Process for coloring thermoplastics

An improved process for coloring polymethacrylates or polyglutarimides in an extruder includes admixing the molten thermoplastic, conveyed directly from the polymerization operation, in an extruder mixing zone with a dispersion of at least one pigment, dye, or colorant in a suitable clear carrier, then conveying the admixed colored thermoplastic into an extrusion zone or injection molding zone.

BACKGROUND OF THE INVENTION 
Since the advent of clear moldable thermoplastics, it has been desired to 
prepare them in colored form, whether transparent, translucent, or opaque. 
Many processes exist in the art which have been evaluated to accomplish 
the coloration uniformly, efficiently, and cheaply. Among these exist 
processes for adding colorants in dry form to the molten thermoplastic, 
for adding colorants in dry form to a solid polymer and mixing in the 
molten state, and for adding colorants in solution or dispersion to a 
solid polymer as it is fed to an extruder for first melting and then 
mixing in the molten state. 
It has been considered potentially cost-effective and energy-conserving if 
the colorant could be added to the molten matrix thermoplastic polymer as 
it is processed upon leaving the reactor wherein it has been prepared. 
Further, the method would offer much easier cleanup and color changes than 
when the colorant is added at the rear of the extruder, such as with the 
feed of pellets to be re-melted. Further, the matrix polymer would receive 
less thermal history and thus avoid potential degradation problems from 
excess heating prior to forming the final object. Further, such a process 
should allow better control and less variability of the colorant content, 
especially when low levels of colorant, such as for toning or optical 
brightening, are involved. 
Such preparative/extrusive manufacturing processes are common for 
methacrylic polymers, that is, polymers with at least 80 weight percent of 
their units derived from methyl methacrylate, especially where the polymer 
is prepared by a continuous technology, unreacted monomer removed by 
devolatilization and recycled, and the devolatilized polymer conveyed 
directly to an extrusion process for forming pellets, film or sheet. Such 
a preparative/extrusive process is also common for polyglutarimides, 
wherein the molten methyl methacrylate polymer is reacted in an extruder 
with a primary amine, such as monomethylamine, to form the glutarimide 
units, optionally treated with an alkylating agent such as triethyl 
orthoformate or dimethyl carbonate to lower the acid and anhydride 
content, and then conveyed directly to an extrusion process for forming 
pellets, film or sheet. 
However, a colorant process where the colorant is added to the molten 
polymer just prior to pelletization or extrudation or molding would offer 
similar advantages over other colorant methods, even if the polymer has 
been formed separately and cooled, then re-melted for coloration. For 
example, clear polymer of methyl methacrylate could be separately prepared 
in an appropriate molecular weight range and then re-melted, colored by 
direct colorant addition after melting, and then processed into colored 
melt-calendered sheet. 
The difficulty with such a direct colorant-addition approach for 
methacrylic and glutarimide polymers is that there have been no known 
effective combination of colorants and non-volatile carriers known which 
meet the requirements for these plastics, viz., (a) compatibility with 
matrix polymer, as judged by visual appearance and good dispersion; (b) 
thermal stability in the melt, as judged by no loss of molecular weight of 
the matrix polymer and no discoloration of the matrix polymer; (c) no 
adverse effect on weathering behavior, as judged by retention of physical 
and appearance properties after accelerated or conventional outdoor 
weathering test, and (d) little or no loss in physical properties of the 
matrix polymer, such as heat distortion temperature. 
European Unexamined Patent Application 478,987 teaches feeding of a liquid 
color concentrate to a polymer melt stream, mixing, and then 
devolatilizing to remove the carrier material. This application teaches 
utility for polyamides and polyesters only, not the polymers of the 
present invention. It further requires a devolatilization step, which is 
not a critical step of the present process. Indeed, the carrier in the 
'987 process is generally water, whereas the present invention utilizes 
carriers of sufficiently high boiling point that under the conditions 
employed for mixing and extruding, the carrier is not volatilized. 
