CA1074191A

CA1074191A - Lightweight glass fiber reinforced polyester laminate and method of making the same

Info

Publication number: CA1074191A
Application number: CA250,123A
Authority: CA
Inventors: Eugene J. Grandlic; Howard P. Cordts
Original assignee: Freeman Chemical Corp
Current assignee: Freeman Chemical Corp
Priority date: 1975-06-06
Filing date: 1976-04-09
Publication date: 1980-03-25
Also published as: BE841539A; FR2313187A1; GB1493547A; DE2618506A1; JPS51148781A

Abstract

ABSTRACT

Low density laminates of glass fiber reinforced unsatu-rated polyester resins are obtained by combining an unsaturated polyester resin syrup with an alpha hydroxy azo blowing agent immediately prior to spraying with randomly oriented chopped glass fibers which are thereupon collected on a substrate. The resin is cured on the substrate to produce a glass fiber reinforced laminate having a low density, that is, less than 75 percent of the density of a similar laminate produced from the same unsaturated polyester resin syrup in the absence of the alpha hydroxy azo blowing agent.
Inert particulate fillers can be incorporated in the unsaturated polyester resin syrup. Alumina trihydrate is a preferred inert particulate filler.
In one method, the unsaturated polyester resin syrup is mixed with the alpha hydroxy azo blowing agent immediately prior to forming a spray of the mixture. In another method, the unsaturated polyester resin syrup is formed into a first distinct spray and impinged upon a second distinct spray of the alpha hydroxy azo blowing agent. The two combined sprays are impinged against chopped glass fibers to produce the laminate.

Description

The invention concerns glass fiber laminates utilizing unsaturated polyester resin syrup. Unsaturated polyester resin syrup and chopped glass Eibers have been combined to produce glass fiber reinforced plastic laminates. See "Polyesters and Their Applications", Bjorksten et al, Reinhold Publishing Corporation, 1956. The resulting laminates have a high strength-to-weight ratio, good resiliency, electrical properties, resistance to corrosion, moldability. Such materials have been employed in aircraft components, boats, automobile components, light transmissive building sheeting, snowmobiles, golf carts, chairs, piping, tanks, window surrounds, bathroom vanities, bathtubs and shower stalls, building panels.
One type of product, known as a lay-up or spray-up laminate, is obtained by spraying the unsaturated polyester resin syrup along with appropriate free radical initiators, accelerators, surfactants, inhibitors, pigments, dyes or fillers against a stream of chopped glass fibers whereby the fibers are wetted and collected on a substrate. Customarily the wetted glass fibers are thereupon rolled or tamped to embed the loose ends of the glass fibers and eliminate gas pockets. The wetted fibers are retained in engagement with s ~-;
the substrate until the resinous components cure. Customarily the free radical initiator and the promoter are selected so that the curing will commence at room temperatures. The curing is exothermic and causes a general increase in the temperature of the laminate. Complete cures in spray-up or lay-up operations normally require about one to four hours before the glass fiber reinforced laminate can be separated from the substrate and withdrawn as a product.

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The use of alpha hydroxy azo blowing agents to producefoamed polyester castings is known. See copending Canada patent application serial number 237,381 filed October 6, 1975, and assigned to the assignee of this invention.
Other prior art attempts to develop foamed polyesters include the use of (a) a mixture of alkali metal carbonate and an organic acid, British 652,770 and U.S. 3,479,303; (b) dissolved vaporizable organic blowiny agents such as trichloro fluoro methane, U.S~ 3,232,893. No successful foamed polyester product has been produced with saturated alkanes or halogenated alkanes because the vaporization oE the blowing agent (a) occurs before the resin has gelled sufficiently to form a foam or (b) occurs after a strong gel has already been formed.
According to the present inVention~ glass fiber rein-forced unsaturated polyester resin articles are obtained i:~ without significant sacrifice in the strength of the product yet with a substantially lowered density which permits production of useful articles with less materials. In ~ 20 general, the density of the glass fiber reinforced laminates of this invention is less than 75 percent of the density of a similar glass fiber rein~orced article formed from the ~; identical unsaturated polyester resin by the methods of the prior art.
According to one embodiment of the present invention an unsaturated polyester resin syrup is formed into a first :-i~
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i; suitable initiator of free radicals, a promoter, inhibitors ~ and surfaFtants. A second stream containing an alpha hydroxy ~ ~ :

