CA1064689A - Rheological agents - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A rheological control agent comprises a particulate blend of hydrogenated castor oil and an oligomeric polyamide derived from hydroxystearic acid, one or more saturated aliphatic .alpha. - omega diprimary diamines containing 2,4,6,8, or 10 carbon atoms and one or more saturated aliphatic .alpha.-omega dicarboxylic acids containing from 2 to 10 carbon atoms and/or hydrogenated dimer acids. The oligomeric amide is suitably derived from the reaction of the hydroxystearic acid, di-carboxylic acid and diamine in such proportions that for each mo?? of hydroxystearic acid there is present 0.05 - 0.5 moles of dicarboxylic acid and a quantity of diamine sufficient to provide from 0.8 to 1.15 amine groups for each carboxyl group in the acid mixture.

Description

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This invention is concerned with improvements in and relating to rheological or flow control agents, i. e. additives which modify the flow properties of surface coatings such as paints, varnishes and printing inks, adhesives and laminating compositions.
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~ 8 The design and control of rheological properties is a `~ most important aspect of the art of formulating surface coatings such as paints, varnishes, and printing inks. In particular, the formulator has resort to a number of devices for imparting and ~ , controlling specific patterns of non-Newtonian flow behaviour which .; 10 may affect the behaviour of the surface coating before, during ` and after application. Means for imparting thixotropy or false-body in various degrees constitute one of the most important groups of such ~ -techniques. Thus it is well known that drying oils and alkyds (which are widely usèd surface coatings media) may be modified by reaction with poiyamide resins to provide thixotropic media which, - inter alla, minimise pigment settlemerlt during storage, facilitate application by brushing, and reduce the tendency of the applied - film to sag. Other means of achieving these or combinations of related effects comprise the use of the so-called flow control additives . .: . .
which are mixed or milled into the surface coating at a later stage ,',: ~ , : of manufacture, for example after the varnish has been manufacturer, i durmg pigmentation, or even after a paint has been pigmented. A
well known eæample of such a flow-control additive is finely powdered hydrogenated castor oil. This has been used mainly in the so-called ; 25 decorative paints in which the principal solvent is white spirit, A
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major disadvantage of this material is its restricted useful temperature range. Thus, in the manufacture of paints and the like it is not uncommon for them to be subjected to temperatures in excess of 60C. as, for example, during the process of ball milling or high speed mixing. Under such c onditions hydrogenated castor oil (HCO) tends to dissolve in the paint medium and to crystallise out on subsequent cooling. The crystals so deposited are much ,j ~
- greater in size than the original micronised particles and are substantially ineffective as rheological control agents. The effect , . .
of the lICO is thus substantially reduced or even completely lost. ~ further disadvantage is that the crystals may be visible in the applied paint film giving rise to the defect known as seediness.
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Industrial paints of the kind normally dried by stoving , , present a special problem because they normally contain relatively powerful solvents such as xylol. The flow control additives of the prior art are substantially ineffective in such systems. This . : ~
is doubly unfortunate because there is a need to control the flow of the paint in the applied film during the actual stoving operation. Thus one wishes to avoid the tendency of the wet paint to run down vertical surfaces of the painted object which is a particularly severe problem ..
during the stoving operation because of the reduction in viscosity caused by the elevated temperature, The flow control additives of .
, the prior art completely fail to do this at the typical stoving temperatures of about 120 C.

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- ~6~6~3 It has now been found, in accordance with the present invention, that an improved rheological agent (e.g. one which may be used at elevated temperature) may be obtained by modifying , HCO by blending it with certain oligomer?ic amides and using the hlend, in finely divided form, as a paint additive. This is .; .
-~ ~ surprising since HCO alone is intolerant of elevated temperatures . . -whilst the oligomeric amides alone are substantially ineffective ` as flow control agents. Decorative paints dlssolved in white spirit and like substantially aliphatic solvents may be modified in such a way that rheological control is exercised and seeding is avoided when the ~: :
- paint has been subjected to temperatures substantially higher than . .~: -those which could safely be used with HCO. It is further possible to modify ~` ~ industrial paints containing powerful solvents such as xyloI to such a degree that flow control during stoving operationæ, typically at . . ; , 100 -120C is achieved.
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The oIigomeric amides used in the agent of the invention are those prepared by reaction of hydroxystearic acid with one or ., . . ~ .
more saturated aliphatic ~alpha-omega diprimary diamines containing -

