CA1062722A - Mixed-terminated polyester plasticizers - Google Patents

Abstract

ABSTRACT
Polyester plasticizers having improved low temperature properties are obtained by terminating the polyester with mixed chain-terminating agents. Mixtures of monobasic acids and monofunctional alcohols are em ployed for this purpose. The polyesters of this invention find particular utility as plasticizers for PVC homopolymers and copolymers.

Description

Z~Z2 The use of polyesters as plasticizers fOT polyvinylchloride ~PVC) homopolymers and copolymers is known. The use of these plastici~ers has grown markedly in recent years due to their low volatility and rasist-ance to migratiDn. There are, ho~ever, certain disadvantages associated with the use of polyes~er plasticizers. The polyesters are not as effi-cient as simple ester plasticizers and large amounts are ~herefore re-quired to achi0ve the same level of flexibility - with resulting decrease in the mechanical properties of the resin. Also, the heretofore known polyesters adversely af~ect the low te~perature properties of the final produc~ as compared to ~he si~ple ester plasticizers.
It is am object of this invsntion to produce mixled-terminated polyesters capable of imparting flexibility and softness for PVC and other resinous materials A fur~her objective is to provide plasticizers having superior low temperature performance, low volatility and resistance to extraction.
We hsve now discovered mixed-terminated polyes~ers which are effective plastici~ers for a variety of resins, including PVC homopolymers and copoly~ersO ~ui~e unexpectedly it has been fouDd that with the poly- -esters terminated in accordance w;th this i~vention it is possible ~o achieve improved low *emyarature performan e of the plasticizsd vinyl resins as compared to identical resins plasticized with polyesters term-inated either solely with monobasic acids or solely with ~onofunctional alcohols. The present mixed-tel~nat0d plasticizeTs, in addition to hav-ing cxcellent low temperature properties9 exhibit low volatility, excellen~
perm~nonce and resistance to exkrac~ion with aqueous and organic solutions.
Accordi~g to one aspect of ~he inven~ion, there is provided a ~ -mix~d-t~rmina~ed polyester ha~ing an average molecular weigh~ between 500 and 5000 obtained by the reaction of (a) an alipha~ic glycol containing 2 to 6 carbon atoms, ~b) an aliphatic dibasic acid containing 4 to 12 carbon ~-atoms or a mîxture o C4 1~ aliphatic dibasic acid and aroma~ic dib~sic .. ..
.. .......
: , : . . . : ., : . :
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acid or anhydride ~hereof, and (c) a mixture of aliphatic monobasic acid con~aining 4 to 18 carbon a~oms and aliphatic monofunctional alcohol con-taining 4 to 18 carbon atoms, the molar ra~io of said monobasic acid to said monofunctional alcohol ranging between 0.5:1.5 and 1.5:0.5.
AnotheT aspect of the invention provides a method of preparing a mixed-terminated polyester having an average molecular weight between 500 and 5000, which comprises reacting (a) an aliphatic glycol containing

2 to 6 carbon atoms, (b) an aliphatic dibasic acid containing 4 to 12 carbon atoms or a mixture of C4 12 aliphatic dibasic acid and aroma~ic dibasic acid or anhydride thereof, and (c) a mixture of aliphatic mono-basic acid containing 4 to 18 carbon atoms and aliphatic monofunctional alcohol containing 4 to 18 carbon atoms, the monobasic acid and the mono-functional alcohol being present in a molar ratio of between 0~5:1.5 and 1.5:0.5. Useful glycols for the preparation of this plasticizer con-tain from about 2 to 6 carbon atoms and the dibasic acid can contain - la ~

,, . .
" '; ..... .. : ' ~

~272~:
from about 4 to 12 carbon atoms. The dibasic acids can be aliphatic or can be a mixture of aliphatic and aromatic, such as phthalic acid or the anhydride thereof. Preferably the dibasic acid will contain from 6 to 10 carbon atoms. The terminating agent consists of a mixture of a monobasic acid containing 4 to 18 carbon atoms and, more preferably, 8 to 18 carbon atoms and a monofunctional alcohol containing 4 to 18 carbon atoms and, more pre~erably, 6 to 12 carbon atoms. The molar ratio of monobasic acid to monofunctional alcohol ranges between 0.5:1.5 to 1.5:0.5. The present polyesters are particularly useful with PVC homopolymers and PVC copolymers having vinyl chloride contents of 50% or more.

