption of energy and processing auxiliaries; 
low process costs in comparison to the new value of the recycled material; 
only environmentally unobjectionable means are used; 
no ecologically objectionable emissions or residues; 
the process can be automated and can be operated continuously. 
The use of the process described is appropriate for all cases in which a 
polycarbonate or polycarbonate blend can be subjected to high-grade 
material recycling by stripping. This is the case in particular for the 
bumpers of the top car class, which consists of this material to the 
extent of more than 50% and more than 4 kg per part. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
The text below reports on a trial operation of a stripping operation for 
bumpers at the Applicant Company (assignee of this application) on a 
semi-industrial scale, on the basis of a flow diagram which is shown in 
the single drawing FIGURE. 
The front and rear bumpers of motor vehicle consist of a large number of 
individual components made from various materials. From the dealerships 
and contract partners of the Applicant Company the old bumpers from 
vehicles involved in accidents which have been repaired are picked up by a 
waste management company, collected centrally and broken down into the 
various individual parts which are sorted by material. The largest 
component of the bumpers hailing from the Applicant Company is the 
panelling; on most types of vehicles this panelling consists of a plastics 
blend of polycarbonate and polybutylene terephthalate (PC/PBT or PBT/PC), 
which is to be stripped in the process example described below. However, 
it should be pointed out now that the stripping process described is also 
suitable for other coated plastics, for example EUPP. 
The aim of reprocessing coated components at a high level is to remove the 
paint completely. For this purpose the bumpers are first precomminuted to 
a pourable size; in this context an advantageous spectrum of sizes is one 
where the greatest edge length or overall length of the parts measures 
less than 20 cm for about 90% by weight of the parts and measures more 
than 1 mm for at least 99% by weight of the parts. Instead of specifying a 
size classification the precommminuted parts can also be classified by 
weight: in this case the aim would be a spectrum of parts in which about 
90% by weight of the parts weighed less than 50 g each and at least 99.5% 
of the parts weighed more than 5 mg each. The approximate limit for the 
upper size of parts is an approximate marker for the region below which a 
fairly homogeneous and ordered pouring and circulation of the parts is 
still possible. Going below the lower limit would be disadvantageous 
because degradation of the parts and/or of the plastic only remains still 
tolerable above this limit. The panelling of used and dismantled bumpers 
was comminuted by passing it a plurality of times through a twin-shaft 
cutter with low thermal and mechanical stress to give uniform chips having 
an edge length of about 25 mm and a narrow spectrum of parts. These parts 
could be treated in the reactor without difficulty. 
The PC/PBT or PBT/PC pieces obtained by comminution are placed in a reactor 
in 50 kg batches. For this purpose it was in fact possible to use a 
finishing-grinding unit from Spaleck which is normally used in the 
manufacture of parts produced en masse, for deburring, cleaning, polishing 
or similar surface treatments. The inner lining of this finishing-grinding 
unit was modified to make it resistant to chemicals and heat. In addition, 
the finishing-grinding unit was supplemented to meet the requirements for 
the reactor with a circulation pump apparatus, a heating device and a 
closure cap for the filling aperture. The contents of the vessel are set 
in a rotationally symmetrical, circulatory movement with a funnel-shaped 
space in the middle by means of the stirring paddles at the bottom of the 
finishing-grinding unit used. In this way the amount of mechanical energy 
supplied to the mass of particles can be adjusted in a simple manner. 
It is in fact conceivable, apart from the necessary addition of chemicals 
and the elevation of temperature, only to treat the precomminuted plastics 
parts alone in the reactor. However, it has proved advantageous to add 
abrasive particles to the mass of parts, these particles being produced 
from a material which is resistant to alkali and glycol, in particular 
from stainless steel plate. However, in comparison with the mass of 
plastics parts these abrasive particles must not be too light and must not 
be too heavy, so that they do not separate out during circulation. It 
appears to be advisable for the co-circulated abrasive particles to have a 
specific apparent density which corresponds to from 0.2 to 5 times, 
preferably from 0.5 to 2 times, the specific apparent density of the 
plastics parts. In this context commercially available packing media for 
washing towers or the like are suitable. As far as the amount added is 
concerned, it is recommended to add the abrasive particles to the 
circulating mass in such a quantity that, based on the overall volume of 
the mass, the proportion of abrasive particles is from about 1 to 20% by 
volume, preferably from 5 to 10% by volume thereof. 