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74~9~1 azo blowing agent is delivered to the mi~in~ ~one. The two streams are combined in such proportions that their combined total includes from 0.5 to 5.0 parts by weiyht of the alpha hydroxy a~o blowing agent for each 100 parts by weight of unsaturated polyester resin. The two streams are mixed to form a combined stream which is immediately sprayed through a nozzle onto chopped glass fibers which are wetted by the combined stream and are collected as a coating on a substrate.
The collected wetted fibers are rolled or tamped and the resin is allowed to cure. After the resin has cured, the resulting laminate can be separated from the substrate or can be recovered in combination with the substrate as a product. The resin possesses useful adhesive properties which may be exploited if the coating is intended to remain on the substrate. Where the substrate is merely a form for shaping the laminate, a release-coating or barrier membrane may be applied to the substrate to thwart the adhesive property.
AGcording to an alternative embodiment the unsaturated ~0 polyester resin syrup, along wi-th free radical initiator, promoter, inhibitor and surfactant, is formed into a first spray. An alpha hydroxy azo blowing agent is formed into a second spray. The two sprays are impinged in a ratio such that the combined sprays contain from 0.5 to 5.0 parts by weight o~ the alpha hydroxy azo blowing agent for each 100 parts by weight of unsaturated polyester resin. The combined sprays impinge upon chopped glass fibers which are collected ; as a coating~ rolled or tamped. Thereupon the resin is allowed to cure as in the first method.

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~9'74~1 The resulting article has a density of 15 to 60 pounds per cubic foot and contains from 15 to 50 weight percent of randomly oriente~ glass fibers conf:;ned in a continuous mass of cellular polymerized unsaturated polyester resin syrup.
The cells of the article are filled with a gas having a greater nitrogen concentration than the atmospheric nitrogen concentration.
Cellular plastic materials have been defined as "a plastic, the apparent density o~ which is decreased sub-stantially by the presence of numerous cells disposed through-out its mass." The Eoaming of the present process arises from the nitrogen gas which is developed from the alpha hydroxy azo blowing agent. The expanding nitrogen gas creates the cells and is confined therein in concentrations which exceed the atmospheric nitrogen concentration.
! The polymerized unsaturated polyester resin is essen-tially free of urethane linkages and allophanate linkages which are an identifying feature of polyurethane foams.
FIGURE 1 is a schematic illustration of apparatus useful in practicing a preferred embodiment of the present invention.
FIGURE 2 is a fragmentary schematic illustration of , equipment for practicing an alternative embodiment of the ~; present inventionO
;~ The three essential ingredients of tAe present invention ! ~ are chopped glass fibers, unsaturated polyester resin syrups and alpha hydroxy azo blowing agents.
Unsaturated Polyester Resin Syrup -~
The unsaturated polyester resin syrup is a solution of , ' ~ :
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an unsaturated polyestex resin in a copolymerizable monomer which is usually styrene but may be other ethylenically unsaturated monomers such as vinyl toluene, divinyl benzene, acrylic acid, methacrylic acid, alkyl acrylates and meth-acrylates, ortho-chlorostyrene, alpha-methylstyrene, ethylene glycol dimethacrylate. The unsaturated polyester resin is formed by polyesterification of polyol and polycarboxylic acid or polycarboxylic acid anhydride at least a portion of which contains ethylenic unsaturation. Typical polyols include glycols such as ethylene glycol~ propylene glycol, butylene glycol, neopentyl glycol, diethylene glycol, di-propylene glycol, polyethylene glycol, polypropylene glycol and the like. The polycarboxylic acid or anhydride may include materials which do not possess ethylenic unsaturation such as phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, adipic acid, succinic acid, glutaric acid, malonic acid, pimelic acid, sorbic acid, halogenated dibasic acids. Typical unsaturated dicarboxylic acids include maleic acid, maleic anhydride, fumaric acid, endo-methylene-tetrahydrophthalic acid or anhydride, itaconic acid and the like. Typically the unsaturated polyester resin is prepared by polyesterification of an excess of polyol with polycarboxylic acid. Typically at least 40 mol percent of the polycarboxylic acid is ethylenically unsatu-rated polycarboxylic acid. The acid and polyol are cooked in the presence of a polyesterification catalyst until the polyesterification is essentially completed as indicated by the acid number of the resulting product being reduced to 30 or less. The unsaturated polyester resin syrup contains , . . . . . . . .