The polyester plasticizers of this invention have average mole-cular weights from 500 to 5000 and, more pre~erablyJ from 600 to 2500 and will generally conform to the structural formula O O O O
,. .. " "
T'-O-R ~ OC-Rl-CO-R-O ~ C-Rl-CO-T"
wherein R represents a bivalent hydrocarbon radical containing from 2 to 6 carbon atoms; Rl represents a bivalent hydrocarbon radical containing ~rom 2 to 10 carbon atoms andJ more preferably, from 4 to 8 carbon atoms;
n represents an m teger such that ~he average molecular weight of the polyester falls within the above-defined ranges, n generally being from 1 to 9; T' is an acyl radical containing 4 to 18 carbon atoms and, more preferably, 8 to 18 carbon atoms; and T" is an alkyl radical containing 4 to 18 earbon atoms, and more preferably 6 to 12 carbon atoms. It will be evident ~o those skilled in the art that ~he molecular weights are average molecular weights. The molecular weight distribution of the polyesters may vary considerably but this is acceptable for the purpose of this invention and does not detract from the desirability of the poly~ster pla~ticizers as long as the average molecular weights fall within the deEined limits. Similarly, the skilled artisan will recognize that the structural formula represents the ideal molecule ~2~
obtained when equimolar amounts of the glycol and dibasic acid are used and the mole ratio of ~he monobasic acid to monofunctional alcohol making up the mixed terminator is 1:1. It is evident, however, that not every molecule will be terminated with both an alkyl and acyl radical due ~o the random nature of the reaction. Furthermore, the formula does not take into account for possible variations in the mole ratio of monobasic acid to monofunctional alcohol.
Glycols useful for the preparation of the polyesters of this invention are aliphatic, branched- or straight-chain glycols containing from 2 to 6 carbon atoms. These include, for example, ethylene glycol, 1,3-prdpanediol, 1,4-butanediol, 1 J 5-pentanediol, 1,6-hexanediol, neo-pentyl glycol, 2-methyl- 1,3-propanediol, 3-methyl-1,5-pentanediol, 2,3-dimethyl-2,3-butanediol, 1,3-butylene glycol, 1,2-butanediol, 1,2-prop-ylene glycol, 1,2-pentanediol, 1,3-pentanediol, l,~-pentanediol and mixtures thereof.
Useful dibasic acids are aliphatic acids containing from ~ to 12 carbon atoms or mixtures of these aliphatic acids with aromatic dibasic acids, such as phthalic acid or the anhydride thereof. Representative alipha~ic dibasi.c acids include adipic acid, pimelic acid, suberic acid, s~bacic acid, azelaic acid, dodecanedioic acid and mixtures thereof. When aromatic acids or anhydrides are included they can constitute up to about 75 wt. % of the total dibasic acid charge, however, they will more gener-ally be present in amounts up to about 50 wt. %. Aliphatic dibasic acids containing from 6 to 10 carbon atoms are particularly useful for this in-vention.
To obtain the improved plas~icizers of this invention it is essential that a mixture of terminating agents be employed in an amount such that the molecular weight of the resulting polyester is between about 500 and 5000. The terminating agent consists of a mixture pf a mon~basic acid and a monof~mctional alcohol. Equally effective are mixtures wherein - , , . ~ . . . ~ ; - ;

" . . ~ . . ..