While thermal and mechanical energy is supplied, the plastics parts are 
treated in the reactor with a liquid mixture of chemicals for about 15 
minutes, the quantity of mixture of chemicals in the reactor being from 1 
to 10% of the volume of the mass to be treated, such that the circulated 
parts are only wetted by the mixture and in no way are suspended in it. 
This mixture of chemicals is continuously passed through the reactor and 
pumped in circulation at a volume flow rate of from about 5 to 10 l/min 
(liters/minute), a chemical reservoir of about 40 l being employed in this 
circulation system. The mixture of chemicals employed is, preferably 
anhydrous, alkalified diethylene glycol or propylene glycol. After this 
period the paint is detached and is present in the chemicals in part in 
the form of solid, suspended particles. 
The chemicals are drained off from the reactor and the paint particles 
present in them are filtered out or centrifuged. The mixture can be 
employed approximately ten times although with each passage a certain 
amount is lost, remaining dispersed over the plastics parts in the 
reactor. 
After being used ten times the mixture of chemicals is passed to a 
rectification plant. A large proportion of the chemicals is recovered 
using this plant, while the reminder is rendered inert in the on-site 
effluent treatment plant which is present at the works. The products from 
this reprocessing operation, water and salt, are classified as not 
hazardous in water and can be discharged into the sewer system without 
problems. 
In order to separate the residual chemicals remaining in the reactor from 
the plastics parts, centrifugation is carried out after each batch and the 
reactor is then flushed with 50 l of process water. Particles of paint 
which have still not been removed from the reactor with the chemicals are 
then flushed out with water. 
The particles of paint present in chemicals and water are filtered out. 
Residual moisture is pressed out of the filter residues in a filter press. 
The filtrates pass back into the liquid circuits, while the "dried" 
residues, comprising about 0.5 kg of paint, 1 kg of water and 0.05 to 0.1 
kg of a mixture of chemicals per batch, have to be disposed of separately. 
After each passage the process water is purified to remove chemicals in a 
neutralization unit. This procedure allows the water to be used five times 
before becoming unusable as flushing liquid. It can then be discharged to 
the on-site effluent treatment plant at the works as unproblematical 
effluent. 
The stripped and flushed PC/PBT or PBT/PC parts are centrifuged, removed 
from the reactor and, after separating off the abrasive particles, are 
placed in a convection oven for drying. The stripped plastics parts are 
subsequently stabilized in an extruder, granulated and packed ready for 
dispatch. 
The operation of the plant for processing PC/PBT requires energy, 
personnel, transport resources, auxiliaries, maintenance and waste 
disposal. Whereas the costs for personnel and maintenance are to a certain 
extent independent of the level of utilization, the costs for energy, 
transport, auxiliaries and disposal are dependent on the throughput and 
enter into the cost calculation. However, experience with the trials 
carried out on the semi-industrial scale by the applicant makes it 
possible to conclude that the recovery costs, including the direct and 
indirect costs of stripping, are about 20 to 40% of the costs of procuring 
the new plastic. 
Material analysis of the plastic recycled in the manner described by melt 
index testing resulted in values which compared with those of the original 
characteristics of the components. The good properties of the stripped and 
reprocessed PC/PBT make it possible to reuse the material, at least as an 
admixture in the production of new bumpers. This application possibility 
is evidence of the high quality of the recycled material, since the 
applicant demands the very highest quality for bumpers. In accordance with 
the cautious estimation of experts, a proportion of about 10% of recycled 
material in the bumper material will have no adverse effect on the quality 
of the components; proportions of recycled material which are 
substantially higher than this would appear to be entirely realistic aims. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.