Eliminating the need for devolatilization offers faster extrusion rates 
and less opportunity for remaining volatiles to cause poor moldings. Thus, 
the process described in the '987 application does not present a 
satisfactory solution to the problem of coloring methacrylic or 
glutarimide polymers in the melt directly after polymerization and/or 
reaction, and so avoiding the less satisfactory processes of feeding 
colorant prior to the polymerization or reaction, or re-melting the 
polymer after adding colorant to isolated pellets. 
European Patent Specification 234,889 teaches that previous attempts to 
disperse colorant dispersions in carriers and then convey to a polymer 
melt have given unsatisfactory mixing, and teaches a solution to the 
problem wherein the colorant/carrier combination is admixed after the 
extrusion of the matrix melt is completed, utilizing a positive 
displacement feed means to form two streams of the melt and adding the 
colorant/dispersant into one stream. Their solution requires sophisticated 
equipment added externally to the end of the extruder. 
We have now found that suitable non-volatile colorant carriers exist which, 
in combination with appropriate feeding and melt-mixing processes, will 
allow a wide number of colorants to be added and dispersed effectively in 
the "additives zone" of an extruder, with subsequent advantages in cost 
and ease of manufacture of colored plastics of high quality. 
SUMMARY OF THE INVENTION 
Thus, we have discovered a process for coloring thermoplastics in an 
extruder comprising: 
a) conducting into an extruder mixing zone a molten thermoplastic selected 
from at least one of (1) a polyglutarimide or (2) a polymer containing at 
least 80 weight percent of units derived from methyl methacrylate; 
b) admixing the molten thermoplastic in the extruder mixing zone with a 
dispersion of at least one pigment, dye, or colorant in a suitable clear 
carrier, the pigment, dye, or colorant being present at from about 0.0001 
parts to about 5 parts per 100 parts of thermoplastic, the suitable clear 
carrier being present at from about 15 parts to about 99.9 parts per 100 
parts of the dispersion, and the suitable clear carrier being present at 
no more than about 1 part per 100 parts of thermoplastic; and 
c) conducting the admixed colored thermoplastic to an extrusion die or 
injection molding zone. 
Further, there may be added at the extruder mixing zone at least one of a 
lubricant, an ultraviolet stabilizer, a hindered amine light stabilizer, 
an anti-oxidant, a stabilizer against degradation by sterilizing 
radiation, or a thermal stabilizer. The admixed colored thermoplastic may 
be opaque or translucent, but preferably is transparent to light, that is, 
with a visual appearance of transparency and with a total white light 
transmission through a standard ASTM molded optical plaque of at least 8%. 
The plaque will show high transparency to those specific wave lengths of 
light not absorbed by the colorant. 
In one aspect of our process, the admixed colored thermoplastic is then 
extruded through an extrusion die into strands and chopped into pellets 
suitable for re-molding. The dispersion of colorant may be pumped into the 
extruder mixing zone, and if so, the flow may be metered into the mixing 
zone for better consistency. With suitable pumping means, the viscosity of 
the dispersion may be above 20,000 cp., although the viscosity of the 
suitable clear colorant per se will be below about 1000 cp. 
Although the colorant, dye, or pigment may be any of those known to the art 
for coloring thermoplastics, it is preferred for good coloration and 
weathering that the colorant is an anthraquinone or perinone dye. Carbon 
black is also useful in this process for producing gray colors. The 
preferred colorants cannot be added in the molten form, as they either 
melt too high or are chemically unstable in the melt. 