- - -~2722 several monobasic acids and/or monofunctional alcohols are present. The molar ratio of monobasic acid~s) to monofunctional alcohol(s) will ran~e from about 0.5:1.5 to 1.5:0.5. Excellent results are ob~ained when this ratio is about 1:1. In the makeup of the mixed terminator, monobasic acids containing from about 4 to 18 carbon atoms and, more preferably, 8 to 18 carbon atoms will be used. Useful aliphatic branched- or straight-chain monobasic acids include such acids as butyric acid, valeric acid, caproic acid, caprylic acid, capric acid, myristic acid, palmitic acid, stearic acid, 2-ethylhexoic acid, isostearic acid and mixtures thereof. The mono-functional alcohols are branched- or straight-chain aliphatic alcohols containing about 4 to 18 carbon atoms and, more preferably, 6 to 12 carbon atoms. These include, for example, n-butanol, isobutanol, isoamyl alcohol, n-hexyl alcohol, n-octyl alcohol, 2-ethylhexanol, 2,2-dimethylpentanol, 2,2,4-trimethylpentanol, lauryl alcohol, m~ristyl alcohol and mixtures thereof.
Superior polyester plastici2ers are obtained when the dibasic acid is adipic acid or azelaic acid, used by themselves or in combi~ation Wit}l phthalic acid; the glycol is 1,2-propylene glycol, 1,3-butylene gl~col, 1,4-butanediol or any combination thereof; and the termina~ing agent is a mixture of ~a~ C8-C18 monobasic acids or mixtures thereof and (b) C6-C12 aliphatic monofunctional alcohols or mixtures thereof. Monobasic acid mixtures and mixtures of monofunctional alcohols are particularly useful ... ... . . .
as they are readily obtainable from commercial sources and impart very favor-able economics to the plasticizers of this invention. Mixtures of pelargonic acid or lauric acid with 2-ethylhexanol also provide extremely useful mixed terminators for this invention. Polyesters having improved properties typically have acid values less than 10 and hydroxyl values less than 20.
Polyesters obtained in accordance with this invention exhibit superior plasticizer properties when used with a variety of resinous pro-ducts but find particular utility for use with PVC homopolymers and copolymers.

27~2 Particular aclvantage is realized with PVC resins having vinyl chloride contents above about 50~ by weight. Polyvinylchloride copolymers for which the present polyesters are useful include any of those obtained when vinyl chloride is polymerized with vinyl bromide, vinyl acetate, vinyl butyrate, vinyl propionate, methylmethacrylate, vinylidine chloride, 2-ethylhexyl acrylate, acrylonitrile, methacrylonitrile, styrene and the like, or combinations of two or more of these comonomers. The present plast-icizers can also be used with butyl rubber, polyvinylidene chloride, chlor-inated PVC, polyvinylbutyral, cellulose acetate, cellulose butyrate or ~he like The plasticizers are readily compatible with the aforementioned polymers and can be utilized at levels ranging from about 10 to 110 parts, but more preferably, are employed at levels from 30 to 80 parts by weight per 100 parts of the PVC homopolymer and copo]ymer. Other compounding in-gredients can also be used in conjunction with the plasticizers of this invention in formulating the finished product. For example, stabilizers to protect the resins against the deleterious effects of oxidative, thermal and photochemical degradation, pigments, dyes, fillers, lubricants and other processing aids can be included with the plasticizer. The physical properties of the formulated resins can be widely varied by manipulating the amounts and types of ingredients employed in compounding the resin. The polyesters of this invention have excellent stability when subjected to processing conditions and do not appreciably degrade when maintained at elevated temperatures for prolonged periods. These products also exhibit low volatility so that losses during processing are minimal. The polyesters are essentially odorless and colorless and do not appreciably to the color of the resulting plasticized sheets. If improvement in the color of the polyester is desired, it can be bleached uith peroxide, hypochlorite or other suitable bleaching agents or dec~lorized using bleaching clays, char-coal or the li~e. Clear, pliable sheets are obtained when the plasticizers ~fiZ72Z
are incorporated into PVC and the polyester does not exude from the sheet upon standing. Most importantly, however, uslng the polyester of the present invention significant improvement in low temperature performance is possible. This is particularly surprising and unexpected since poly-esters prepared in an identical manner and derived from the same glycols and dibasic acids but terminated either solely with monobasic acids or solely with monofunctional alcohols do not exhibit the same superior low temperature characteristics. Although the chain-terminated agents employed in the preparation of the polyester plasticizers of this invention are not in themselves novel and they have been used individually as chain-stopping agents, it is completely unexpected that by utilizing a mixture of these terminating agents superior low temperature properties are possible.
The reaction of the dibasic acid, glycol and terminating agents is conducted in conventional equipment using established esterification procedures. Typically, the reactants are charged to a suitable esterifi-cation kettle as a unit charge and heated to about 150-250C at atmospheric pressure until the reaction is substantially complete - usually three to eight hours. The esterification can be promoted using suitable catalysts such as phosphoric acid, sulfuric acid, p-toluenesulfonic acid, methane-sulfonic acid, dibutyl tin o~ide, tetrabutyl titanite, zinc acetate and the like. The amount of catalyst can vary but usually will range from about 0.01 to 1.0 percent by weight of the total reactant charge. ~hile the reaction may be conducted entirely at atmospheric pressure it is generally more desirable to reduce the pressure to about 2-50 mm Hg. at 200-250C during the final stages of the reaction to remove the last traces of water and strip off excess glycol or other volatiles which may be present. In this way acid values less than 10 and hydroxyl values less than 20 are obtained.
Inert diluents such as benzene, toluene, x~lene or the like can be used as `
the reaction medium for conducting the reaction, however, it is more usual to charge an excess of the glycol based on the stoichiometric or eqùivalent . . ~, , . ., , : .
.. , , , , . . . . ~ ,, . , .. ; .: .:- . ::