2, , 6, 8 or lO carbon atoms together with one or more saturated aliphatic alpha-omega dicarboxylic acids containing 2 - 12 carbon :. . : : .
-,1 atoms and/or hydrogenated dimer acids, (i.e. acids obtained by the hydrogenation of the acids obtained by the thermal dimerisation j of unsaturated fatty acids). A preferred group of such acids , s comprises those containing 2, 4, 6, 8 or 10 or 12 carbon atoms.
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Accorcling to the invention, therefore, theré
is provided a rheological or flow control agent comprising a blend in - particulate form of hydrogenated castor oil and an oligomeric amide derived from hydroxystearic acid, one or more saturated ~,; 5 aliphatic alpha-omega diprimary diamines containing 2, 4, 6, 8 or 10 carbon atoms and one or more saturated aliphatic alpha-omega dicarboxylic acids containing from Z to 12 carbon atoms and/or hydrogenated dimer acids.
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The oligomeric amides are prepared by reacting the components together in known manner, typically by heating under conditions of distillation of the evolved water at temperature of about 200C.
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~ Proportions of reactants are such that for each . ~ i . .
mole of hydroxystearic acid there is present 0. 05 to 0, 5 moles of dicarboxylic acid and a quantity of diamine such as to provide 0, 8 to 1,15 amine groups for each carboxyl group present in the acid mixture. It is preferred to use 0, 95 to 1. 05 amine groups . .
per carboxyl group since unreacted carboxyl or amine groups tend to : .
~ diminish the flow-control properties of our products.

, - It is not ~nece6sary for the hydroxystearic acid to be of ,, .
high purity: the crude material produced, for example, by the :.~ .... . .
hydrolysis of HCO may be used, and this is a considerable economic advantage, It is to be understood that functional '' ... .

. , 3L1~ 9 derivatives of the reactants, capable of reacting to form the oligomeric amides may be used in place of the acids and amines themselves. For example, the methyl esters may be used in place - of the acids, when the by-product of the reaction will bé methanol ~ 5 instead of water.
.: . . , The oligorneric amides are blended with HCO by , melting together in a weight ratio 9: 1 to 1: 9, preferàbly 4 :1 to 1: 4, the mixed melt is cooled and allowed to solidify, and the powdered blend is finely powdered by means, for examplej of a fluid energy mill or microniser. The powdered blend preferably has ; an average particle size of less than 25 microns and most~preferably not more than 20% by weight of the powder, particularly not more than 10% by weight of the powder has a particle size above 25 microns.

The rheological control agents of the invention may be lS used in a very wide variety of surface coating compositions, e.g. paints, .,~ . .. .
varnishes, printmg mks, adheslves and laminating agents? based on a - wlde variety of resin systems:such as acrylic resins, alkyd and modified :: ........................................... . . . .
alkyd resins, oleoresins, epoxy resins, polyester resins and chlorinated ; rubbers. The surface coating composition will normally also contain a solvent for the resin component, e. g. an aliphatic or aromatic hydrocarbon such as white spirit, solvent naptha or xylene, an ~a fr.~ rf~) ~ aliphatic alcohol or ether such as butanol or a "Cellosolv~ or ester thereof; or a phenol. It is not nececessary however that the ~''`~ " ,`
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coating system contains a solvent and thus9 for example, the surface coating cornposition may be of the solventless type, e.g.
an epoxy resin/polyamide solventless coating composition or an . , unsaturated polyester based laminating composition.
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The finely powdered blend is incorporated in the ' ' ' ' . -surface coating composition in a known manner. A convenierrt procedure consists in first preparing an approximate 15~o w¦w dispersion of the powdered additive in an organic medium miscible . , , - , with the surface coating medium to be modified. In the case of a . :
decorative paint this will normally be white spirit. Dispersion of ~ the additive in the organic medium is effected by high speed stirring,~
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- durirlg which a temperature of about 35 C will typically be attained, and allowing the dispersion to cool overnight. This dispersion i~
; ~: - . , then added to the paint mill-base prior to grinding and the manufacture of the paint is completed in the conventional manner.
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- The quantity of additive used depends on the precise ; composition of the additive, the degree of flow control desired and :., - , .. . . .
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1~ i8~9 the conditions under which it is to be operative. For the flow control of decorative paints a typical concentration of solid additive would be about 0. 5% calculated on the resin content of the paint.

