Thickening agents for low pressure molding compositions

Low pressure molding compositions comprising a thermosetting resin and a thermoplastic resin are thickened with a combination thickening agent within a specific concentration range to retard rapid initial thickening of the molding compositions.

This invention relates to moldable compositions. 
More specifically, this invention relates to thickening agents for low 
pressure molding compositions. 
In one of its more specific aspects, this invention facilitates the 
production of sheet and bulk molding compounds by providing compositions 
moldable at low pressures which compositions comprise at least one 
alkaline-earth metal hydroxide and zinc oxide as a combination thickening 
agent. 
The production and use of sheet and bulk molding compounds are well known. 
Such molding compounds are based upon unsaturated polyester resin systems 
and are widely used in the production of automobile body parts. Usually 
such systems are based upon a combination of an unsaturated polyester 
resin contained in an monomer and a thermoplastic soluble in the monomer. 
These materials are blended with thickening agents, crosslinking 
catalysts, release agents and, optionally, colorants, inhibitors and the 
like. 
This resulting molding composition in the form of a paste can then be 
processed through equipment to produce sheet or bulk molding compounds. 
For example, sheet molding compound can be produced by laying down a first 
layer of the paste on a first layer of polyethylene film or the equivalent 
thereof, laying on this first layer of the paste filler reinforcements 
such as chopped glass fibers in an amount up to about 30 percent by weight 
of the total weight of the paste, and laying thereover a second layer of 
the paste. The two layers of the paste with the filler reinforcements 
sandwiched therebetween are then topped with a second sheet of 
polyethylene film and the resulting composite (sheet molding compound) is 
stored. 
As used herein, the term "working time" will be understood to mean that 
period during which the molding composition remain in that viscosity range 
within which the molding composition will satisfactorily wet-out and 
impregnate reinforcing material, in particular, glass fibers. 
The prior art teaches molding compounds which are suitable for molding at 
low pressures. Ideally, and in this respect the prior art low pressure 
molding compounds are deficient, the working time should provide 
flexibility to compensate for minor process interruptions which frequently 
occur in the processing of the molding compositions. Accordingly, time is 
required in excess of the amount of time needed to wet-out and impregnate 
reinforcements, so that if a minor process interruption occurs, the 
interruption can be corrected and processing of the molding composition 
continued. Otherwise, a minor process interruption requires the scraping 
of the molding composition. 
The present invention provides a combination thickening agent which, when 
employed in low pressure molding compositions, reduces initial thickening 
rates and extends working time. 
According to this invention, there is provided a thickening agent 
consisting essentially of at least one alkaline-earth metal hydroxide and 
zinc oxide. 
Also according to this invention, there is provided a low pressure molding 
composition comprising an unsaturated, crosslinkable polyester, a monomer, 
a thermoplastic soluble in the monomer and a thickening agent consisting 
essentially of at least one alkaline-earth metal hydroxide and zinc oxide, 
the thickening agent being present in an effective amount to retard the 
rate of thickening of the molding composition as compared to the rate at 
which the molding composition thickens in the absence of the thickening 
agent or in the presence of other thickening agents, such as, calcium 
hydroxide. 
This invention also provides a method for retarding the rate of thickening 
of low pressure molding compositions which method comprises the 
incorporation of a thickening agent consisting essentially of at least one 
alkaline earth metal hydroxide and zinc oxide into a low pressure molding 
composition. 
This invention also provides a molding compound comprising a low pressure 
molding composition comprising an unsaturated, crosslinkable polyester, a 
monomer, a thermoplastic soluble in the monomer and a thickening agent 
consisting essentially of at least one alkaline-earth metal hydroxide and 
zinc oxide, the thickening agent being present in an amount sufficient to 
retard the rate of thickening of the molding composition as compared to 
the rate at which the molding composition thickens in the absence of the 
thickening agent or in the presence of other thickening agents, such as, 
calcium hydroxide. 
As used herein, molding compositions viscosities unless otherwise stated, 
refer to Brookfield viscosities, Model HBT with Helipath stand and T-A 
spindle at 5 rpm and 90.degree. F., this method and its values being 
familiar to those skilled in the art. 