i;27;~2 amount calculated for the acid present. The excess glycol is dlstilled from the reactor as the esterification reaction is carried to completion and may be recycled, if desired. Usually up to about 25% by weight ex-cess glycol wiil suffice for this purpose, however, larger excesses can be used.
The following Examples illustrate the invention more fully, how-ever, they are not intended as a limitation on the scope thereof. In the Examples all parts and percentages are on a weight basis unless otherwise indicated.
EXAMPLE I
A reactor equipped with a suitable stirrer, thermometer and dis-tillation column topped with a condenser was charged as follows:
Azelaic acid 2 moles 1,2-propylene glycol 2 moles Lauric Acid 1 mole 2-Ethylhexanol 1 mole About 25% excess of the hydroxyl-containing materials were charged to drive the reaction. The reaction mixture was then slowly heated with agitation while removing the ~ater of ~eaction. A small amount (0.02%) H3P02 was added to the reac*ion mixture to catalyze the reaction. When the rate of reaction slowed, as evidenced by a marked reduction in the rate of water evolution, 0.03% butyl titanate catalyst was added and the pressure gradually reduced to 20 torr to achieve a controlled rate of stripping ~removal of water and excess hydroxy-containing matcrials). When an acid value of 0.4 was xeached the reaction mixture was allowed to cool to room temperature and filtered using a diatomaceous earth filter aid to remove catalyst residue and other impurities. The final polyester product had an acid value of 0.3, hydroxyl value of 5.3 and 100F viscosity of 80.2. The average molecular weight of the polyester was 770.

The polyester was used as a plasticizer for PVC homopolymers and Z72~, copolymers without further modification or processing. To demonstrate the superior low temperature properties obtainable with this plasticizer a PVC formulation was prepared as follows:
PVC resin (Geon 102 EPP5)100 parts Triphenyl phosphite 1 part Mixed barium-cadium soaps 2 parts Polyester plasticizer 56 parts These~ingredients were blended for about 10 minutes on a standard two-roll mill heated to about 170aC. The sheets obtained from the mill were then pressed to uniform thickness using a chrome-plated ASTM mold ~170C for 6 minutes; 1500 psi). Suitable specimens were out from these sheets for the various tests.
The brittle point of the piasticized PVC was -38C as determined with the Scott Brittleness Tester in accordance with modified ASTM test procedure D 746-57 T. Physical properties as determLned using an Instron Tester ~pull rate 5 inches per minute) were as follows: 100% modulus ~psi) 1150; tensile strength ~psi) 2800; and 330% elongation.
EXAMPLE II
Following the same procedure as described in Example I, a mixed-terminated polyester was prepared by the reaction of two moles adipic acid, two moles 1,2-propylene glycol, one mole lauric acid and one mole lauryl alcohol. The resulting polyester product had an average molecular weight of about 742, an acid value of 0.3 and 100F viscosity of 71.7. The plastîcized PVC containing 56 phr of this polyester and formulated in . ..
accordance with the procedure of Example I had a brittle point of -38C

and modulus ~100%) of 1475 psi. An identical formulation plasticized with 50 phr dioctyl phthalate had a brittle point of -30C.