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
A "suitable clear carrier" is defined as meeting the following criteria: 
a) when admixed with the matrix polymer at up to the 1% level, no 
deleterious effect on the color or clarity of the resulting blend; 
b) a viscosity below about 1000 cp., or an ability of the carrier to be 
pumped at a uniform rate into the extruder at a temperature which does not 
cause any turbulence or bubbles in the pumped colorant dispersion; 
c) a decrease in thermal stability of the blended matrix polymer plus 0.5 
weight percent of the carrier (no colorant) of no more than about 10 
centigrade degrees (versus unmodified matrix polymer) as judged by TGA 
(thermogravimetric analysis) (conditions: heating 20.degree. C./minute; 
temperature for 2% weight loss) or by a spiral flow test. In this 
non-standard spiral flow molding test, metering temperature is measured 
until all moldings of the matrix fill the 3 mm. mold without any visible 
decomposition of the matrix polymer, and the metering temperature of the 
matrix polymer plus carrier shall be no more than about 10 degrees C. 
lower than for the unmodified matrix polymer; 
d) little decrease in accelerated weatherability stability of the blended 
matrix polymer plus 0.5 weight percent of the carrier (no colorant) as 
judged by visual appearance and lack of color shift or haze formation (SAE 
test method J 576) versus unmodified matrix polymer, after modified Xenon 
arc or EMMAQUA testing; 
e) sufficiently low volatility that the blend can be extruded and molded 
into bubble-free objects without requiring a devolatilization step after 
the mixing operation. 
Certain low molecular weight unsaturated polyesters (of unknown 
composition) are commercially available which meet the above requirements, 
but acceptable carriers are not limited to unsaturated polyesters. 
Surprisingly the unsaturated polyesters do not cross-link or cure under 
the mixing conditions, as demonstrated by the observation that there is 
essentially no effect on the melt viscosity of the blend when they are 
present at such low levels. Such acceptable carriers may include mineral 
oils, linseed oil, castor oil, epoxidized soybean oil, and aluminum or 
zinc stearates, any of which must meet the above criteria. 
The polymer to be colored (which will be called the matrix polymer) may be 
a polymer of methyl methacrylate in which at least about 80% of the units 
are derived from methyl methacrylate. Preferred compositions for molding 
or extrusion into sheet are from about 89 to about 99 weight percent 
methyl methacrylate, the remaining monomers being a lower alkyl acrylate, 
such as methyl, ethyl, or butyl acrylate. Other monomers such as styrene, 
a-methylstyrene, vinyl toluene, vinyl acetate, acrylonitrile, maleic 
anhydride, and the like, may also be useful in forming the matrix polymer. 
The matrix polymer also may be a polyglutarimide. Polyglutarimides, 
otherwise known as polymethacrylimides, are polymers which contain the 
unit 
##STR1## 
wherein R.sub.1 and R.sub.2 may be the same or different and are hydrogen 
or lower alkyl of up to four carbon atoms, such as methyl, ethyl, or 
butyl, and R.sub.3 is H or alkyl, such as methyl, butyl, dodecyl, and the 
like. The polyglutarimides best known to the art are those where R.sub.1 
and R.sub.2 are methyl and R.sub.3 is hydrogen, methyl, other lower alkyl, 
or cyclohexyl. Particularly preferred are those polymers derived from 
poly(methyl methacrylate) and monomethylamine, where R.sub.1 =R.sub.2 
=R.sub.3 =methyl. The polyglutarimide may also contain units derived from 
a lower alkyl or cyclohexyl methacrylate, from methacrylic acid, or of 
methacrylic anhydride. The matrix polyglutarimide may also be treated by 
an alkylating agent to lower the acid and anhydride content, preferably 
prepared by the method of U.S. Pat. No. 4,727,117, which patent is herein 
incorporated by reference. A preferred method for preparing the 
glutarimide is that of Kopchik, U.S. Pat. No. 4,246,374, wherein the 
polymer is prepared from a polymethacrylate and an amine in an extruder, 
and may be conveyed directly to a zone for coloring by the method of the 
present invention, or may be first treated with an alkylating agent, then 
conveyed to the mixing zone. 
