MANUFACTURE OF MOLDED COMPOSITE PRODUCTS FROM SCRAP PLASTICS

FIELD OF THE INVENTION

This invention relates to waste product recycling processes,

and more particularly to processes for the recovery and reuse of scrap

plastics materials.

BACKGROUND OF THE INVENTION

Scrap plastics materials as collected from refuse sites,

manufacturing operation wastes, household wastes, "fluff' from shredded

automobiles and the like are commonly complex mixtures of many diverse

waste materials - paper, thermoplastic products, cured thermosetting

products, metals, fibrous products, etc. It is, difficult and commonly

uneconomic to proceed through one or more sorting and separating steps

before the recycling process. Plastics materials are particularly difficult in

this regard, since successful recycling and reuse of plastics materials in

useful products may require the sorting of the scrap products into different

types and grades of such plastics.

Particularly difficult materials to handle in complex scrap

mixtures are cured thermoset resins. Conventionally, these will not melt

for remolding purposes, even after separation and isolation from scrap

mixtures. Also, they are commonly associated with fibrous reinforcements

such as glass fibres, which are equally difficult to separate and reuse.

Accordingly, there is a need for a process which will permit

the recycling and reuse into useful products, of complex mixtures of waste

materials which include in their composition substantial quantities of cured

thermoset plastics materials.

DISCUSSION OF THE PRIOR ART

Attempts have been made in the past to make chipboard-like

products using thermoplastics-contaimng scrap materials as the binder or

glue therein. As is well known, chipboard is conventionally made of wood

chips and liquid/powder uncured thermoset resins. The resins, which act

as the binder or glue, are pressed into chipboard products in continuous or

discontinuous processes, and subsequently cured under heat and pressure so

that the wood chips become held together by the polymerized and cured

resins (melamine, phenolics, polyurethanes, etc.). Attempts to use mixed

thermoplastics scrap resins, sometimes contaminated with other substances

such as paper, metals, textiles, wood, etc. have focused on extrusion,

kneading and injection molding processes. These allow recycling of

thermoplastic waste directly into finished or semi-finished products, without

separation of the components of the waste or intensive washing thereof.

U.S. Patent 4,187,352 Klobbie, for example, discloses a

process in which unsorted thermoplastic synthetic resin waste material is

formed into an article having the working and processing properties of

wood by subjecting the mixture to a mixing operation in a housing

including a screw/kneading member so that it is extruded into a finished

product.

The disadvantage of such kneading/extrusion/injection molding

processes is that the resulting products are of widely variable and

inconsistent properties because of the incompatibility of the ingredients of

the mixture with one another. When fillers and reinforcements such as

wood chips, glass fibres and the like are added to such mixtures of

materials and then the mixture is subjected to an injection molding or

extrusion process, the strength properties thereof are substantially reduced,

largely due to the mechanical destruction or impairment of the fibres during

the extrusion or injection molding process.

U.S. Patent 3,956,541 Pringle, and its companion patents

numbers 4,016,232 and 4,016,233, disclose a process for making flexible

structural members, namely cable reels, using scrap wire and cable

insulation, namely polyvinylchloride and polyethylene,, and possibly using

other scrap materials as well. The scraps are shredded in combination with

the wire remnants, and the wire is separated from the shredded insulation.

This scrap is then mixed with phenolic resin, zinc stearate and wood filler,

and compression molded to form flexible objects. A non-homogeneous

product is formed, containing discreet zones of thermoplastic material,

acting like an impact modifier to the otherwise brittle phenolics/wood

compound.

U.S. Patent 4,279,790 Nakajima, and its companion Patent

4,339,363, describe the preparation of composite material compositions of

waste paper, thermoplastic resins and other additives, mixed together as the

paper is dried from a slurry condition. The inclusion of synthetic rubber,

normally a thermoset, is suggested in this patent. The final products are

formed by injection molding.

U.S. Patent 4,364,979 Dutton discloses a substitute for wood

in particle boards, which comprises waste materials such as chicory roots

and coffee grounds as obtained from an instant coffee manufacturing plant.

The binder which is proposed is new, previously unused thermoset resin

such as urea-formaldehyde resin.

