Method for producing dyed moulding materials

Details Method for producing dyed moulding materials Method for producing dyed moulding materials

2000-02-29

2001-12-11

Sanders, Kriellion A. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C523S348000

Reexamination Certificate

active

06329448

ABSTRACT:

The invention relates to an improved process for preparing pigmented thermoplastic molding compositions.
The pigmentation of thermoplastics by means of screw compounders has been known for a long time.
For this, pellets of thermoplastic are often metered together with a pigment powder mixture into the hopper of a screw compounder. A disadvantage of this procedure is the circumstance that fine pigment powder particles can become attached to the hopper walls and thus form pigment layers which break away at irregular intervals or are dragged free by the pellets.
A result of this is that undesirable variations in concentration of the pigment can occur in the pigmented polymer.
U.S. Pat. No. 4,937,034 and U.S. Pat. No. 3,892,390 describe apparatus and processes in which design features in the feed hopper of the screw compounder are used to shorten the distances which the pigment powder must fall.
U.S. Pat. No. 4,937,034 describes simultaneous metering of additive concentrate and polymer particles into an extruder, the hopper located over the feed section of the pigment-mixing extruder having a hole through which the additive concentrate is conveyed into the hopper by means of an ancillary metering screw. To prevent the flow of additive from being influenced by the main polymer flow, the additive flow is shielded from the polymer particle flow by a metal guide plate. Both individual flows reach the feed section of the extruder via a drop path which is common to both.
U.S. Pat. No. 3,892,390 discloses an apparatus in which the feed hopper of a pigment-mixing extruder has a hole through which additives reach the hopper by means of an attached conveying screw. The thermoplastic component thus reaches the feed section of the extruder screw together with the additives.
In the processes mentioned, parts of the extruder hopper become coated with pigment powders, causing deviations from consistent color and lengthening changeover times at color changes.
Another version of the process, frequently used for coloration with pigment powders, consists in mixing thermoplastic pellets with the pigment powders and, if desired, other additives in a premixer. With the aid of oils or water or other liquids, the surface of the thermoplastic pellets is coated with a low-molecular-weight film, so that the additives adhere to the pellets.
A disadvantage of this process is the additional introduction of a low-molecular-weight substance, which gives undesirable changes of shade and, for example, of mechanical properties of the pigmented molding compositions.
In this process also, the hopper of the pigment-mixing extruder and upstream pipelines become coated with powders which break off from the pipeline walls and hopper at irregular intervals and cause deviations from color consistency and high cleaning costs at color changes.
It is an object of the present invention to provide an improved preparation process for pigmented thermoplastic molding compositions which gives better color consistency (quality of pigmentation) and lower cleaning costs and time losses at color changes.
We have found that this object is achieved by means of a continuous process for preparing pigmented thermoplastic molding compositions in an extruder, which comprises
I) in a first zone
A) metering in from 10 to 99.999% by weight of a thermoplastic polymer into an extruder and
II) in a subsequent zone at temperatures below the respective glass transition temperature of the thermoplastic A)
B) adding from 0.001 to 60% by weight of a colorant
and
then heating the thermoplastic/colorant mixture in the subsequent zones, homogenizing the polymer melt and extruding the resultant mixture.
By means of the novel process, color pigment powders and thermoplastic particles are metered separately into the pigment-mixing extruder; in this method, the thermoplastic material reaches the pigment-mixing extruder first, via the hopper, and the pigment powders are only introduced downstream by means of a stuffing screw or other suitable conveying equipment via an opening in the extruder housing.
The pigment powder reaches the extruder at the point where the thermoplastic material is still solid. Since at this point the channels of the screw are filled with thermoplastic pellets or thermoplastic granules, reverse flow of powder into the hopper is avoided, so that there are hardly any deposits of pigment there which can bring about downtime at color changes.
The pigment metering point of the pigment-mixing extruder is continuously swept by the pigment metering point, so that hardly any deposits can form here either.
Since the plastic is as yet unmelted at the point where the pigment is metered in, sticking and crust formation are avoided. At color changes, the pigment powder stuffing screw is exchanged for another clean stuffing screw. This can markedly reduce the time which elapses between the production of two differently pigmented plastics.
In principle, the advantage appears with plastics of any type. Suitable thermoplastics A) are listed, for example, in Kunststoff-Taschenbuch (ed. Saechtling), 1989 edition, where sources of supply are also mentioned. Processes for preparing such thermoplastics are known per se to the person skilled in the art. Some preferred types of plastic will be described in somewhat greater detail below.
1. Polyoxymethylene homo- or copolymers
Such polymers are known per se to the person skilled in the art and are described in the literature.
Very generally, these polymers have at least 50 mol % of recurring units —CH
2
O— in their main polymer chain.
The homopolymers are generally prepared by polymerizing formaldehyde or trioxane, preferably in the presence of suitable catalysts.
For the purposes of the invention, polyoxymethylene copolymers are preferred as component A, in particular those which, besides the recurring —CH
2
O— units also have up to 50 mol %, preferably from 0.1 to 20 mol %, in particular from 0.3 to 10 mol % and very particularly preferably from 2 to 6 mol %, of recurring units
where R
1
to R
4
, independently of one another, are hydrogen, C
1
-C
4
-alkyl or halo-substituted alkyl having from 1 to 4 carbon atoms, and R
5
is —CH
2
—, —CH
2
O—, or methylene substituted with C
1
-C
4
-alkyl or with C
1
-C
4
-haloalkyl, or is a corresponding oxymethylene group, and n is from 0 to 3. These groups may be advantageously introduced into the copolymers by ring-opening of cyclic ethers. Preferred cyclic ethers are those of the formula
where R
1
to R
5
and n are as mentioned above. Cyclic ethers mentioned merely as examples are ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepan, and as comonomers also linear oligo- and polyformals, such as polydioxolane or polydioxepan.
Other polymers suitable as component A) are oxymethylene terpolymers, prepared, for example, by reacting trioxane and one of the cyclic ethers described above with a third monomer, preferably bifunctional compounds of the formula
and/or
where Z is a chemical bond, —O—, —ORO— (R=C
1
-C
8
-alkylene or C
2
-C
8
-cycloalkylene).
Preferred monomers of this type are ethylene diglycide, diglycidyl ethers and diethers of glycidyls and formaldehyde, dioxane or trioxane in a molar ratio of 2:1 and diethers of 2 mol of glycidyl compound and 1 mol of an aliphatic diol having from 2 to 8 carbon atoms, for example the diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, 1,3-cyclobutanediol, 1,2-propanediol and 1,4-cyclohexanediol, to mention just a few examples.
Processes for preparing the homo- and copolymers described above are known to the person skilled in the art and are described in the literature, and so further details are not necessary here.
The preferred polyoxymethylene copolymers have melting points of at least 150° C. and molecular weights (weight-average) M
w
in the range from 5000 to 200,000, preferably from 7000 to 150,000.
End-group-stabilized polyoxymethylene polymers which have C—C bonds at the chain ends are particularly preferred.
2. Polycarbonate

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