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FIELD OF THE INVENTION
This application relates to introduction of plastics additives into
polymeric materials, and more particularly, to a process for the spray
application of plastics additives to polymeric materials.
BACKGROUND
In the manufacture of products using thermoplastic resins such as
polyolefins, various additives are generally included in the resin to
affect color, to ease processability, and to inhibit oxidation and other
types of degradation, to stabilize the physical characteristics of the
resin and thus prolong the life expectancy of the product.
For maximum effectiveness, it is important that any additive be uniformly
distributed in the thermoplastic resin. Poorly distributed additives may
contribute to unsatisfactory properties in the final product, such as
reduced tensile and tear strengths, reduced resistance to low temperature
flexing, reduced elongation characteristics, reduced breakdown voltage
strengths of dielectric materials, and electrical losses caused by an
increased power factor and increased dielectric constant.
The physical form of plastics additives can determine the efficiency and
economics of their introduction into the thermoplastic material. For
example, fine powders tend to be fairly readily dispersed but are
difficult to handle and can cause environmental problems. They are also
difficult to introduce continuously into process equipment.
In practice, a number of procedures have been employed to incorporate solid
additives into polymeric systems. Conventional additive delivery systems
use dry additive feeding and mixing with polymer resins, in which dry
plastic additives are metered and mixed with polymer particles in blenders
or mixers. Alternatively, dry additives are mixed with a resin as it is
processed through a pelletizer, extruder, or compounding device. In
another process, the additives are melted and coated on plastic resin
particles before introducing them into an extruder. High melting additives
are difficult to control in this technology. In yet another process, resin
particles are coated with an aqueous emulsion of the additives, then
dried. This procedure is not suitable for hydrolyzable additives such as
many phosphite antioxidants, and the water must ultimately be removed,
resulting in complexity and expense. Other methods involve, for instance,
dissolving additives in one or more of the components of the mixture to be
polymerized before the polymer is formed, or mixing the additive in a
solution, suspension, or emulsion of the polymer and then removing the
solvent or suspending agent.
The literature also contains descriptions of spray procedures for
introducing plastics additives into polymeric materials, and for spraying
various other materials in supercritical carbon dioxide. A number of these
references are discussed briefly below.
U.S. Pat. No. 5,007,961 and corresponding PCT application WO 90/02770
disclose aqueous systems for applying additives to polymeric particles, as
well as methods for applying such additive systems such as spraying,
wiping, or dipping, and polymeric particles treated with such additive
systems. The additive systems comprise an emulsified wax, surfactant,
base, one or more functional polymer additives, and water.
European patent application 411,628 discloses stabilizing polyolefins in
non-extruded as-polymerized particle form by depositing on the particles a
mixture of stabilizers including one or more organic phosphites or
phosphonites and one or more phenolic antioxidants. Optional ingredients
are thioethers, organic polysulfides, hindered amine light stabilizers,
benzophenone and benzotriazole derivatives, and diluents such as
paraffins, cycloparaffins, epoxidized soybean or linseed oil, silicone
oils, and olefin oligomers. The stabilizer mixtures are applied, in a
melted state or in a liquid state by virtue of containing liquid
phosphites or phosphonites, by a continuous or batch mixer optionally
equipped with a spraying mechanism.
U.S. Pat. No. 5,041,310 of Williams discloses a coating composition
comprising a mixture of polymer additives, gelling agent, and oil, which
is applied as a liquid to the surface of particles of polymer, and caused
to gel.
U.S. Pat. No. 4,960,617 discloses a process for post-reactor stabilization
of polyolefins by melting a polyolefin wax, blending at least one additive
into the resulting melt, fluidizing polyolefin particles to be stabilized
with hot gas, and spraying the liquid polyolefin wax containing at least
one additive on the fluidized polyolefin particles.
U.S. Pat. No. 4,882,107 discloses a method and apparatus for spraying a
solution, suspension, or dispersion of a mold release material in a
supercritical fluid such as supercritical carbon dioxide onto the surface
of a mold, to coat it with the release agent.
U.S. Pat. Nos. 4,923,720 and 5,027,742 and Chemical Abstract 113:154288p
disclose a process and apparatus in which supercritical fluids such as
supercritical carbon dioxide are used to reduce the viscosities of viscous
coating compositions to permit their application as liquid sprays.
U.S. Pat. No. 5,066,522 discloses the use of supercritical fluids such as
supercritical carbon dioxide as diluents in liquid spray applications of
adhesives.