The term "colorant" is used herein to refer to any additive which alters 
the appearance of a transparent polymer with relatively little loss in 
light transmission or increase in haze. A dye is a colorant soluble in the 
matrix; a pigment is an insoluble colorant which can be well-dispersed in 
the matrix polymer, but may also be used when at a larger particle size to 
introduce controlled translucency or opacity into the thermoplastic matrix 
polymer. Titanium dioxide is an example of an opacifying pigment. Optical 
brighteners also are examples of such "colorants". The colorants 
themselves are not novel, having been used in blends with methyl 
methacrylate polymers, but admixed by other methods. 
The colorant, dye, or pigment is normally separately dispersed in the 
suitable clear carrier by a process such as high-shear mixing. The mixing 
may optionally be carried out with heating (to hasten dissolution or 
dispersion), or with cooling (to keep from overheating the carrier due to 
heat imparted by high-shear mixing. Other mixing methods, such as a 3-roll 
mill, a pebble mill. a steel ball mill, or a horizontal or vertical media 
mill, may also be employed. 
The level of the pigment, dye, or colorant is normally from about 0.0001 
parts to about 5 parts per 100 parts of thermoplastic, and the suitable 
clear carrier is normally present at from about 15 parts to about 99.9 
parts per 100 parts of the dispersion; however, under certain conditions, 
higher or lower levels of both colorant and carrier may be employed, such 
as when a very low level of colorant for use as a toner is desired. The 
dispersion will normally be added to the melt at the extruder mixing zone 
by a pumping process, and a metering device may be present to assure 
constant flow. The dispersion may be heated to reduce its viscosity while 
pumping, but usually it is preferred to add it at about room temperature 
to avoid over-heating in the mixing zone. The suitable clear carrier 
should be present at no more than about 1 part per 100 parts of 
thermoplastic to avoid plasticization of the matrix polymer. 
The extruder configuration at the melt-mixing zone may be any of a 
conventional type used for admixing of plastics. However, because it has 
been found advantageous in the manufacture of polymethacrylates and 
polyglutarimides to conduct (a) the devolatilization of the methacrylate 
polymer with recycle of monomer or (b) the imidization and acid-reduction 
process of the formation of the polyglutarimide in a twin-screw extruder, 
such twin-screw extruders are preferred. The twin-screw extruder may be of 
the co-rotating or contra-rotating variety, and the screws may be 
tangential or slightly separated. 
Good mixing of the colorant dispersion into the melt is required. Such 
mixing can be accomplished with a mixing screw design, preferably 
involving a paddle configuration, which is suitable for dispersant of 
other additives, such as lubricants, fed in a molten form. Inadequate 
mixing, such as by lowering the number of mixing paddles, will result in 
surging problems due to the presence of two separate phases in the melt. 
Other mixing methods known to the art may be applied in the mixing; many of 
these are taught in C. Rauwendaal, "Mixing in Polymer Processing", M. 
Dekker, NYC (1991). Thus, cavity mixers, slotted flight mixers, blister 
rings, and the like may be employed, with configurations such as taught in 
Rauwendaal on pages 164, 176, 180, 182, and 185-7. It will take limited 
experimentation by one of ordinary skill in mixing solid additives into 
the molten matrix polymer to determine whether adequate extrduer mixing of 
the colorant/carrier blend is occurring. 
Other additives may be compounded essentially simultaneously with the 
coloring dispersion. Normally such additives are melted and conveyed to 
the mixing zone of the extruder. It is preferred that two entrance ports 
be present, one for the coloring dispersion and one for the other 
additives, for ease in cleaning and for ease in conversion to production 
of non-colored materials. Such additives will usually be present at levels 
of from about 0.1 parts to about 2 parts based on 100 parts of the 
thermoplastic polymer. One or more additives may be present. Such types of 
additives include a lubricant, such as a long-chain aliphatic alcohol, a 
long-chain fatty acid, an amide of a long-chain fatty acid, or an ester of 
a long-chain fatty acid, and the like; an ultraviolet stabilizer, such as 
a benzotriazole, a salicylate ester, a cyanoacrylate, and the like; a 
hindered amine light stabilizer, such as a derivative of 
1,1,6,6-tetramethylpiperidine; a stabilizer against degradation by 
sterilizing radiation, such as butyl lactate; or a thermal stabilizer, 
such as an organic phosphite, a thioester, and the like. For certain end 
uses, chemicals which are near infra-red absorbers may also be added at 
this point. 