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Optimum selection of additive and of additive , - :
` ~ 5 content may be carried out by simple routine tests well known in the art. A typical procedure applicable to decorative paints will .. ~ . -:
first be described.
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The basis of the tests may be a sîmple white ~ ~ :
paint pigmented with titanium dioxide at a pigment:binder ratio ~-of 0. 75:i. A suitable grade of titanium dioxide is Tioxide R-CR2 (Tioxide is a Regis~ered Trade Mark). A suitable binder is a long oil linseed allyd such as Synolac S0W tSynolac is a Registered~

Trade Mark). The solvent is white spirit, and 0, 5% lead and 0. 05% cobalt, calculated orl alkyd solids, are incorporated in the form o~their naphthenates as driers, ,;,. - : : _ ' ' ' ~

The pigment and an equal weight of medium are , ~ , . .
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incorporated to provide what is known in the art as the mill base.

: To this is ~added a quantlty of the 15% dispersion of the nOw control additive such as to provide an additive concentration of, say, ,., , : .
20 0. 5% on the total al}~d resin solids of the finished paint. This mixture =~ is ground on a single roll mill to a Hegmann (fineness of grind) ., , , .
gauge reading of 8~, blended with the remaining medium and adjusted to a viscosity of 4 poises at 25 C. and 2500 sec 1 by addition of white spirit.
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" ''' ~L~6~6~9 This procedure may be repeated at different concentrations of additive and with different additives to find the optimum formulation appropriate to a particular paint cornposition under given conditions, ~' "' ' .
The paints may be tested after they have been ; heated to different temperatures, e. g. 40, 80, 120, and subsequently allowed to cool to room temperature for 24 hours, by being drawn down using a Hegmann gauge to evaluate seediness. A Hegmann reading below 6 indicates seeding at a deleterious level, Flow control in terms of sag resistance may conveniently be assessed by means of the well-known dye-line sag test, In this the paint is sheared for about 5 minutes in a laboratory mixer to break down , .
its structure and then quickly applied to a 12" x 4" glass plate ~: . ' , ' carrying on its surface a line of white spirit soluble dye . ~ , .
parallel to the short side. The paint is applied as a wedge lS film of wet thickness 0. 002" to 0, 006" by means of a wedge applicator and the plate is then stood vertically. Any sagging of the paint is .
indicate~d by a downward movement of the dye-line. An arbitrary measure of sag resistance may be taken as the film thickness at : , which a sag of 3mm below the original dye-line is observed. (The - 20 higher the figure, the greater the sag-resistance), .: .

-; The adaption of the forgoing test procedure to industrial stoving paints will be obvious to those skilled in the art, -, However, in these systems the dye-line sag resistance test is less satisfactory because the powerful solvents present tend to cause blurring .