European patent application 350,910 discloses liquid spray application of
coatings with supercritical fluids as diluents, and spraying from an
orifice.
Production of fine powders in inorganic oxides and certain drugs by rapid
expansion of supercritical fluid solutions has been reported. See Chemical
Abstracts 108:155263k, 105:197085x, 105:63102s, and 104:227104b. Graphite
has also been produced in a micro-powder form by wetting it with liquid
CO.sub.2 then vaporizing the CO.sub.2 at a temperature and pressure above
the critical point of CO.sub.2 gas. See Japanese patent publication
62/265111. However, it does not appear that particle size reduction of
plastics additives in nonvolatile liquid matrices upon spraying in
supercritical CO.sub.2 has been reported.
Despite the progress made in applying polymer additives to polymers in
spray processes, indicated by the references discussed above, prior art
processes generally suffer from certain deficiencies. Some liquid systems
have high viscosities which make them difficult to atomize without
heating, dilution, use of a high amount of atomization gas, and/or use of
relatively high pressures for spraying. Systems which involve the spraying
of materials which are solids under standard conditions can experience
difficulties related to handling or melting of the solids, and plugging of
lines as a result of resolidification of the solid materials in vessels,
piping, and the spray nozzle. Such operational difficulties can make the
spraying operation inefficient, adversely affecting not only its
economics, but also the control of the amounts of the additives and the
uniformity of their application to the polymer being treated. An improved
spray process for applying plastics additives to polymers would be very
desirable. Such a process is the subject of this application.
SUMMARY
The process of the present invention involves the steps of 1) forming in a
closed pressurized system a mixture comprising: a) at least one polymer
additive material which is a solid under standard conditions of
temperature and pressure; b) at least one liquid carrier material capable
of dissolving, suspending, or dispersing the polymer additive material;
and c) at least one viscosity reducing material which is i) a fluid under
the pressure of the closed pressurized system, ii) at least partially
soluble in the liquid carrier material, iii) present in the mixture in an
amount which is effective to cause the mixture to have a viscosity which
renders it sprayable, and iv) volatile under standard conditions of
temperature and pressure; and 2) spraying the mixture onto a polymeric
substrate.
This process enables manufacturers of plastic items to introduce mixtures
of plastics additives onto resins in a convenient liquid form, thereby
avoiding the problems of handling, dusting, agglomeration, and metering or
measuring associated with dry solids. No volatile solvent or water is
incorporated into the polymeric substrate. Reduction of solid particle
size occurs upon spraying of a number of plastics additives. The spraying
aspect of the process provides both improved control of the amounts of
additives applied and the uniformity of their incorporation into the
polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more completely understood from a consideration of
the following detailed description taken in conjunction with the drawing,
in which:
FIG. 1 is a schematic diagram of apparatus which may be employed in the
spray process of the invention; and
FIG. 2 is a schematic diagram of apparatus for continuous spraying.
DETAILED DESCRIPTION
Polymer additive materials, otherwise referred to as polymer additives, are
materials which are suitable for inclusion in polymers to affect their
properties or processing characteristics. In other words, they are
compounds which affect or modify the properties of a polymeric system of
which they are a part. Depending of their chemical constitutions, they may
act as antioxidants, neutralizers, metal or catalyst deactivators, slip
agents, light stabilizers, antiblocking agents, colorants, lubricants,
flame retardants, coupling agents, processing aids, antistatic agents,
nucleating agents, blowing agents, etc.
Examples of antioxidants include, but are not limited to: hindered phenols,
phosphites, and propionates. Examples of hindered phenols are
1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene;
octadecyl-3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate; tetrakis
[methylene-3(3',5'-di-tert-butyl-4'-hydroxyphenyl)-propionate]methane);
4,4'-butylidene-bis(5-methyl-2-t-butyl)phenol; and
2,2'-ethylidene-bis-(4,6-di-tert-butylphenol). Examples of phosphite
andioxidants are tris(2,4-di-tert-butyl-phenyl)phosphite;
bis(2,4-di-t-butyl-phenyl) pentaerythritol diphosphite; and
2,2'-ethilidene-bis(4,6-di-t-butylphenyl)fluorophosphite. Examples of
propionate antioxidants are dilaurel thiodipropionate and distearyl
thiodipropionate.