The polymers so colored may be directly conveyed to a molding apparatus or 
to a sheet or film extruder. Usually, they are extruded through a die into 
strands and chopped into pellets suitable for re-molding. The pellets, if 
of suitably high molecular weight, may be re-extruded into sheet or film, 
such as by a melt calendering process. 
The polymers so colored are useful in many applications where the good 
weathering, clarity, and high heat distortion of the basic matrix polymer 
is desirable. Such applications include automotive lighting parts, such as 
tail-light lenses, head-light lenses and housings, glazing, and the like; 
also lenses, windows, protective coverings, sky-lights, appliance parts, 
and the like.

EXAMPLES 
Example 1 
Coloring of a Polyglutarimide 
A polymer containing ca. 76 weight percent of N-methyl dimethylglutarimide 
units, below 0.5% of combined acid and anhydride units, the remainder 
being units derived from methyl methacrylate, is prepared by the method of 
U.S. Pat. No. 4,727,117, from a polymer of methyl methacrylate of MW ca. 
150,000 weight-average and monomethylamine, followed by alkylation with 
dimethyl carbonate. The molten polymer, at a melt feed rate of 165 
grams/minute and a melt temperature of 280.degree. C. to 310 .degree. C., 
is conveyed in a twin-screw contra-rotating, tangential extruder of screw 
outer diameter 0.80 inches (20.32 mm.), barrel diameter 0.812 inches 
(20.62 mm.), after passing a final devolatilization zone, to a mixing 
zone. 
At this zone, the main screw has four mixing paddles which are 0.3 inches 
(7.62 mm) long, and three paddles on an auxiliary screw used to feed the 
additives. The paddles have a perpendicular flat face at the surface in 
the direction of the screw rotation. Each paddle has eight segments spaced 
around the screw, the second set of segments being off-spaced by 22.5 
degrees from the first. At this point, the screw root is enlarged to ca. 
0.8 in (20.32 mm.) in diameter, and the height of the paddles is ca. 0.11 
inches (2.79 mm.). 
A mixture of equal parts of the colorant perinone pigment known as Solvent 
Red 135 (Chem. Abstr. #4612-023-8) and of polyester carrier, a viscous 
yellow liquid (ca. 550 cp.) described as an unsaturated polyester by the 
supplier, is prepared externally with a high-shear, high-speed mixer until 
a unform dispersion was judged by eye to have been achieved. The blend 
could then be passed through a heat-exchanger so it could be fed hot, but 
in the present experiments, a heat exchanger is not used. The mix is fed 
to the additives conveyer at a rate to add 0.5 weight % of carrier to the 
polymer. The dispersion of colorant and carrier is added to the main 
polymer melt at the mixing zone, and the well-mixed molten polymer then 
conveyed through a pelletizing die and into a water bath. 
When compared to a polymer of similar composition produced without colorant 
and the colorant then being admixed in a dry form and the polymer 
re-extruded to obtain uniformity, the consistency of coloring is 
equivalent. Accelerated weathering results are similar for both polymers. 
The heat distortion temperature is decreased by about 1.degree. C., and 
the thermal stability decreased (measured by TGA) by about 8.degree. C. 
relative to the uncolored product or the product colored with dry 
colorants. Commercially acceptable product can be prepared with a polymer 
exhibiting these slight decreases. 
Example 2 
Study of the Dispersing of Colors 
The polyester carrier from Example 1 is separately shown by blending 
experiments in an extruder to be compatible with both the polyglutarimide 
of Example 1 and with commercial "acrylic" molding powders of MW ca. 