U.S. Patent 4,396,566 Brinkmann discloses a process for the

continuous manufacture of sheeting from thermoplastic synthetic resins, in

which the resin is used in the form of particles and passed continuously

through a preheating zone, and then through a treatment zone in which it

is pressed and compacted to form a visually appealing flexible sheet

material. The possibility of using waste strips of thermoplastic synthetic

resin is disclosed, but no use of fillers is suggested.

U.S. Patent 4,427,818 Prusinski, discloses building blocks

made from contaminated scrap materials by a process of mixing and

heating, then cooling in molds. It relates to the use of a widely varying

composition including thermoplastic resins, but does not disclose the use of

scraps containing mixtures of thermoplastic and cured thermosetting resins.

It is an object of the present invention to provide a novel

process for the recycle and reuse of scrap materials containing mixtures of

thermoplastic and cured thermosetting resins.

It is a further and more specific object of the present invention

to provide a process of making sheet and board materials of the chipboard

type, having reinforcing materials bound together by scrap thermoplastic-

thermosetting resin mixtures.

SUMMARY OF THE INVENTION

It has now been found that scrap mixtures containing both

thermoplastic and cured thermosetting resins in substantial quantities can be

used as the binder in preformed products containing reinforcing materials

and/or fillers, in a process of compression molding/laminating of the

mixtures thereof, provided that the scrap material mixture is first reduced

to a fine particle, powder size and is homogenized to form a macro

homogeneous powder mixture, before it is mixed and intimately dispersed

with the filler or reinforcement prior to compression molding. The process

of the invention allows the utilization of mixed scrap material, without the

need for separation of the individual components thereof, and the formation

there from of chipboard-like products of good physical properties, avoiding

the deterioration of properties normally obtained by injection molding or

extrusion of such products.

Thus, according to the present invention, there is provided a

process of recycling and molding scrap plastics material into products of

predetermined shape, the scrap material comprising a mixture of

thermoplastic material and thermosetting material and optionally containing

other organic or inorganic ingredients, the process comprising the steps of:

(a) shredding and milling the mixture to reduce it to a fine particle size

such that substantially all the particles thereof have a maximum

dimension not greater than about 1 mm;

(b) homogenizing the fine particle size mixture so formed into a free-

flowing, macrohomogeneous powdered form;

(c) warming the homogenized mixture so formed to an elevated

temperature which is at least 80°C but is one at which the mixture

maintains its free-flowing condition;

(d) dry blending the warm mixture with at least one additive material

selected from reinforcing materials and fillers in an amount such that

the blend contains at least 10% by weight of homogenized

thermoplastic components mixture, and forming an intimately

dispersed blend thereof;

(e) compression molding the blend at elevated temperatures and

pressures into a product of predetermined shape.

By the term "macrohomogeneous" as used herein, there is

meant a product which, on sampling and analyzing several macro

extractable aliquots, e.g. of weight ten grams each, substantially identical

analytical results for components are obtained, even though analysis of

micro samples under a microscope may reveal the presence of different

particles in different quantities. The macrohomogeneous product formed

during the process of the present invention is a powdered, free-flowing

particulate material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The scrap materials which can be recycled and used in the

present invention can be of very wide and diverse composition. They

should preferably contain a minimum of 20% by weight of thermoplastics

materials, such as polyethylene, polypropylene, polystyrene, impact

polystyrene, polyvinyl chloride, acrylonitrile-butadiene-styrene resins,

expanded polypropylene, polyamides such as nylon 66, polyesters such as

polyethylene terephthalate or polybutylene terephthalate, polyacrylates,

polymethylmethacrylates, polyacrylonitrile, etc., and mixtures of two or

more thereof; in fact, typical plastics and plastics mixtures which would be

found in a random sampling of household wastes and industrial plastics

scraps. They can be contaminated with (i.e. contain about 2-5% by weight)

or in fact contain substantial quantities of (10-30% or even up to 50% by

weight) cured thermoset plastics scrap, e.g. polyester thermoset, epoxy,

polyurethane, melamine, urea-formaldehyde, cross-linked or cured

polybutadiene polyisoprene, poly(butadiene-styrene), butyl, ethylene-

propylene-diene rubbers, SMC (sheet molding compounds), S-RIM

(structural resin injection moldings), RTM (resin transfer moldings), RRIM

(reinforced resin injection molding-thermoset resins reinforced with fibres

of glass, Kevlar, carbon, etc.) and mixtures of two or more thereof. They

can contain other scrap materials also, such as waste paper, cellulosic

fibres, rayons, clay, ceramics, glass, metals such as steel, aluminum and

brass, and vegetable materials as commonly found in household and

industrial wastes.