Examples of neutralizers/catalyst deactivators include, but are not limited
to: zinc oxide, zinc stearate, fatty amines and fatty amides such as those
sold by a division of Witco Chemical Company under the KEMAMINE label;
3,5-bis(1,1-dimethyl-ethyl)-4-hydroxy-benzene propanoic acid;
2-[3[3,5-bis-(1,1-dimethylethyl)-4-hydroxy phenyl]-1-oxopropyl]hydrazide;
molecular sieve, and hydrotalcites.
Examples of slip agents include, but are not limited to: erucamide,
oleamide, and ethylene bis-stearamide.
Examples of light stabilizers include, but are not limited to: benzophenone
stabilizers, such as those sold under the tradenames CYASORB-UV 2018
(American Cyanamid), UVINUL M40 and UVINUL 490 (BASF Corporation),
hindered amine compounds such as those containing tetraalkyl-piperidinyl
functionality, including UV absorbers marketed by Ciba Geigy under the
tradenames TINUVIN 144, TINUVIN 326, TINUVIN 327, TINUVIN P, TINUVIN
622LD, and TINUVIN 770
(N,N-diphenyl-N,N-di-2-naphthyl-p-phenylene-diamine), American Cyanamid's
CYANOX 3346, and FAIRMONT MIXXIM AO-30.
Examples of blowing agents are: azodicarbonamide and sodium bicarbonate. An
example of a nucleating agent is dibenzylidine sorbitol.
Examples of antiblocking agents are diatomaceous silica, clay, and talc.
Examples of colorants are titanium dioxide, carbon black, and organic dye
pigments.
Examples of lubricants are organomodified polydimethylsilioxanes such as
UCARSIL PA-1 processing aid and polyalkylene glycols such as UCON.RTM.
lubricant LB-285, available from Union Carbide Chemicals and Plastics
Company Inc., and calcium stearate.
Examples of processing aids are calcium stearate and organomodified
polydimethylsilioxanes such as UCARSIL.RTM. PA-1 processing aid.
Examples of antistatic agents are glycerol monostearates, etholated amines,
polyethylene glycol esters, and quaternary ammonium compounds.
Standard conditions of temperature and pressure means 25.degree. C. and one
atmosphere pressure.
Liquid carrier materials useful in the process of the invention, otherwise
referred to as liquid carriers, are materials which are capable of
dissolving, suspending, or dispersing polymer additives. They may be
functional or nonfunctional fluids, and are substantially nonvolatile
under standard conditions of temperature and pressure.
Examples of functional liquid carriers are organomodified polysiloxanes
such as Union Carbide's UCARSIL.RTM. PA-1 processing aid, liquid phosphite
stabilizers such as Borg Warner's WESTON 399B, alpha tocopherol (vitamin
E), ditridecylthiopropionate, trisnonylphenylphosphite, ethoxylated fatty
amines, alkylated diphenylamines, and alkyllauryl polyether phosphate
esters. Examples of nonfunctional liquid carriers include, but are not
limited to: polyethers such as polyethylene glycols and polyalkylene
glycol lubricating oils such as Union Carbide's UCON.RTM. lubricant
LB-285; hydrocarbons such as mineral oils, poly alpha olefins,
polypropylene oils; and polyesters such as sorbitan monooleate and
glycerol trioleate. These are relatively low surface energy materials.
Viscosity reducing materials suitable for use in this invention are
compressed fluids such as supercritical fluids and subcritical compressed
fluids.
As used herein, the term "compressed fluid" means a fluid which may be in
its gaseous state, its liquid state, or a combination thereof, or is a
supercritical fluid, depending upon (1) the particular temperature and
pressure to which it is subjected upon admixture with the solvent-borne
composition that is to be sprayed, (2) the vapor pressure of the fluid at
that particular temperature, and (3) the critical temperature and pressure
of the fluid, but which is in its gaseous state at the standard conditions
of zero degrees Celsius temperature and one atmosphere absolute pressure.
As used herein, a "supercritical fluid" is a material that is at a
temperature and pressure such that it is at, above, or slightly below its
critical point. As used herein, the critical point is the transition point
at which the liquid and gaseous states merge into each other and
represents the combination of the critical temperature and critical
pressure for a given substance. The critical temperature as used herein is
defined as the temperature above which a gas cannot be liquefied by an
increase in pressure. The critical pressure as used herein is defined as
that pressure which is just sufficient to cause the appearance of two
phases at the critical temperature.