120,000 and containing 90 to 99 weight percent methyl methacrylate. 
Separately it is shown that stable uniform dispersions of many commercial 
colorants, dyes, and pigments may be prepared by admixture with various 
ratios of the carrier unsaturated polyester. 
Example 3 
Addition of Other Additives 
The process of Example 1 is followed. The port where the colorant/carrier 
mixture is introduced leads to a molten pumped stream of stearyl alcohol 
(0.3 weight % on polymer) and 2-(2-hydroxy-5-t-octylphenyl)benzotriazole 
ultraviolet stabilizer (0.25% on polymer). The co-mixed stream is then fed 
to the mixing zone of the extruder as in Example 1. The resulting 
polyglutarimide will have acceptable molding characteristics similar to an 
uncolored polyglutarimide so lubricated with stearyl alcohol and 
stabilized, and the coloring behavior will be similar to the polymer of 
Example 1. 
Example 4 
Other Colorants 
In a manner similar to Example 3, are prepared the following: 
a) mixture of dye and carrier as in Example 1, but fed at a rate to add 
0.50 lbs. colorant plus carrier to 100 lbs. of matrix polymer; 
b) a mixture of carbon black (ca. 10% of blend) and polyester fed at a rate 
to add 0.10 lbs. colorant plus carrier to 100 lbs. of matrix polymer. 
Example 5 
Use of the Process with Poly(methyl methacrylate) 
A polymer of methyl methacrylate 97/ethyl acrylate 3 is prepared in a CFSTR 
at a temperature of ca. 160.degree. C.-180.degree. C. to about 50-60% 
conversion in the presence of ca. 0.2% n-dodecyl mercaptan and of ca. 
100-300 ppm of a polymerization initiator known to be effective in that 
temperature range, such as di-t-butyl peroxide, di-t-amyl peroxide, 
t-butyl; peracetate, t-butyl peroctoate, 
2,5-dimethyl-2,5-di(t-butylperoxy)hexane, dicumyl peroxide, 
di-(t-amylcyclohexanone ketal) peroxide, methyl ethyl ketone peroxide; 
t-butyl perbenzoate; and the like; mercaptan and initiator are added with 
the monomer charge. The molten polymer is then devolatilized and conveyed 
at a melt temperature of 260.degree. C. to 290.degree. C. and a rate of 
120-150 grams/minute to the mixing zone of an extruder equipped as in 
Example 1. A mixture similar to that prepared in Example 1 of colorant and 
carrier is fed to the polymer melt. Colorant feed rates are adjusted 
downward to obtain the same concentration of the colorant in the matrix 
polymer. The resulting polymer is anticipated to exhibit good consistency 
and color behavior. 
Example 6 
Use of the Process of Claim 3 on Larger Equipment 
Similar results will be obtained when the reaction equipment is of similar 
design but larger. In the present case the barrels are 4.5 inches (114.3 
mm.) in diameter, the melt feed rate is 2400 lbs. (1091 kg.) hour, and a 
melt pump is placed in the system after the mixing zone to aid conveyance 
to the pelletizing die. 
Example 7 
Illustration of Coloring of Reprocessed Polymer 
A polymer is prepared in as manner similar to that of Example 5, but the 
melt is conveyed directly through any extruder mixing zone without 
addition of colorant dispersion. The extrudate is stranded, cooled by a 
water batch and chopped into pellets. The resulting pellets are then added 
to the feed hopper of a single-screw extruder equipped with an extruder 
mixing zone, means for feeding a colorant dispersion to the extruder 
mixing zone, and a sheet die producing 50 mil (1.27 mm) sheet. A colorant 
dispersion similar to that used in Example 5 is added to the molten 
polymer at the extruder mixing zone, and the colored polymer extruded 
through an appropriate sheet die into sheet, which is anticipated to be of 
uniform color.