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about 1 part by weight of copolymerizab:Le monomer for every 1 to 10 parts by weight of -the unsaturated polyester resin.
Normally the unsaturated polyester resin syrup contains 25 to 45 weight percent monomer and 75 to 55 weight percent unsaturated polyester. In connection with the present invention, when the monomer content is below about 25 percent, the resulting syrup is difficult to spray. When the monomer content exceeds 45 weight percent the monomer tends to separate out as a distinct phase from the foaming, ungelled resin.
Initiator A high temperature initiator is preferably employed in the present process. The initiator is provided from 0.2 to

2 percent by weight of the unsaturated polyester resin syrup. Useful initiators include tertiary butyl perbenzoate, ditertiary butyl perben~oate, cumene hydroperoxide, methyl-ethylketone peroxide and tertiary butyl hydroperoxide.
These initiators are ineffective at room temperature but become effective when the temperature is increased, particu-larly in the presence of metal salt promoters.
A polymerization initiator is not required in the polyester resin syrup. Instead the resin can be cured by means of the alpha hydro~y azo blowing agent alone although the resulting product tends to be soft, friable and somewhat crumbly. The additional initiator is preferred in the unsaturated polyester resin syrup. Peroxy type initiators, if employed, are used in the unsaturated polyester resin syrup. Other types of initiators may be combined with the alpha hydroxy azo blowing agent or with the unsaturated polyester resin syrup.

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Metal Salt Promoters The present compositions preferably contain metal salt promoters in amounts from 1 to 100 parts by weight per million (p.p.m.) of the weight of the unsaturated polyester resin syrup. A preferred promoter is copper naphthenate solution although cobalt salts, vanadium salts, manganese salts, calcium salts, magnesium salts can also be employed~
Surfactants In order to produce a foamed product having relatively uniform foam cell sizes, a small quantity of a silicone surfactant is employed in the amount of 0.1 to 2.5 weight percent of the unsaturated polyester resin syrup.
Inhibitors Customarily unsaturated polyester resin syrups are provided with polymerization inhibitors to forestall unin- -tended premature gelation.
Fillers The unsaturated polyester resin syrup may be filled with inert materials such as pigments, dyes, aluminum oxide trihydrate, silica, limestone, ground glass, clays, mica and thixotropic additives such as silica aerogel.
Where thixotropic additives are employed they are used in quantities ranging from about 1/2 to about 3 percent by weight of the unsaturated polyester resin syrup. The other inert fillers may be employed up to about an equal weight of the unsaturated polyester resin syrup.
A particularly preferred inorganic filler is powdered aluminum oxide trihydrate from about 70 to 110 percent of - the weight of the unsaturated polyester resin syrup. At ' ::

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concentrations above about 110 percent by weight aluminum oxide trihydrate, the resulting mixture cannot be sprayed.
At concentrations below about 70 percent by weight aluminum oxide trihydrate, no substantial benefit is observed.
However when the aluminum oxide trihydrate is employed from about 70 to 110 percent by weight of the unsaturated poly-ester resin syrup, the resulting larninate exhibits outstandiny flame spread ratings and smoke inde~ ratings.
he ~lpha__H~d oxy Azo Blowing Agent Alpha hydroxy azo blowing agents have the following general formula:

HO-C-N=N-R3 wherein Rl and R2 are lower alkyl groups having from 1 to 4 carbon atoms and R3 is a tertiary alkyl group having ~ to 8 carbon atoms or an aromatic substituted tertiary alkyl group having 9 to 12 carbon atoms. Examples of the R3 substituent are the tertiary-butyl radical and the alpha-cumyl radical.
A preferred alpha hydroxy azo blowing agent is identi-fied as blowing agent I having the following formula:

HO-C-N=N-C(CH3)3 2H5 . '.
The azo blow:ing agent may be employed full strength or it may be diluted with inert solvents which will not impede the polymerization of the polyester resin syrup. Such solvents include mineral oil and styrene for exampleO
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Glass Fibers The glass fibers employed in the present reinforced articles are normally the type which are conventionally used in glass fiber reinforced articles, that is, chopped glass roving with an average length of about 3/~ inch to about 4 inches although lengths ranging from 1/2 inch to 1 inch are normally preferred. Commercial glass roving i8 usually coated with a silane sizing to improve bond with khe resins.
The glass fibers normally constitute from about 5 to 30 weight percent of the product.
The Present Process As shown in FIGURE 1, a tank 10 is supplied with an unsaturated polyester resin syrup containing suitable quanti-ties of free radical initiator, promoter, surfactant, and, if desired, inert fillers. A tank 11 contains the alpha hydroxy azo blowing agent in liquid form. A spool 12 contains glass fiber rovings of the type customarily employed in producing glass fiber reinforced plastic laminates.
Unsaturated polyester resin syrup from the tank 10 is delivered through a tube 13 to a spray gun 1~. The alpha hydroxy azo blowing agent is delivered from the tank 11 through a tube 15 to the spray gun 14. A supply of pressur-ized air is delivered through the tube 16 to the spray gun j 14. Within the spray gun 14 there is a mixing chamber where the unsaturated polyester resin syrup and the alpha hydroxy a~o blowing agent are mixed and formed into a spray 17 by the atomization from the compressed air in the spray gun 14.
Concurrently the glass fiber roving 18 is drawn from the spool 12 and chopped in a chopper 19 to form a descending : ;, _ 9 _ .
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cloud 20 of chopped glass fibers which impinge the spray 17 and thereby become wetted with the spray mixture. The wetted ~ibers 17 fall onto a substrate 21 and are collected as a coating 22. The exposed surface 23 of the substrate 21 is coated with a suitable mold release agent to facilitate subsequent separation of the coating 22 from the substrate 21. Alternatively the substrate 21 may be a ~inal product which will contain the coating 22 in its completed form. An example i5 where the substrate 21 is a premolded sheet of 10 acrylic resin shaped into the form of a bathtub. The coating 22, as depos~ted, contains randomly oriented glass fibers and objectionable air pockets. Accordingly, it is preferred that the coating 22 be rolled and/or tamped to embed any projecting glass fibers and to minimize the unintended porosity resulting from unwanted air pockets. The rolling and tamping should be carried out promptly inasmuch as the mixture of unsaturated polyester resin syrup and alpha hydroxy azo blowing agent commences gelation almost immedi-ately upon mixing. The coating 22, following rolling and/or 20 tamping, should be allowed to remain quiescent ~or a suitable time to allow the coating to develop an e~otherm, to complete cure and commence cooling.
Example 1 - A commercially available unsaturated poly-ester resin is fabricated from 105 mol parts propylene glycol, 50 mol parts isophthalic acid and 50 mol parts ~-maleic anhydride. The three ingredients are cooked to a final acid value between 17 and 25, measured at 60 percent solids content in methyl cellosolve solvent. 73 parts by weight of the described unsaturated polyester resin is ' ~ :
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combined with 27 parts by weight of styrene to produce an unsaturated polyester resin syrup having an acid value of 27 to 33.
The unsaturated polyester resin was combined with 0.5 weight percent tertiary butyl perbenzoate as a free radical initiator, 1 percent by weight silicone fluid as a surfactant, 4-1/2 parts per million by weight of copper naphthenate as a promoter. No filler was employed in this example. The chopped glass roving was a type known in the trade as 60-end roving chopped into lengths of l-1/2 to 3 inches. The alpha hydroxy azo blowing agent I was employed at a flow rate of 1 percent by weight of the unsaturated polyester resin flow rate. Both the unsaturated polyester resin and the alpha hydroxy azo blowing agent I were introduced into a spra~ gun ~-combined with a glass roving chopper. The chopped glass fibers, wetted with the combined spray, were applied against a drum lid mold which was previously coated with a mold release agent. ~he spray was applied in three passes over the surface. The coated drum lid was rolled with a glass compacter roller and allowed to rise freely at room tempera-ture. The laminate was demolded from the drum lid after about 15 minutes. The laminate had a flexural strength of 4100 psi, a flexural modulus of 0.26 x 106 psi, a tensile strength of 2900 psi, a tensile modulus of 0.217 x 106 psi, ~ a density of 26 pounds per cubic foot and a closed cell ;~ content of about 92 percent. The laminate contained 26 percent by weight glass fibers. The starting thickness of the laminate after compacter rolling was 0.125 inch. After ;~` curing the final :Laminate thickness was 0.432 inch.