Examples of viscosity reducing materials which may be employed as
supercritical fluids in the process of the invention include, but are not
necessarily limited to, carbon dioxide, ammonia, nitrous oxide, xenon,
krypton, chlorotrifluoromethane, monofluoromethane, methane, ethane,
ethylene, propane, and pentane. The critical parameters of these materials
are listed in Table 1.
TABLE 1
______________________________________
Critical Parameters of Supercritical Fluids
Boiling Critical Critical
Critical
Point Temp. Pressure
Density
Compound (.degree.C.)
(.degree.C.)
(bar) (g/ml)
______________________________________
Carbon Dioxide
-78.5 31.3 72.9 0.448
Ammonia -33.35 132.4 112.5 0.235
Nitrous Oxide
-88.56 36.5 71.7 0.45
Xenon -108.2 16.6 57.6 0.118
Krypton -153.2 -63.8 54.3 0.091
Chlorotrifluoro-
-31.2 28.0 38.7 0.579
methane
Monofluoro-
-78.4 44.6 58.0 0.3
methane
Methane -164.0 -82.1 45.8 0.2
Ethane -88.63 32.28 48.1 0.203
Ethylene -103.7 9.21 49.7 0.218
Propane -42.1 96.67 41.9 0.217
Pentane 36.1 196.6 33.3 0.232
______________________________________
Examples of viscosity reducing materials which may be employed as high
pressure subcritical compressed fluids include, but are not necessarily
limited to, carbon dioxide, ammonia, nitrous oxide, xenon,
chlorotrifluoromethane, monofluoromethane, ethane, and propane.
Carbon dioxide (CO.sub.2) and nitrous oxide (N.sub.2 O) are preferred
viscosity reducing materials for the practice of the present invention due
to their low critical temperatures, low toxicities, nonflammability, and
low cost. Carbon dioxide is the most preferred viscosity reducing material
because of its low cost, availability, and environmental acceptability.
Mixtures of any of the above mentioned materials are also within the scope
of the invention.
The purpose of the viscosity reducing material is to reduce the viscosity
of the mixture of polymer additive and liquid carrier to a point where it
is sprayable, thus permitting relatively high levels of additives to be
used in the composition to be sprayed, and to provide this function in an
environmentally benign way. To fulfill this function, the viscosity
reducing material must be a fluid under the system conditions of
temperature and pressure, at least partially soluble in the liquid
carrier, and present in an effective amount. Since it is not intended that
the viscosity reducing material become part of the treated plastic, it
should be volatile.
The step of spraying may be accomplished using any appropriate equipment
capable of handling and spraying mixtures of liquids and solids under
pressure.
The process of the invention may be employed to introduce plastics
additives into or onto any polymeric material, those with low surface
energies being preferred. Examples of polymeric materials which may be
treated are the following: polyolefins such as high density polyethylene
(HDPE), linear low density polyethylene (LLDPE), low density polyethylene
(LDPE), polypropylenes, polyacrylates and polymethacrylates, poly(vinyl
chloride), and polystyrene; polyesters; polyamides such as nylons;
cellulose acetates; polycarbonates; and crystalline and elastomeric
copolymers of ethylene with propylene and/or other C.sub.3 -C.sub.8
straight or branched chain alpha olefins such as 1-butene, 1-pentene,
1-hexene, 1-octene, 4-methyl-1-pentene, 3-methyl-1-butene; terpolymers of
alpha olefins and dienes; acrylonitrile-butadiene-styrene terpolymers;
heterophasic polymers of propylene and other olefin polymers and/or
copolymers; and mixtures thereof. Numerous other materials will occur to
those skilled in the art. The process is most useful for introducing
additives into polymeric materials which are in solid form, and preferably
in particulate form.
FIG. 1 shows a schematic diagram of apparatus suitable for practicing the
spraying process of the invention in a batch mode. A pressurizable mixer
10 is equipped with means for mixing 12 which may be driven by a motor 14.
Pressurizable mixer 10 is additionally provided with heating means 16.