To demons~rate the superior low temperature properties obtainable .. . .
with mi~xed-terminated polyesters as compared to polyesters terminated wîth .. ;~ . .
all monobasic acid or all monofunctional alcohol the following comparative '.:' '~ ' -8- ~
,. .

, ~ , ; . , . ~ . ., , . .:
. , . .: .. . .
, . , . . ~ ,, , .. .. , , . , :

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ex~criments were conduc-ted in an identical manner. In the first experiment a polyester was preparcd from two moles adipic acid, three moles 1,2-propylene glycol and two moles lauric acid. This polyester had an average molecular weight of 816, 100F viscosity of 111.1 centistokes, and when formulated with PVC in accordance with the above recipe gave a brittle point of -28C. In another experiment a polyester was prepared from three moles adipic acid, two moles l,2-propylene glycol and two moles lauryl alcohol. The resulting solid product ~AV 1.2) had an average molecular weight of 846 and PVC plasticized therewith had a brittle point of -33C.
EXAMPLES rII-IX
A series of mixed-terminated polyester plasticizers were prepared using the above-described procedure. In these Examples the dibasic acid was adiplc acid while the glycols and mixed terminators were varied. Table I sets forth the type and molar amount of each of the reactants employed.
The acid value, hydroxyl value and 100F viscosity of the resulting products are also reported. These polyesters were all formulated with PVC homopoly-mer in accordance ~ith the recipe and procedure of the previous Examples.
Modulus, tensile, 7~'~

X ¦ N N ~ ~/ 00 1~ ~ O O 11~ 1~
O ~ o O U~ N N
i~ ~ N

N N ~1 ~ Ci~ 00 ~ O
t~ O oo '~ O U~
::. O _~ N

O O ; ~ ~ ~ ~OD ~1 N
0 5~ ~CO N ~ I

¦ ~`1 N ~ ~1 00 t` N O O O 0~1 O C~
N ~I N

I ~ ~ ; I ~~-` 0~ 0 0 0 0 O O
~ : :' E~ ~1 ~ ~ ~ ~ o o o ~ - , ..
C; Lf' t'') . ' ¦ t~l t~l _I ~ O Gtl 1-- 0 0 0 N
H I ; ~ ~ ~o c:~ o ~ N

~> o ~ ~ ~ ¢
o _~ ~ o a~ o o b~ ~ ~ ¢ ~ O ~: ~ -~0 ~ , ,~ ., x æ X ~ ~ O ~

x ~" $,~ p, cn h ~ X C~ o\ Vl ~

t.Ll rl O O ~ N~ r-l ~ ~Ll t~ U.l ~ O ~ H O ~ O rl r ~_ Z r~1 ~1 Z; ~ 1 N ~ N ~ O ~ O
.:

.. . .

-10- . .