The moldable compositions of this invention, will be comprised, preferably, 
of the following components: 
(a) an unsaturated, crosslinkable polyester resin in an amount within the 
range of from about 55 to about 80 parts by weight per 100 parts by weight 
of resin. The polyester resin will be employed in the form of a 
monomer-containing syrup having a solids or resin content of approximately 
67 percent by weight. The polyester resin will have an acid number, or 
value, within the range of from about 15 to about 45, and preferably about 
30. It will have a molecular weight within the range of from about 1,300 
to about 2,500. (As used herein, "molecular weight" refers to weight 
average molecular weight); 
(b) a thermoplastic resin having an acid number up to about 3.5, the 
thermoplastic being present in the composition in an amount within the 
range of from about 20 to about 45 parts by weight per 100 parts by weight 
of resin. The thermoplastic resin will be employed in the form of a 
monomer-containing syrup having a solids content of approximately 33 
percent. The thermoplastic syrup will have a preferred acid number of 
about 3.0. The thermoplastic will have a molecular weight within the range 
of from about 80,000 to about 350,000, preferably within the range of from 
about 100,000 to 250,000 with a molecular weight of about 160,000 being 
preferred; 
(c) a monomer in which the acid-functional thermoplastic is soluble; 
(d) a mold release agent, or internal lubricant, in an amount of from about 
0.5 to about 5 parts by weight per 100 parts by weight of the resin; 
(e) one or more fillers in an amount within the range of from about 25 to 
about 180 parts by weight per 100 parts by weight of the resin; 
(f) chopped glass fibers in an amount within the range of from about 20 to 
about 150 parts by weight per 100 parts by weight of the resin; 
(g) a crosslinking catalyst in an amount within the range of from about 0.1 
to about 3 parts by weight per 100 parts by weight of the resin; and, 
(h) a thickening agent consisting essentially of at least one 
alkaline-earth metal hydroxide and zinc oxide in an amount within the 
range of from about 1 to about 3.5 parts by weight per 100 parts by weight 
of the resin. 
In the preferred embodiment of this invention, the zinc oxide will be 
present in an amount within the range of from about 1 to about 6 parts per 
part by weight of alkaline-earth metal hydroxide. 
The unsaturated polyesters which can be employed in this invention are the 
poly condensation-products of at least one a, b, - ethylenically 
unsaturated dicarboxylic acid (which term as used herein includes the 
corresponding anhydrides) and dihydric alcohols or oxides. 
Suitable unsaturated dicarboxylic acids include maleic anhydride, fumaric 
acid, itaconic acid, citraconic acid and chloromaleic acid and the like, 
and mixtures thereof. Preferred materials are maleic anhydride and fumaric 
acid. A minor proportion of the unsaturated dicarboxylic acid, that is, up 
to about 25 mole percent, can be replaced by saturated carboxylic acids 
such as ortho-phthalic acid, succinic acid, adipic acid, sebacic acid, 
methyl-succinic acid and the like, and their mixtures. 
Suitable dihydric glycols and oxides which can be employed include 
1,2-propaneldiol, dipropylene glycol, ethylene glycol, propylene glycol, 
diethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 
triethylene glycol, tripropylene glycol, ethylene oxide and the like, and 
mixtures thereof. 
The unsaturated polyester will be soluble in the after-defined monomers in 
which it will be crosslinkable during the molding process to a thermoset, 
continuous phase in which the thermoplastic will exist as the disperse 
phase. 
The thermoplastic is a polymer or copolymer or mixture thereof which is 
prepared employing a free-radical initiator and at least one ethylenically 
unsaturated monomer. Suitable monomers include alkyl methacrylates and 
alkyl acrylates in which the alkyl group contains up to about 18 carbon 
atoms, including alkyl groups chosen from methyl, ethyl, n-propyl, 
iso-propyl, n-butyl, isobutyl, 2-ethyhexyl, stearyl and the like, and 
mixtures thereof. Also suitable are cyclic methacrylates and acrylates 
wherein the cyclic group is chosen from cyclohexyl, benzyl, bicyclic 
groups such as isobornyl, bornyl, fenchyl, isofenchyl, and the like. 
Particularly suitable are monovinyl aromatic compounds such as styrene, 
substituted styrenes such as a-methyl styrene, vinyl toluene, 
tert-butylstyrene, halogen substituted styrenes such as chlorostyrene, 
dichlorostyrene and the like, and mixtures thereof. Also employable are 
acrylonitrile, methacrylonitrile, and mixtures of vinyl chloride and vinyl 
acetate. Cellulose acetate butyrate and cellulose acetate propioniate can 
also be used. 
Preferred thermoplastics are formed from about 80 to about 99.9 weight 
percent styrene copolymerized with acrylic acid and, or, methacrylic acid 
in an amount within the range of from about 0.05 to about 1.9 weight 
percent; or with acrylonitrile in an amount within the range of from about 
1.3 to about 20 weight percent; or with hydroxyethyl acrylate in an amount 
within the range of from about 1.3 to about 20 weight percent. Suitable 
thermoplastic compositions also include graft polymers of styrene and 
polybutadiene, styrene and styrene-butadiene polymers and the like 
containing from about 70 to about 97 weight percent styrene and from about 
3 to about 30 weight percent of the elastomeric group. 