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of the dye-line. A more satisfactory test, readily applicable to the assessment of behaviour in the stoving oven, is the following.
The substrate is a steel panel 12" x 4" with a line of 1/4" diameter holes, 1" apart, drilled parallel to and 1" from one of the long edges. With the panel in a horizontal position, a wedge film of paint is applied, by spraying, ranging from 40 to 100 microns dry film thickness across the panel. The panel is then rapidly placed in the stoviny oven in an upright position with the row of holes uppermost. When stoving is complete sags ^: ~
or tears of varying length, increasing with film thickness, depend from the holes. An arbitrary measure of sagging in the stove may be taken as the film thickness measured in microns just " helow a 1 cm. sag. A high figure denotes good sag resistance.
A typical paint system appropriate to this test pro~
I cedure and used in our evaluations is based on titanium dioxide! (supplied under the trademark"Runa RH472'~, aplasticising alkyd : , .:. . .
resin (supplied under the trademark "Synolac 123X") and a butyl-ated melamine formaldehyde resin (supplied under the trademark `
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"Beetle BE615"). The solids ratio of alkyd to melamine resin is 70:30 and the pigment:binder ratio is 0.8:1.
Again it is convenient to prepare a 15% dispersion of the flow control additive but, with a paint solvent system largely composed of xylol, xylol is used to prepare the dispersion. -~ -i Addition is again conveniently made to the mill base before `~
grinding, :
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~64689 grinding is to a Hegmann gauge reading of 8+ but final thinning is to a viscosity of 3 poises at 25C, 2500 sec 1 using a xylol-butanol mixture corresponding approximately to the solvent composition in the resin mixture. In the presence of such relatively powerful solvents, and for flow control at the temperatures encountered in stoving, higher levels of additive are normally required and 1-4% by weight calculated on the resin solids ' content of the paint is typical. -: , .
In order that the invention may be well understood , .. :
the following preparations and Examples are given by way of illustration only. ~ ~
` . 1: ': ~ ' Preparations 1 -15, .
The oligomeric amides detailed in Table 1 were all prepared by heating I mole of hydroxystearic acid with the designated dicarboxylic acid and diamine in the molar proportions stated, at ... . .
200 C. under conditions of distillation of the liberated water for about 4 hours or until the reaction was substantially complete as , indicated by measurement of acid value.
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Oligomeric Dicarboxylic Dicarboxylic Diamine Diamine amide No. acid acid moles molec :
Sebacic 0,1 Ethylene 0. 6 .. :
diamine ~ ~
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2 Sebacic 0. 25 Ethylene 0. 75 dlamine ~ : ;
; 5 ` 3 Adipic 0. 25 Ethylene ~ 0, 75 ~- . diamine ' ~ 4 Suberic 0. 25 Ethylene 0. 75 :` diamine :~ .
., Sebacic. : 0, 5 Ethylene 1. 0 diamine ~ -.
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. ~ : 6 Sebacic 0.1 Hexamethylene :
diamine ~ 0. 6 '~" Oxalic ~ 0. 5 ~ Ethylene 0. 75 -diamme ~:

~' 10 : 8 Azelaic: ID. 5 :Ethylene 1. 0 diamine 9 : Succinic ~ 0.-1 ~ Ethylene ~ ~ 0. 6 dlamine~ :
: Hydrogenated ~ 0,1 Ethylene 0, 6 dlmer ~ ~ ~ diamine ~ ~
Hydrogenated : ~ 0. 25 Ethylene 0. 75 -~ ~:
dimer : ~ : diamine : ;
. `: 12 Dodecane di~ic 0. 05 Ethylene : 0. 55 diamine ,. . .
¢'~ ~ ~: 15 13 Dodecane dioic 0. 5 Ethylene 1. 0 ~ :
. ~ ~ . diamine 14 Sebacic 0. 1 1, 4 Diamino ,: ~ butane 0. 6 ~ , ,: :
Glutaric 0.1 Ethylene 0, 6 diamine .

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Examples 1 -15 oligomeric amide - MCO blends.