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~Z~Z2 elongation and brittle point are reported for each Eormula~ed resin in the table.
EX~PLE X
A mixed-terminated polyester having an average molecular weight of about 1050 was prepared by reacting three moles adipic acid, three moles 1,2-propylene glycol, one mole 2-ethylhexanol and one mole of a mixture of aliphatic acids containing about 35% myristic acid, 55%
palmitic acid and 10% stearic acid. The reaction was conducted in accord-ance with the already described procedures. The resulting liquid poly-ester had an acid value of 2.2, hydroxyl value of 11.1 and 100F
viscosity of 82 centistokes. Fifty-six parts of this polyester was milled with 100 parts PVC homopolymer and 3 parts stabilizer and clear pliable sheets obtained. The vinyl composition had a brittle point of -2gC and a tensile strength of 3000 psi at 320% elongation. The vola-tility of the plasticizer at 90C was only 3.2% as determined by ASTM D
1203-55.
To demonstrate the improved low temperature properties obtained with the polyester of this invention, comparative examples were conducted.
In one of the comparisions three moles adipic acid, two moles 1,2-propylene glycol and two moles 2-ethylhexanol were reacted to obtain a polyester o~
average molecular weight 850. PVC homopolymer plasticized therewith (56 phr plasticizer) had a brittle point of -~7C. In another comparative test, two moles adipic acid, three moles 1,2-propylene glycol and two moles of the mixed aliphatic acids were reacted. This polyester had a molecular weight of 1032. PVC homopolymer plasticized with 56 phr of the plasticizer had a brittle point of -8C.
~XAMPL~'Xr ~ :
To demonstrate the versatility of this invention and the ability to obtain polyesters using aromatic anyhdrides a polyester was prepared by reacting three moles adipic acidJ one mole phthalic anhydride and four ;Z722 moles 1,2-propylene glycol. This polyester was terminated with a mixture of one mole 2-ethylhexanol and one mole of the mixed aliphatic monobasic acids of Example X. The resulting polyester had a molecular weight of 1276, an acid value of 0.6, hydroxyl value of 11.6 and 100F viscosity of 231 centistokes. PVC resin plasticized with 56 phr thereof had a brittle point of -22 and tensile of 3100 psi at 265% elongation. Vola-tility of the polyester plasticizer at 90C was only 2.1%. For compara-tive purposes, polyesters were prepared from ~a) three moles adipic acid, one mole phthalic anhydride, three moles l,2-propylene glycol and two moles 2-ethylhexanol and ~b) two moles adipic acid, one mole phthalic ac~d and four moles l,2-propylene glycol and two moles of the mixed aliphatic monobasic acids. These polyesters had respective molecular weights of 1074 and 1256. PVC homopolymers plasticized at 56 phr level with ~a) and ~b) had brittle points of -17C and -2C, respectively.
To further demonstrate the uti]ity of these mixed-terminated plasticizers and their resistance to extraction with various aqueous and organic solutions, 2 1/2" diameter specirnen were stamped from 20 mil sheets of the plasticized resin and extracted for 2~ hours to determine the weight loss. Upon extraction with soapy water at 90C 12.8% weight loss :
was observed. 5.6% Weight loss was obtained after 24 hours extraction with hexane at room temperature. Extraction with mineral oil (Atreol #9 white oil) at 50C resulted in only 2.7% weight loss.
EXAMPLE XII
By esterifying one mole adipic acid, one mole phthalic anhydride, -~ two moles 1>2-propylene glycol and a terminating mixture consisting of one mole lauric acld and one mole 2-ethylhexanol, a polyester having an acid value of 0.4, hydroxyl value of 5.9 and 100F viscosity of 209.2 was obtained. This product was employed to plasticize PVC homopolymer and copolymer at 56 phr level. The resulting plasticized vinyl resin composit-ions had superior brittle points as compared to identical resins plasticized 6Z7;Z~2 with polyesters terminated solely with lauric acid or solely with 2-ethyl-hexanol. The polyester exhibited low volatility, excellent permanence and additionally had excellent resistance to extraction with soapy waterl hexane and mineral oil. Mechanical properties of the plasticized vinyl resins were acceptable for general purpose usage.
The above Examples clearly demonstrate that superior plasticizers are obtained when polyesters are terminated with a mixed terminating agent consisting of a mixture of monobasic acid and monofunctional alcohol. The much improved low temperature properties are evident. ~urthermore, it can readily be seen that the plasticizers of this invention have excellent per-manence and are resistant to extraction. These plasticizers do not impart objectionable color to the vinyl resins and do not exude from the resin.
The polyesters have very low volatility and can beprocessed at elevated temperatures with minimal loss due to volatilization.
The present mixed-terminated polyesters are useful in a variety of applications. They may be used in extruded products and plastisol applica-tions. Because of their permanence and superior low temperature properties they are acceptable for use in outdoor furniture, :Eootwear and in automotive applications. They may also be used wi~h coated fabrics and for metallic coatings. The presen~ plasticizers also find application in synthetic rubbers such as neoprene and butadien~/acrylonitrile rubbers.