As mentioned, the thermoplastic will have an acid number of up to about 
3.5, the acid functionality being incorporated in the thermoplastic 
employing suitable carboxylic or other acid-functional substituents such 
as acrylic acid, methacrylic acid and the like. The preferred 
thermoplastic comprises the reaction product of about 100 parts by weight 
of styrene and about 0.05 parts by weight acrylic acid, the acid 
functionality being incorporated in the thermoplastic as a component of 
the monomer system used to prepare the polymer. 
The monomer will be one in which the thermoplastic is soluble and which is 
copolymerizable with the unsaturated polyester to produce a crosslinked, 
thermoset matrix. The monomer is a liquid monomer, or mixture of monomers, 
having at least one polymerizable reactive, ethylenically unsaturated 
(--C.dbd.C&lt;) group per molecule. The monomer system can be chosen from the 
group consisting of styrene, substituted styrenes such as vinyl toluene, 
tert-butyl styrene, lower (i.e. C.sub.2 to C.sub.4) alkyl esters of 
acrylic and methacrylic acids, a-methyl styrene, cyclic acrylates, 
methacrylates such as cyclohexyl methacrylate and acrylate, benzyl 
methacrylate and acrylate and the like, bicyclic methacrylates and 
acrylates such as isobornyl methacrylate and acrylate, halogenated 
styrenes such as chlorostyrene, dichlorostyrene, 1,3-butanediol 
dimethacrylate, diallyl phthalate and the like, and mixtures thereof. 
The mold release agents which can be used in this invention are those well 
known in the art and include stearates of zinc, calcium, aluminum and the 
like. 
Various types and concentrations of fillers can be employed in the present 
invention, these including clay, talc, mica, alumina trihydrate, calcium 
carbonate, silicas, carbon black, glass fibers and the like. In the 
composition of the present invention, calcium carbonate and chopped glass 
fibers are employed in the preferred embodiment. 
The crosslinking catalysts will be such as are conventionally employed. 
These comprise free radical catalysts which do not degrade until molding 
temperatures are attained but which provide fast cure after gelation. 
Among the catalysts which can be employed are benzoyl peroxide, tert-butyl 
perbenzoate, tert-butyl peroxide, tert-butyl peroctoate, lauryl peroxide, 
and the like. 
The thickening agent employed in this invention will consist essentially of 
one or more alkaline-earth metal hydroxides in combination with zinc 
oxide. Particularly suitable alkaline-earth metal hydroxides are calcium 
hydroxide, barium hydroxide, magnesium hydroxide and strontium hydroxide.

The following examples illustrate the invention but are not to be 
considered as limiting the invention to the specific materials inasmuch as 
the invention is employable with all resins normally used to produce 
molding compounds. 
EXAMPLE I 
A thermosetting polyester resin syrup was prepared from the following 
material in substantially the following manner. 
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Materials Pounds 
______________________________________ 
Propylene Glycol 428.6 
Isophthalic Acid 85* 131.5 
Maleic Anhydride 439.8 
Toluhydroquinone (Solid) 
0.33 
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*(Isophthalic Acid 85 - 85% Isophthalic Acid, 15% Terephthalic acid) 
The polyester resin syrup was prepared by charging all of the propylene 
glycol and all of the Isophthalic Acid 85 into the reactor while using a 
continuous nitrogen sparge. The temperature of the contents of the reactor 
was raised to 200.degree. F., agitation started and the nitrogen sparge 
continued overnight. Next, all of the maleic anhydride was added to the 
reactor contents and the temperature of the contents was raised to 
420.degree. F. and about 6 hours after the first distillate, when the 
reactor contents had reached an acid value of about 40, about 0.09 pound 
of solid toluhydroquinone was added. The reactor contents were heated for 
about 2 hours, the contents reached an acid value of about 32 and the 
contents of the reactor were then cooled to 350.degree. F. 
In another reactor, 428.6 pounds of styrene and 0.24 pound of solid 
toluhydroquinone were mixed and heated to a temperature of 135.degree. F. 
Thereafter, 890.7 pounds of polyester resin, produced as described above 
and at a temperature of about 200.degree. F. were added to the 
styrene-toluhydroquinone mixture to produce a thinned polyester syrup 
which had a viscosity (LVT, #4, 60 rpm, 77.degree. F.) in the range of 
2,000-2,500 cps, a maximum water content of about 0.08 weight percent, an 
acid value of from about 18-22 and a monomer content within the range of 
about 32-35 weight percent. 