Attached to mixer 10 via lines 18, 20, and 22 is a tank 24 for holding a
mixture of materials to be sprayed. Tank 24 is provided with an air inlet
26 and a vent 28. A pump 30 such as a gear pump is provided between lines
20 and 22 to move material to be sprayed from tank 24 into mixer 10. Also
attached to mixer 10 is a container 32 of viscosity reducing material,
container 32 being connected via line 34 to a gas booster pump 36, the
output of which is connected via line 38 to a surge tank 40, which is in
turn connected via line 42 to a liquids pump 44 whose output is connected
via line 46 to a second surge tank 48, the output of which is connected
via line 50 to line 18 and thence into mixer 10. Mixer 10 may optionally
be provided with a recirculating loop shown in the figure by lines 52, 54,
and 56. Pump 58 is provided in the recirculation loop for recirculating
the contents of mixer 10, and sprayer 60 is connected to the recirculation
loop via lines 62 and 64. Mixer 10 is also provided with pressurization
means 66, such as, for instance, a source of nitrogen gas. If a
recirculating loop is not used, mixer 10 is connected to sprayer 60 via
lines 56 and 64 and valve 74, and lines 52, 54, pump 58, and values 68, 70
and 72 are either absent or shut down.
FIG. 2 shows an alternative spraying apparatus, for continuous spraying. In
this unit a jacketed static mixer 80 is employed for mixing the additive
slurry with the viscosity reducing material. Following mixer 80 is an air
driven power mixer 82 which is in turn connected to sprayer 60, which in
this instance is controlled by an air operated solenoid and timer 84.
Depending on the material being sprayed, it may only be necessary for the
spray apparatus to possess one mixer, 80 or 82. In a single mixer
apparatus, mixer 80 is preferred. As before, the mixture of materials to
be sprayed is held in tank 24, which is provided with an inlet 26 and a
vent 28. Tank 24 is connected via line 22 to pump 30, which is in this
instance connected to a variable speed drive 86. The outlet of pump 30
connects to line 20, which is in turn connected to static mixer 80. A
container 32 of viscosity reducing material is connected via line 88 to
the inlet of an air driven gas booster pump 90, which is in turn connected
to a small surge tank 92, which is connected to an air driven liquids pump
94 via line 96. This is in turn connected to a second small surge tank 98
which is connected via line 100 to a needle valve 102. The exit side of
needle valve 102 is connected via line 104 to a back pressure regulator
106, which is in turn connected via line 108 to line 20 leading into
static mixer 80.
Examples of processing equipment used for mixing include, but are not
limited to, static mixers, power mixers, and other mechanical mixing
devices, as well as recirculators for use with a closed system. Examples
of mechanical mixing devices are the Kenics Static Mixer model 37-08-135
and the Graco Hydra-Cat Power Mixer model 207-388 series F. Pumps 30 and
58 are preferably gear pumps such as those made by Viking and Zenith.
Pumps 36 and 44 of FIG. 1 correspond to pumps 90 and 94 of FIG. 2. The gas
booster pumps 36 and 90 are preferably air driven gas booster pumps such
as those made by Haskel. Liquids pumps 44 and 94 are also preferably but
not necessarily air driven. Heating of process fluids can be provided for
in any of the various ways known to the art, or in the case of a static
mixer, a heating jacket may be employed as illustrated. Close coupling of
the mixing apparatus to the spraying operation is preferred to maintain
well mixed fluid for spraying. This is particularly important when the
viscosity reducing material is used in an amount higher than is soluble in
the mixture of polymer additive and liquid carrier.
Examples of spray units include, but are not limited to, the plural
component or airless types manufactured by Binks, Nordson, Graco, and
Spraying Systems.
Examples of apparatus which may be employed to handle the polymeric
materials being spray treated include ribbon blenders, Henshall mixers, a
resin drop zone, and a conveying line.
Referring to FIG. 1, in practice, one or more polymer additive materials
are suspended or dispersed in a liquid carrier material to form a mixture
having a paste-like consistency. This is initially charged to tank 24,
from which it is conveyed through lines 22, 20, and 18 into mixer 10.
Mixer 10 is sealed and pressurized, and viscosity reducing material from
container 32 is introduced to mixer 10 under pressure and there mixed with
the polymer additive material and liquid carrier material to form a
sprayable composition. Spraying may be carried out with or without the
circulating loop. Where the circulating loop is employed, valves 68, 70,
and 74 are open and valve 72 is closed. When the circulating loop is not
employed, valves 68, 70, and 72 are closed and valve 74 is open.
The operation of the apparatus shown in FIG. 2 is substantially similar to
that of FIG. 1, except that the materials to be sprayed are provided
continuously to mixers 80 and/or 82. The viscosity reducing material,
preferably CO.sub.2, is preferably supplied as a liquid from any suitable
source, such as a cylinder or tank. It is pressurized by the gas booster
pump, then pressurized by the liquids pump to a final desired pressure.