The mixed scrap material is first shredded and milled so as to

reduce its particle size below 1 mm maximum dimension, and preferably

below 0.5 mm maximum dimension. This can be done in a conventional

shredder or milling machine. The material is then subsequently

advantageously screened, to remove therefrom particles having a size

greater than 1 mm, i.e. materials whose particle size has not been

sufficiently reduced by the conventional shredding and milling process. It

is normally economically advantageous to remove such particles rather than

to expend excess energies on a special milling process which will reduce all

of the particles to the 1 mm size or less.

Next, a homogenization process takes place using conventional

high speed mixing and homogenizing apparatus. This can be done in a dry

blender, or in a wet slurry form, e.g. using an aqueous slurry which can

also serve as a cleaning and separation medium, to dissolve away some of

the components of the mixture which are water soluble and which would

not contribute to the properties of the final product. As a result of this

process, a macrohomogeneous, powdered material is formed of the mixed

scrap. If a wet homogenizing process has been adopted, the mixture is

dried. In any event, a substantially free-flowing powdered particulate

material is obtained.

Next, the homogenized material is warmed, to an elevated

temperature at which it still retains its free-flowing characteristics. This

temperature is suitably at least 80°C, but should not be above the softening

point of the majority of the thermoplastic components making up a

substantial proportion of the thermoplastic portion of the mixture.

Typically, the temperature will be between 80°C and 160°C.

Then the warmed homogenized scrap mixture is blended with a reinforcing material and/or a filler material. This is accomplished by dry

blending, e.g. using a dry blender apparatus.

The finished products produced by the processes of the present

invention largely derive their physical properties from the reinforcing

materials or fillers which are incorporated therein at this stage. When a

product of low tensile strength is required, e.g. for use as a floor tile, then

fillers alone may be used as the additive material, e.g. additional cured

thermoset powder, wood powder, clays and the like. To improve the

surface bonding of the fillers to the thermoplastic portions of the scrap

mixture, and thereby enhance the strength of the final products, the fillers

may bepre-treated on the surface, or otherwise impregnated, with adhesion-

enhancing chemicals such as a combination of different peroxides. Also,

metallic powder, ferrites or other metallic oxides can be added to the fillers

to provide, in the interphase between the fillers and the thermoplastic

portion of the mixture, an area which can subsequently be heated in an

electromagnetic high frequency field for promoting this bonding. In

another alternative, this interphase can be enriched with highly polar

substances so as to enable the compound to be heated in a microwave

heating station, again to improve the adhesive bond between the filler

surface and the thermoplastic resin component of the mixture.

The fillers can also be impregnated with uncured liquid/solid

thermoset resins (such as phenolics, melamine, polyurethanes etc.) so that

when additional heat is applied during the compression molding/laminating

process to soften or melt the thermoplastic portion of the blend, this will

also initiate the curing process of the uncured thermoset resins. This adds

additional strength to the product at higher temperatures, thus adding

temperature resistance to the finished product. It also reduces production

cycle time and increases production output, by allowing the removal of the

product from the die, belt or rolls whilst still hot. Because of its improved

high temperature strength, such a product can be removed from the forming

machinery at a higher temperature compared to a product relying on the

adhesion of the thermoplastic portion only, since the strength of such a

product only develops at a lower temperature, when the thermoplastic

portion solidifies.