The oligomeric amides and HCO in the stated proportions were heated together until just molten and well mixed. The melt was allowed to cool and solidify. The solidified mixture was broken up in a mortar and then comminuted by treatment in a laboratory grinder. The grinding was less efficient ~-than that which can be obtained in a microniser and the flow- ~ `
contro] effectiveness of the products was therefore below ~`
optimum. Even so, the products were found to be superior to the micronised flow-control additives of the prior art (see following sections) .
Typical blends are listed in Table 2.
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Blend No,Oligomeric Oiigomeric HCO Capillary mp of blend ~, amide No. amide wt. % wt. % C
1 1 '35 65 . 120-130 2 2 ; ~ 35 65` ~ ~ 170-190 .:
~ ~ 3 3 35 - 65 160-1~5 ~ ~
, -' 165~215 , 9 ~ 35 65 , 123 130 8 12 35 65 ~ 125-135 g 13 35 65 205-235 14 35~ 65 160-175 11 1 10 ~0 114-170 ~?~
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12 1 25 7S ) 128-170 13 1 35 65 ) 158-170 14 1 60 40 ) 158-170 -(X 15 6 ;35 6i5 ) 143-155 : , ' ' ' ~ ' ' Effect of temperature on the dyeline sag resistance of decorative paints modified with t flow control additives of the invention.) ,~ . .
'' ' ', , ,/ The simple Tioxide R-CR2/Synolac 50W paint .
described above was prepared using different powdered blends as , flow control additive at the level of 0. 5% on alkyd soIids. After manufacture the paints were heated~to ~7arious temperatures to simulate various degrees of overheating and allowed to mature at room temperatur~e for 24 hours before testing ior sag resistance ;
by the dye-Ilne method described. Results in Table 4 are given as wet film thickness in thou~ands of an mch~at which a 3 mm sag occurred.
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Blend No. Dyeline sag after heating to l ;
Room temp. ~ l20r, (no heating~
1 2,5 3,2 3,5 2,7 2 ~ 3.0 3.7 4.3 3.0 . ~- .

3 2. 5 3. 0 4. 9 5. 9

4 2,5 3,0 3,5 4, g 2. 5 2, 9 2, 9 3. 3 6 3.0 3.4 3,1 3,7 7 . 2.5 2,5 2,9 2.8 l3 ~, ... , , . ' ~64f~

8 2.6 2.5 2,8 2.5 9 2,5 2.8 3,2 3,7 2.5 2.5 5.8 5.8 11 2.5 3.~ 6.0 6.0 12 2,6 3.5 6.~ 6.0 13 2.8 3.5 4.5 ~.0 14 3,0 4.0 3.3 3.7 - ~ 15 2.8 3,5 3,5 4,5 Control A 3. 0 3. 5 3, 0 2. 5 Control B 2, 5 3, 0 4. 4 2, 6 i Control C 2,5 2.5 2,5 2.5 (~ontrol A was HCO, micronised Control B was a commercial product according to U. S, Patent - No. 3, 203, 820 micronised .
Control C was the original paint without llow control additive.

Eff~ct of temperature on seeding of decorative pa nts modified with `~
: , (flow control additives of the invention).

The paints were prepared, heated and matured as described above. They were then drawn down on a Hegmann gauge, Readings of 8 or higher (that of the original paint) were recorded ~ -as "smooth", 6-8 as "slightly seedy", below 6 as "seedy".
Occasionally the action of the drawdown blade caused a further film defect: the paint film was disrupted and became discontinuous.
In such cases the result "broken filrn" is recorded.

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TABLE 5.
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Blend No, Condition of film on Hegmann drawdown after heating to F~oom temp. 40C 80C 120C

(no heating) : 1 Smooth Smooth Smooth Smooth : 2 Smooth Smooth Smooth Smooth :
3 Smooth Smooth Smooth Smooth :.
4 Smooth Smooth Smooth Smooth Smooth Smooth Smooth Broken :
film.
6 : Smooth ~ Smooth ~ Smooth Smooth 7 Smooth Smooth ~ . Sl. seedy Sl. seedy 8 Smooth ~ ~ Smooth~ Smooth ~ Sl. seedy 9 Smooth : ~; Smooth: ~ Smooth Smooth 15: 10 Smooth~ : Smooth:: Smooth Smooth Smootb ~~ Smooth~ Smootb Broke 12 : Smooth ~ ;: SmoothSmooth Smooth 13 ~ ~~ Smooth ~ ; Smooth : Smooth Smooth , .
14 Smooth~ Smooth~ Smoath ~ Broken Film Smooth Smooth Smooth Smooth : :
Control A Smooth Smooth Seedy Seedy ~:
.~ Control B Smooth Smooth Broken Seedy film , . .
Control C Smooth Smooth Smooth Smooth se of a typical additive in an industrial stoving paint, The paint was prepared by the procedure described according to the ~,., ~, '.
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,, ~6~1689 following formulation~ (parts by weight). ~-Titanium dioxide (supplied under the trademark Runa RH472)26.60 Plasticising alkyd in xylol (Synolac 123) 46.65 :~