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_ample 2 - A conventional lamlnate was prepared from the unsaturated polyester resin described in Example 1 combined with an equal weight of powdered aluminum oxide trihydrate. The 50-50 mixture of aluminum oxide trihydrate and unsa-turated polyester resin syrup was sprayed with chopped glass fibers to produce a laminate having a ~inal density after cure of about 90 pounds per cubic foot.
Example 3 - The filled unsaturated polyester resin syrup of Example 2 including an equal weight of aluminum oxide trihydrate was combined in a spray gun as described in Example 1 with 1 percent by weight of alpha hydroxy azo blowing agent type I. Chopped glass fibers wetted with the combined spray produced a laminate having a density of about 45 pounds per cubic foot. This laminate was subjected to combustion tests in an Underwriters' Laboratories test and yielded a smoke index of 182 and a flame spread of 44. The glass content in the panel was 20 percent by weight. The panel had a thickness from about 1/4 inch to about 1/2 inch.
The laminate of Example 3 had a density (45 p.c.f.) about 50 percent of the density of the Example 2 laminate (90 p.c.f.).
Exampla 4 - The present foamed resinous materials were .
employed in the construction of a number of boats, one of which will be described. A 21 foot long inboard motor boat hull was prepared from a commercial hull mold which itself was fabricated from reinforced polyester materials. Initially a mold release agent was applied to the surface of the hull mold by wipi~y with a rag~ Thereafter a pigmented gel coat was sprayed over the entire surface and allowed to cure.

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Therea~ter a conventional unsaturated polyester resin and glass fiber spray-up coating was applied over the cured gel coat in an average thickness of abo~lt 80 mils. This spray-up coating was allowed to cure. Thereafter a number of pre-cut wooden reinforcing stringers and cross-pieces were applied against the cured spray-up coating by press fitting.
Thereafter a polyester resin foam and ylass fiber coating of this invention was applied to the original spray-up coating to an initial thickness of about 15 mils prior to foaming. After foaming the coating had a thickness of about 40 mils. The foam-glass fiber coating covered the si~e surfaces of the wooden stringers and cross-pieces and served as a binder for retaining them permanently in position.
After the foamed resin coating had been cured, a further skin coating of unsaturated polyester resin and glass fibers was applied as a spray-up coating in a thickness of about 80 mils. The total thickness of the boat hull was 0.4 to 0.5 inch. The total weight of the boat hull including wooden stringers and cross-pieces was about 400 pounds.
Corresponding boats are made commercially from conven-tional polyester resin and glass spray-up techniques in two separate spray-up operations having a total thickness of 0.25 to 0.375 inch. A conventional boat hull, if overturned, will sink in water. The boat hull manufactured in accordance with this invention had the same total weight as the conven-tional boat hull but floated when overturned.
The foamed polyester glass spray-up employed a polyester fabricated from a 50/50 weight percent mixture of isophithalic acid and maleic anhydride esterified with a slight excess of : ' i........ . , ~ . ~..... . . . . . . . .
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propylene glycol to an acid value of about :L8 at 60 percent solids. The polyester is mixed into a syrup containing 40 percent by weight styrene, 0.5 ~eight percent of a silicone surfactant, 0.7 weight percent tertiary butyl perbenzoate and 6 parts per million of copper naphthenate. This material is combined with 1.75 weight percent (based on the weight of the polyester syrup) oE the azo blowing agent I. The chopped glass fibers constitute 1~ percent of the weight of the foam layer. Two samples of the foamed g:Lass layer taken at different times showed densities of 23 pounds per cubic foot and 34 pounds per cubic foot.
'rhe foamed polyester-glass fiber coating served to adhere the wooden stringers and wooden cross-pieces firmly to the boat hull surfaces.
General __ Inhibitors which are customarily employed with unsatu-rated polyester syrups are not efEective in the presence of alpha hydroxy azo blowing agents. In a typical unsaturated polyester resin syrup, the addition of 1,000 p.p.m~ hydro-20 quinone will extend the gel time about three-fold. Where an alpha hydroxy azo blowing agent is employed, 1,000 p.p.m.
hydroquinone extends the gel time by only about 10 percent.
Alternative Method Referring to FIGURE 2, an alternative method for making the present spray-up, lay-up laminates is illustrate~ which employs two different spray nozzles identified by the numerals 24, 25. Compressed air from a tube 16A is delivered to both of the nozzles 24, 25. The unsaturated polyester resin syrup is delivered through a tube 13A to the nozzle 24. The ~, ,~ :