Most commonly, it is required to provide products with

improved physical properties over those conferred by the scrap material,

and so it is usual to dry blend the mixed scrap material with reinforcing

material as well as or instead of fillers. Appropriate reinforcing materials

include thermoset/composite chips containing glass fibres or other

reinforcements as typically found in SMC-, S-RIM-, RTM-, RRIM-, etc.

molded products, glass fibres, jute fibres, mineral fibres, synthetic

polymeric fibres, carbon fibres, natural textile fibres. It is preferred that

the fibrous reinforcing materials have a minimum length of about 10 mm

in order to act as a reinforcement. Fibres longer than about 30 mm can be

used, but beyond that length no further improvement in reinforcement is

obtained.

The compositions according to the invention preferably contain

a minimum of 10 parts by weight of thermoplastic material derived from

the macro-homogeneous scrap, with correspondingly 90 parts by weight of

total other material namely other components of the scrap including residues

of thermoset materials, added reinforcing materials and/or added fillers.

They preferably contain a maximum of 85 parts by weight of thermoplastic

material derived from the macro-homogeneous scrap, but correspondingly

15 parts by weight of total other materials.

When filler alone is being used, without reinforcement, an

amount of 70 parts by weight of the filler (i.e. total of all non-thermoplastic

and non-reinforcing materials), thereby providing for a minimum 30 parts

by weight of scrap-derived thermoplastics, should not be exceeded. A

preferred range of all non-thermoplastic components is from 25 - 70% by

weight of the total blend. A typical composition according to the invention

comprises 25 parts by weight of mixed thermoplastic derived from the

homogenized scrap, 25 parts by weight of other scrap material including

thermosets, and 50 parts by weight of optionally pre-treated reinforcing

material.

The reinforcing materials can, if desired, be chemically pre-

treated on their surfaces, as described in connection with the fillers, to

improve the bonding between them and the thermoplastic component of the

mixed scrap, in the final products. Such chemical surface pre-treatment

may also enable the use of microwave or high frequency fields to heat the

compound prior to molding, and may also serve to impregnate the product

with uncured thermoset resins, both of which factors can enhance the

properties of the final product, and can also speed up the production cycle

by means described above in connection with the fillers.

The reinforcing materials are preheated to at least the same

temperature as that to which the mixed thermoplastic scrap is preheated,

prior to blending these two materials together. This avoids the undesirable

effects of cooling of the homogenized scrap mixture on contact with the

reinforcements. In fact, in some instances, there is advantage to heating the

reinforcing material to a temperature above the softening-melting point of

the thermoplastic components of the mixed homogenized scrap material, so

that the thermoplastic particles therein will stick to the surface of the

reinforcing material or fillers, and coat them. When this is done, a

molding compound is prepared which can be directly processed in a

compression mold or die, or a double belt press or calender.

In any event, by one method or another, the mixture of

homogenized scrap material and reinforcements and optionally filler is

intimately dry blended together and subsequently delivered to a compression

molding apparatus. The apparatus may comprise a conventional

compression molding die, a double belt press, or a calendering unit.

Compression molding processes using a conventional compression molding

die are conducted batch-wise or discontinuously, with the mold being filled

with pre-heated molding compound, closed and subjected to appropriate

heat and pressure for a sufficient time to fuse into a solid product

conforming to the shape of the mold itself. Then the product is cooled to

a temperature which allows the product to be removed from the mold or

die.

When such a process is accompanied by a preheating step to

above the softening point of the thermoplastic components, typically at a

temperature above 160°C, prior to entry of the molding composition into

the mold cavity, then the compression molding can proceed directly.

When a double belt press or calendering unit is used in the

process of the present invention, the molding process is conducted

continuously. Then, the preheating of the molding compound can be

conducted continuously on a conveyer belt as it is fed to the first zone of

the double belt press or the first pair of rollers of the calendering unit. The

same typical preheating temperatures as mentioned in the discontinuous

process determines whether the compound can be directly cooled down in

the first zone or pair of rollers, or whether additional heat has to be added

to soften or melt the thermoplastic components or initiate the curing process

of added uncured thermoset resins on the surface of the reinforcements.

The product is subsequently cooled down to a temperature at which it can

conveniently be handled in its semi-finished form.