Butylated melamine-formaldehyde ; resin in n-butanol (supplied under the trademark Beetle 615)16.60 15% dispersion of additive (Blend No. 2) 2.20 Xylol 4-75 n-butanol 3.20 10This was thinned to a viscosity of 3 poises at 25C.
2500 sec 1 using a mixture of 3 pbw xyIol and 1 pbw n-butanol. ~;~
The level of additive employed was 1% w/w on res~in solids.
Seeding tests were carried out in the aforesaid manner against Control D and Control E with the following results.
(Control D contained 1% on resin solids o a commercial additive according to U.S. Patent No. 3,203,820, micronised; Control E
.1 : '-contained no additive).

; Additive Condition of film on Hegmann ~, drawdown after heating to ~

Room temp. 40C 80C 120C
- ~0 2~-22C
(no heating) l Blend No. ;
?~ 2 Smooth Smooth Smooth Smooth -~
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Control D Smooth Smooth Seedy Seedy ;~ ~ ~
., Control E Smooth Smooth Smooth Smooth 1, ~no additive) ., . :, .~, , .
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In the stoving te~t using the drilled steel panels, film thicknesses measured just below a 1 cm sag were Blend No. 2 80 microns : .
Control D 65 microns Control E 50 microns ~::
The action of the additive of the invention and its uperiority ov.er the products of the prior art is thus f~learly .
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demonstrat.ed~

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Example 16 A thermosetting hydroxy acrylic coating composition is prepared from titanium dio~ide (supplied under the trademark "Runa" RH 472) ahydroxy acrylicresin (suppliedunder thetxademark '!Synocryl" 823S) and a butylated melamine formaldehyde resin (supplied under the trademark "seetle" BE 615). The solids ratio of acrylic polymer to melamine resin is 4:1 and the pigment binder ratio is 0.8:1Ø The paint is made up following the pro~
cedure outlined above for the plasticising alkyd resin composi- ~ ~;
tion bu-t using a 3:1 by weight xylol/butanol solvent blend as -thinner. The Elow control additive (blend 2) is added as a 15~ -dispersion in xylol. (1% by weight of the additive based on the total solid resin content of the paint). Using the imprved test described above the following results are obtained.

film thickness below a 1 cm sag ` Hydroxy acrylic paint with no flow control additive 45-50 Hydroxy acrylic paint with flow control additive 80-85 Example 17 A high ~uild chlorinated rubber finish was prepared according to the following formulation:

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Titanium dioxide (supplied under the trademark (Runa RH 472) 12.6 Blanc Fixe 12.6 Chlorinated paraffin plasticiser (supplied under the tr~demark Cereclor 42) 6.1 Chlorinated paraffin plasticiser (supplied under the trademark Cereclor 70) 12.2 Chlorinated rubber (supplied under the trade-mark Alloprene R10) 18.4 Xylol 28.8 ~

15% Dispersion of flow control additive (Blend ~ ;
`~ No. 2) in xylol 9.3 :~ 10 The paint was prepared by high speed stirring in the course of which the temperature rose to 55C. This formulation was compared with two others ! (i) halving the level of flow control additive (ii) omitting the flow control additive ¦~
applying the coatings by brush to abraded steel panels at film thicknesses of 4 thou. The panels were then stored vertically.
Sagging was assessed visuallyi the coating containing no additive ~ -sagges badly at both 4 thou and 10 thou film thickness; the remaining films showed no visual evidence of sagging at either film thickness.

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Example 1~
., Two laminating compositions were prepared from a commercially available unsaturated polyester composition(Synolac ~; ~ 6 34 5) comprising 67% of an unsaturated polyester resin .
In the first composition 0. 5 parts by weight of a flow control additive (Blend No. 2) was milled in 10 parts by weight of the unsaturated polyester solution by high speed dispersion. During the course of this mi2~ing the temperature rose to 40/45 C. This paste was then let down with 90 parts by weight of unsaturated polyester solution.