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alpha hydroxy azo blowing agent is delivered through a tube 15A to the spray nozzle 25 whence it emanates as a spray 26.
The unsaturated polyester resin syrup is delivered as a spray 27 from the nozzle 24. The two sprays 26, 27 combine to produce a composite spray 28 wherein the alpha hydroxy azo blowing agent is intimately admixed with the unsaturated polyester resin syrup. The combined spray 28 wets a descending cloud 20A of glass fibers to form a coating 22A on the substrate 2lA.
Some of the useful products which can be fabricated from the present process include one-piece tub and shower stall units, bathroom vanities, automobile fenders and hoods, cold molded formed shapes such as building window surrounds and cornices, backing for thermoformed acrylic sheeting, snowmobile shrouds, barge covers, bui.lding infill panels, mobile homes, liners for railroad boxcars, tote boxes, silo roofs and panels, tanks and the like, boat hulls, surfboards.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A glass fiber reinforced article containing 15 to 50 percent by weight of randomly oriented glass fibers confined in a continuous mass of cellular polymerized un-saturated polyester resin syrup, said article being a density of 15 to 50 pounds per cubic foot, wherein the said cells contain a gas having a greater nitrogen content than the atmosphere.

2. The glass fiber reinforced article of Claim 1 substantially free of urethane and allophanate linkages.

3. The glass fiber reinforced article of Claim 1 containing 70 to 110 percent by weight of powdered aluminum oxide trihydrate based upon the weight of said polymerized unsaturated polyester resin syrup in the said article.

4. A method of producing a glass fiber reinforced plastic laminate comprising:
delivering to a mixing zone a first stream of liquid polymerizable unsaturated polyester resin syrup containing a free radical initiator and a metal salt accelerator;
delivering a second stream of an alpha hydroxy azo blowing agent to the said mixing zone at a rate such that the combined first and second stream contains 0.5 to 5.0 parts by weight of the said alpha hydroxy azo blowing agent for each 100 parts by weight of unsaturated polyester resin syrup;

mixing the two streams in the said mixing zone and immediately spraying the resulting mixture onto a stream of chopped glass fibers;
collecting the said chopped glass fibers, wetted with the said mixture, as a coating on a substrate; and curing the said coating to produce a glass fiber rein-forced plastic laminate having a density less than 75 percent of the density of a corresponding laminate which would be obtained in the absence of the said alpha hydroxy azo blowing agent;
the said alpha hydroxy azo blowing agent having the formula wherein R1 and R2 are lower alkyl groups having from 1 to 4 carbon atoms and R3 is a tertiary alkyl group having 4 to 8 carbon atoms or an aromatic substituted tertiary alkyl group having 9 to 12 carbon atoms.

5. The method of producing a glass fiber reinforced plastic laminate comprising:
forming a first spray of polymerizable unsaturated polyester resin syrup containing a free radical initiator and a metal salt accelerator;
forming a second spray, distinct from the said first spray, from an alpha hydroxy azo blowing agent;
impinging said second spray and said first spray to produce a combined spray containing 0.5 to 5.0 parts by weight of the said alpha hydroxy azo blowing agent for each 100 parts by weight of the said unsaturated polyester resin syrup;
directing the said combined spray against a descending stream of chopped glass fibers and collecting the chopped glass fibers wetted by the combined spray as a coating on a substrate;
curing the coating on the said substrate to produce a laminate having a density less than 75 percent of the density which would be obtained from practicing the process in the absence of the said alpha hydroxy azo blowing agent;
the said alpha hydroxy azo blowing agent having the formula wherein R1 and R2 are lower alkyl groups having from 1 to 4 carbon atoms and R3 is a tertiary alkyl group having 4 to 8 carbon atoms or an aromatic substituted tertiary alkyl group having 9 to 12 carbon atoms.

6. The method of Claim 4 wherein the said alpha hydroxy azo blowing agent has the formula

7. The method of Claim 5 wherein the said alpha hydroxy azo blowing agent has the formula

8. The method of Claim 4 wherein the said first stream includes powdered aluminum oxide trihydrate in an amount sufficient to constitute 70 to 110 weight percent of the said unsaturated polyester resin syrup.

9. The method of Claim 5 wherein the said first stream includes powdered aluminum oxide trihydrate in an amount sufficient to constitute 70 to 110 weight percent of the said unsaturated polyester resin syrup.