EXAMPLE II 
This example illustrates the preparation of a thermoplastic resin, 
subsequently blended with the thermosetting resin produced in Example I, 
as herein described. 
The materials of the thermoplastic resin and the method preparation were as 
follows: 
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Materials Pounds 
______________________________________ 
Styrene 7085 
Acrylic Acid 34 
Soloprene 1205C* 808 
Poly BDR-45-HT 457 
Benzoyl Peroxide 12 
Toluhydroquinone (Solid) 
3.2 
______________________________________ 
*Soloprene 1205C - a 25/75 styrene-butadiene block copolymer available 
from Phillips Petroleum Co. 
**Poly BDR-45-HT - a polybutadiene polymer available from Arco Chemical 
Co. 
The total amounts of all the materials except the benzoyl peroxide and the 
toluhydroquinone were charged to the reactor. The contents of the reactor 
were purged with nitrogen and heated to 163.degree. F. At 163.degree. F., 
the benzoyl peroxide was added to the reactor contents. The reactor 
contents were cooled and at 35% solids content, the toluhydroquinone was 
added to the reactor contents. The contents of the reactor were recovered 
as a thermoplastic resin and had a viscosity (LVT, #4, 60 rpm, 77.degree. 
F.) of from about 4,000-7,000, an acid value of from 2.5 to 3.5, a solids 
content of from about 30.5 to 33.5 and a maximum water content of 0.08 
weight percent. 
The thermoplastic prepared above was incorporated into a series of molding 
compositions with the thermosetting resin produced in Example I, the 
principal difference in each molding composition being in the ratio of 
calcium hydroxide to zinc oxide employed as the thickening agent. The 
molding composition formulations, eight in all, were prepared as follows, 
all components being expressed in parts by weight: 
TABLE I 
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MOLDING COMPOSITION FORMULATIONS 
Composition No. 
I (Control) 
II III IV V VI VII VIII (Control) 
__________________________________________________________________________ 
Materials (parts by weight) 
Resin of Example I 
60 60 60 60 60 60 60 60 
Thermoplastic of Example II 
40 40 40 40 40 40 40 40 
tert-butyl-perbenzoate 
1 1 1 1 1 1 1 1 
Zinc Stearate 4 4 4 4 4 4 4 4 
Calcium carbonate 
150 150 150 150 150 150 150 150 
Ca(OH).sub.2 0.5 .5 .5 .5 .5 .5 .5 1.25 
ZnO 0 .5 1.0 1.5 2.0 2.5 3.0 0 
__________________________________________________________________________ 
Viscosity determinations were made on all compositions with the results 
shown in Table II. 
TABLE II 
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Molding Composition No. 
I (Control) 
II III IV V VI VII VIII (Control) 
__________________________________________________________________________ 
ZnO, parts by weight 
0 0.5 1.0 1.5 2.0 2.5 3.0 0 
Viscosity (in M cps) 
Initial 36.5 39 40 39 39.7 
58.4 39 40 
After 15 minutes 
39.7 45.1 
51.2 
53.8 
55.7 
56.6 70.1 49.6 
After 30 minutes 
42.6 51.5 
59.8 
63.4 
71.7 
84.5 99.7 71.4 
After 60 minutes 
48.0 68.2 
92.8 
109.1 
139.5 
201.6 
251.2 
267.8 
After 120 minutes 
57.9 122.9 
217.6 
358.4 
771.2 
1200 1400 4200.0 
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It will be seen from the drawing, which is a graphic representation of the 
viscosity data of Table II, that the viscosity rise of the compositions 
employing a thickener which consists essentially of at least one 
alkaline-earth metal hydroxide and zinc oxide provide molding compositions 
having extended working times. 
Molding Composition No. I (Control) is included to establish a base curve; 
Molding Composition No. I does not comprise zinc oxide and is not a 
composition according to this invention. Furthermore, Molding Composition 
No. I is not suitable to produce molding compounds since its viscosity 
rise is such that it would never achieve a molding viscosity. 
Molding Composition No. VIII (Control) represents a low pressure molding 
composition according to the prior art and is included to demonstrate the 
rapid viscosity rise in a molding composition which does not employ a 
thickening agent which consists essentially of at least one alkaline-earth 
metal hydroxide and zinc oxide. 
Molding Compositions Nos. II-VII are compositions according to the present 
invention and demonstrate that low pressure molding compositions which 
comprise a thickening agent consisting essentially of an alkaline-earth 
metal hydroxide and zinc oxide, employed within the range of from about 
one to about six parts of zinc oxide per part by weight of alkaline-earth 
metal hydroxide, possess extended working times. 
It will be evident from the foregoing that various modifications can be 
made to this invention. Such, however, are considered as being within the 
scope of this invention.