These compression molding/laminating processes, using

standard compression molding molds or dies, double belt presses or

calendering units, are highly advantageous in comparison with

extrusion/kneading or injection molding processes when applied to products

produced by the present invention. Because the products contain inherently

incompatible plastics ingredients blended together to give mutual

reinforcement, they are, in injection molding and extrusion processes,

subjected to excessive heat, friction and shear, which tend to degrade the

polymeric materials, as well as to destroy mechanically the added

reinforcing additives such as glass fibres and other fibrous reinforcing

materials. In addition, the molding compounds which can be used in

injection molding and extrusion/kneading processes can only tolerate limited

amounts of fillers or reinforcements or other contaminants, because they are

required to "flow" in a thermoplastic melt. These factors all contribute to

reduced mechanical properties such as brittieness and low modules of

elasticity, resulting from the damage to the reinforcing material, and the

small amounts of reinforcing material which must necessarily be present.

The specific preparation of the molding compound and the ingredients used

therein, in the present invention employing compression/laminating

processes, do not subject the reinforcing materials and filler materials to

such mechanical destruction. Moreover, up to 90% by weight of non-

thermoplastic ingredients can be present. Accordingly, superior products

result. Molding compositions prepared from mixed scrap materials

according to the present invention i.e. including in the process the steps of

size reduction and homogenization as described above, optionally in

addition using pre-treated reinforcements, can be simply advantageously

used in such compression laminating processes to yield high quality

products.

The invention will be further described, for illustrative

purposes, in the following specific examples.

EXAMPLE 1- PRODUCπON OF BUILDING PANELS

A mixture of scrap materials obtained from a spaceframe

vehicle and from household/industrial mixed thermoplastic wastes is used

according to the process of the present invention in manufacture of building

panels. The vehicle is clad with SMC body panels and has RRIM fenders

and S-RIM bumper beams.

The space frame vehicle is initially dissembled into three

component categories, namely:

(1) tires and seating upholstery, to be collected for incineration;

(2) SMC body panels, R-RIM fenders and S-RIM bumper beans,

for use as the building panel reinforcement as described below;

and

(3) the remaining spaceframe vehicle is then conventionally

recycled and the metal components and other heavy parts are separated.

The resulting "fluff" containing a mixture of thermoplastic materials

(polyolefins, polyamides, thermoplastic polyesters, styrenics, PVC,

acrylics, polycarbonates etc.), cured thermoset materials (typically

phenolics, epoxies, natural and synthetic rubbers; polyurethanes, silicones

and others) and other organic and inorganic contaminants, is enriched by

mixing with other contaminated thermoplastic household/industrial scrap,

to give a mixture containing about 70% by weight thermoplastic materials,

30% by weight cured thermoset materials, and other minor amounts of

organic and inorganic contaminants.

This mixture is shredded and milled, screened to separate and

recycle particles larger than 1 mm, and then fed to a dry blender.

Homogenization takes place to form a free-flowing macro-homogeneous

powder of mixed scrap thermoplastic and cured thermoset materials, along

with other organic and inorganic materials. This macro-homogeneous

powder is then pre-heated to 140°C.

In the meantime, the disassembled SMC body panels, R-RIM

fenders and S-RIM bumper beams are delivered to a conventional cutting

and shredding machine where their size is reduced, and then subjected to

a first screening. Particles of maximum dimension less then 30 mm pass

this first screening whilst the larger particles are recycled to the cutting and

shredding machine. The particles which pass the first screen are then

subjected to a second screening. Particles of size less than 10 mm are

separated in the second screening and conveyed to the mixing station where

the above-described fluff mixture and household/industrial scrap is being

mixed together, the entire mixture then proceeding to the screening step to

separate the 1 mm particles, followed by homogenization to the

macrohomogeneous powder form.

Meanwhile, the particles of thermoset scrap and reinforcing

fibre residues which did not pass the second screening, i.e. those of particle

size 10-30 mm, are mixed and pre-treated with 8 % by weight of uncured

phenolic resin in liquid form, to act as the reinforcement in the final

product.

This impregnated reinforcement is preheated to 140°C, and

then the preheated reinforcement and the macrohomogeneous powder,

preheated to the same temperature of 140°C, are brought together in a dry

blender and intimately mixed together at a temperature of 140°C. The

proportions of substances in the mixture are arranged to give a final blend

containing 40% by weight of mixed thermoplastics and 20% of mixed cured

thermosets, and other organic and inorganic substances from the

macrohomogeneous powder, 36% by weight of reinforcement, and 4% by

weight of uncured phenolics resin liquid.