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A second composition was an unmodified aliquot of the unsaturated polyester solution.
':, , - The viscosity of these two compositions was measured on ~

a Brookfield Viscometer model RVT using a number 2 spindle, at ~ ~-25 C.
Sample with Sample without additive additiv~_ Viscosity (spindle speed 5 rpm) 9. 2p 4. 0p Viscosity (spindle speed 5 rpm) 4 . 2p 4, 05p Thixotropic index (viscoslty at 5rpm) 2 19 (viscosity R t 50 rpm) , ': :
The behaviour of these two compositions was exemplified in a laminate of two layers of 1, 5OZ ft 2 at a reson to glass ratio of 2. 5: 1.
The sample with additive showed only slight signs of drainage in areas , .
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: ~ i where resin excess at the surface; the sample without additive exhibited severe drainage under similar conditions.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A rheological control agent comprising a particulate blend of hydrogenated castor oil and an oligomeric polyamide derived from hydroxystearic acid, at least one saturated ali-phatic .alpha.- omega diprimary diamine containing 2, 4, 6, 8 or 10 carbon atoms and at least one saturated aliphatic .alpha.-omega dicarboxylic acid containing from 2 to carbon atoms and/or hydrogenated dimer acid in such proportions that for each mole of hydroxystearic acid there is present 0.05 - 0.5 moles of dicarboxylic acid and a quantity of diamine sufficient to provide from 0.8 to 1.15 amine groups for each carboxyl group in the acid mixture.

2. A rheological control agent as claimed in claim 1 in which the dicarboxylic acid contains 2, 4, 6, 8 or 12 carbon atoms.

3. A rheological control agent as claimed in claim 1 in which the quantity of diamine is sufficient to provide from 0.95 to 1.05 amine groups for each carboxyl group in the acid mixture.

4. A rheological control agent as claimed in claim 1 in which the hydroxystearic acid is one produced by the hydrolysis of hydrogenated castor oil.

5. A rheological control agent as claimed in claim 1 in which the weight ratio of hydrogenated caster oil to oligomeric amide is from 9 :1 to 1 :9.

6. A rheological control agent as claimed in claim 1 in which the weight ratio of hydrogenated caster oil to oligo-meric amide is from 4 :1 to 1 :4.

7. A dispersion of a rheological control agent in an organic medium, said rheological control agent comprising a particulate blend of hydrogenated castor oil and an oligomeric polyamide derived from hydroxystearic acid, at least one saturated aliphatic .alpha.-omega diprimary diamine containing 2, 4, 6, 8 or 10 carbon atoms and at least one saturated aliphatic a-omega dicarboxylic acid containing from 2 to 10 carbon atoms and/or hydrogenated dimer acid in such proportions that for each mole of hydroxystearic acid there is present 0.05 - 0.5 moles of dicar-boxylic acid and a quantity of diamine sufficient to provide from 0.8 to 1.15 amine groups for each carboxyl group in the acid mixture.

8. A dispersion as claimed in claim 7 in which the organic medium is an aliphatic or aromatic hydrocarbon.

9. A dispersion as claimed in claim 7 containing about 15% w/w of rheological control agent.

10. A surface coating composition comprising an organic binder and having dispersed therein a rheological control agent comprising a particulate blend of hydrogenated caster oil and an oligomeric polyamide derivatve from hydroxystearic acid, at least one saturated aliphatic .alpha.-omega diprimary diamine containing:
2, 4, 6, 8 or 10 carbon atoms and at least one saturated aliphatic .alpha.-omega dicarboxylic acid containing from 2 to 10 carbon atoms and/or hydrogenated dimer acid in such proportions that for each mole of hydroxystearic acid there is present 0.05 - 0.5 moles of dicarboxylic acid and a quantity of diamine sufficient to provide from 0.8 to 1.15 amine groups for each carboxyl group in the acid mixture.

11. A composition as claimed in claim 10 containing about 0.5% of rheological control agent.