Surge tanks are placed in the delivery line to dampen the flow and
pressure pulsations resulting from intermittent flow from the
reciprocating pump. The flow rate of the viscosity reducing material is
adjusted by setting the air pressure to the liquids pump, adjusting the
needle valve 102, and adjusting the back pressure to control the flow
through the needle valve. The back pressure regulator is adjusted above
the desired spraying pressure to allow for CO.sub.2 delivery and mixing
with the slurry. Additional apparatus for measuring and controlling
CO.sub.2 flow rate to the system may also be included.
Low average spraying rates using the apparatus of FIG. 1 or FIG. 2 can be
maintained by intermittent spraying, rapidly opening and closing the spray
orifice in sprayer 60, as illustrated in FIG. 2. Such intermittent
operation of the sprayer can be achieved by air operated solenoids or
electronic solenoids in an automatic spray gun assembly. Cycle rates of
180 cycles per minute are commercially available with air operated
solenoids, and cycle rates up to 1800 cycles per minute are commercially
available with electronic solenoids. Sprayers with good positive shutoff
control are preferred for this intermittent mode of operation.
In the inventors' experience, a Nordson automatic spray gun gives the most
preferred intermittent spray operation.
Upon being sprayed in accordance with the process of the invention, many of
the solid polymer additive materials undergo substantial particle size
reduction, which improves the potential for obtaining uniform coatings of
the additives on polymer particles being treated. The superior dispersion
ultimately achieved in the resin enables the desired degrees of polymer
stabilization to be achieved at lower levels of added stabilizers than
would be required if the stabilizers were added by other means.
An alternative means for providing finely divided solid additives for
coating polymer particles is to subdivide the additive materials in a
process such as dry or wet milling prior to spraying them onto the
polymer. However, pumpable slurries containing small particle size solids
are more viscous than slurries containing an optimized size distribution
of solid particles, thus presenting practical difficulties in conveying
slurries containing high concentrations of such small additive particles.
The additive particle size reduction which can be achieved for many solid
plastics additives in the process of this invention thus allows for
delivery of additive mixtures in which the initial slurry can be optimized
for maximum solids loading, while ultimately producing spray which
contains solid particles of plastics additives in a much reduced particle
size.
The process of the invention is not restricted to mixtures of one polymer
additive, one liquid carrier material, and one viscosity reducing
material. The ultimate mixture to be sprayed onto the polymeric substrate
may include multiple solid plastics additives, multiple liquid carrier
materials, and multiple viscosity reducing materials. It may also
optionally include one or more liquid additive components. The mixture to
be sprayed may originate as one or more stable slurries of plastics
additives and liquid carrier materials, which are combined to form the
ultimate sprayable mixture. When several slurries of additives are to be
combined, this may readily be accomplished by having each in a separate
holding tank and introducing it via its own pumping system and transfer
line. Other variations on the theme will occur to those skilled in the
art.
The pressure to be employed in the process of the invention needs to be
high enough to allow production of a thinned effervescent spray from the
additive mixture. Operating pressure may be in the range of 13.8 to 346
bar, preferably 35.5 to 208 bar, and most preferably 49.3 to 139 bar.
The temperature to be employed in the process may range from ambient to the
stability limit of the materials being sprayed, preferably from ambient
temperature to 100.degree. C., and most preferably from ambient
temperature to 60.degree. C. Increasing temperature reduces the viscosity
of the mixture to be sprayed, thus increasing the spray quality.
The concentration of solid plastics additives in the slurry of additive and
liquid carrier material, prior to addition of the viscosity reducing
material, may range from low values such as 5% by weight, to as high as
approximately 70% by weight. Slurries containing solids loadings above 70%
by weight are not readily pumped or metered. Slurries containing low
solids concentrations need to be manufactured in a form that is
gravametrically stable or settling and variation of mixture concentrations
can result. Alternatively, unstable suspensions can be mixed just prior to
spraying. The preferred range of solids concentrations for the slurry is
15% to 70% by weight, and the most preferred range is 25% to 65% by
weight.
The size of the spray nozzle of sprayer 48 needs to be large enough to
allow slurry particles to pass through without plugging. The minimum
nozzle size will depend on the size of the largest particles in the
slurry. Large orifice nozzles are preferred to minimize the chance for
orifice plugging, and to allow for spraying of slurries which contain
large particles. Smaller orifice sizes are preferred for maintaining low
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