The final blend is conveyed, with heating, to a double belt

press, where it is subjected to a pressure of 1 kp/cm ² , at a temperature of

190°C, to form a building panel of about 12.5 mm thickness. Prior to

exiting the double belt press, the panels are cooled to 120°C, at which

temperature they are sufficiently rigid and self-supporting. They are then

subjected to conventional finishing processes, namely edge trimming and

cutting to length, and left to cool ready for shipping of final product. All

the edge trimmings and other waste can be recycled by adding it to the

household/industrial waste, prior to shredding and screening.

EXAMPLE 2 - PRODUCTION OF EXTERIOR SIDING

Exterior siding is prepared from mixed household and

industrial waste containing thermoplastics contaminated with other organic

and inorganic substances, and including cured thermoset resins and

woodchips.

A mixture of household and industrial waste material

comprising about 90% by weight of mixed thermoplastics (polyolefins,

styremics, polyesters, polyamides, etc.) along with 10% contaminants,

predominantly cured thermoset resins such as phenolics, polyesters,

polyurethanes, natural and synthetic rubbers, silicones, etc., but also

including small amounts of paper, aluminum, dust, etc., is shredded, milled

and screened, with recycling of the particles of size greater than 1 mm.

The screened material is mixed and homogenized to produce a

macrohomogeneous product as in example 1, and then preheated to 140°C.

Meanwhile, wood chips of a typical dimension of 10 - 30 mm,

to act as a reinforcing material, are heated to 140°C.

The macrohomogeneous powdered scrap and reinforcement,

both preheated to 140°C, are intimately dry blended together, maintaining

the temperature at 140°C, with continuous low shear mixing to avoid

damaging the wood fibres, in relative proportions of 80 parts by weight

reinforcement and 20 parts by weight of macrohomogenized scrap. This

produces a final blend containing 18% mixed thermoplastics, 2% cured

thermoset and other organic and inorganic scrap, and 80% by weight of

wood chips.

A thin (0.3 mm) aluminum sheet is placed into a compression

die heated by steam to 190°C. The powdered, blended mixture is fed to

the compression die, where it is compression molded into sheet form at

elevated temperature. After compression molding, it is removed from the

compression die, supported by the thin aluminum sheet, where it is left to

cool and harden.

EXAMPLE 3 - PRODUCTION OF FLOOR TILES

Floor tiles, requiring little tensile strength, are made according

to the process of the invention as follows:

Scrap wire and cable insulation consisting of 80% by weight

of thermoplastics (such as polyethylene, polypropylene, polyvinylchloride,

polyamides) and 20% scrap thermoset wastes (such, as crosslinked

polyolefins, cured silicones and synthetic and natural rubber) is shredded,

pulverized and screened, with particles of size greater than 1 mm being

recycled. The screened material is homogenized and preheated to 140°C

as described in the previous examples, to form a macrohomogeneous

powder mixture.

Meanwhile, typical SMC parts as used for automotive body

panels, to act as the filler is prepared, consisting essentially of 30% glass

fibres, 30% cured thermoset polyester resin and 40% inorganic filler (eg.

calcium carbonate). The mixture is shredded, milled and screened, the

particles larger than 0.3 mm being recycled, and the screened product is

homogenized and heated to 140°C.

Next, the preheated thermoplastic and thermoset containing

macrohomogeneous mixture is intimately blended with and the filler, in

equal proportions whilst being kept at 140°C. The final blend contains, by

weight, 40% thermoplastic, 10% flexible thermoset, 15% cured thermoset,

20% inorganic fillers and 15% glass powder. It is fed at 140°C between stacks of pairs of calendering rolls which heat the mixture to 190°C and

press it into flat sheets of the desired thickness. At the last nip, a

decorating film or a textile backing film may optionally be applied to a

surface. Then the tiles are finished by cooling and cutting. All the cuttings

and other waste can be added again to the filler portion prior to shredding

and milling.