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Claims (17)

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

1. A mixed-terminated polyester having an average molecular weight between 500 and 5000 obtained by the reaction of (a) an aliphatic glycol containing 2 to 6 carbon atoms, (b) an aliphatic dibasic acid containing 4 to 12 carbon atoms or a mixture of C4-12 aliphatic dibasic acid and aromatic dibasic acid or anhydride thereof, and (c) a mixture of aliphatic monobasic acid containing 4 to 18 carbon atoms and aliphatic monofunctional alcohol containing 4 to 18 carbon atoms, the molar ratio of said monobasic acid to said monofunctional alcohol ranging between 0.5:1.5 and 1.5:0.5.

2. The mixed terminated polyester of Claim 1 having an average molecular weight from 600 to 2500, an acid value less than 10 and a hydroxyl value less than 20.

3. The mixed-terminated polyester of Claim 1 wherein (b) contains 6 to 10 carbon atoms and (c) is a mixture of C8-18 monobasic acids and C6 12 monofunctional alcohols.

4. The mixed-terminated polyester of Claim 3 wherein (a) is 1,2-propylene glycol, 1,3-butylene glycol or 1,4-butanediol and (b) is adipic acid or azelaic acid.

5. The mixed terminated polyester of Claim 4 having an average molecular weight of 600-2500, an acid value less than 10 and a hydroxyl value less than 20.

6. The mixed-terminated polyester of Claim 5 wherein the molar ratio of monobasic acid to monofunctional alcohol is 1:1.

7. The mixed-terminated polyester of Claim 5 wherein (c) is a mix-ture of pelargonic acid and 2-ethylhexanol.

8. The mixed-terminated polyester of Claim 5 wherein (c) is a mix-ture of lauric acid and 2-ethylhexanol.

9. A mixed-terminated polyester plasticizer characterized by having improved low temperature properties and having an average molecular weight between 500 and 5000, an acid value less than 10, hydroxyl value less than 20 and corresponding to the formula wherein R is a bivalent hydrocarbon radical containing from 2 to 6 carbon atoms, R1 is a bivalent hydrocarbon radical containing from 2 to 10 carbon atoms, T' is an acyl radical containing 4 to 18 carbon atoms, T" is an alkyl radical containing from 4 to 18 carbon atoms, n is a positive integer from 1 to 9 and consisting essentially of the reaction product of 2 to 10 moles of (a) an aliphatic glycol containing 2 to 6 carbon atoms, with an equimolar amount of (b) an aliphatic dibasic acid containing 4 to 12 carbon atoms or a mixture of C4-C12 aliphatic dibasic acid and aromatic dibasic acid or anhydride thereof, and terminated with (c) a mixture of an aliphatic monobasic acid con-taining 4 to 18 carbon atoms and an aliphatic monofunctional alcohol contain-ing 4 to 18 carbon atoms, the molar ratio of said monobasic and monofunctional alcohol in the mixture ranging between 0.5:1.5 and 1.5:0.5.

10. A plasticized vinyl resin composition comprising a polyvinylchloride homopolymer or copolymer and 10 to 110 parts, per 100 parts resin, of the mixed-terminated polyester of claim 1 or 9.

11. A method of preparing a mixed-terminated polyester having an average molecular weight between 500 and 5000, which comprises reacting (a) an aliphatic glycol containing 2 to 6 carbon atoms, (b) an aliphatic dibasic acid contain-ing 4 to 12 carbon atoms or a mixture of C4-12 aliphatic dibasic acid and aromatic dibasic acid or anhydride thereof, and (c) a mixture of aliphatic monobasic acid containing 4 to 18 carbon atoms and aliphatic monofunctional alcohol containing 4 to 18 carbon atoms, the monobasic acid and the monofunc-tional alcohol being present in a molar ratio of between 0.5:1.5 and 1.5:0.5.

12. A method according to claim 11, wherein (b) contains 6 to 10 carbon atoms and (c) is a mixture of C8-18 monobasic acids and C6-12 monofunctional alcohols.

13. A method according to claim 11 or 12, wherein (a) is 1,2-propylene glycol, 1,3-butylene glycol or 1,4-butanediol and (b) is adipic acid or azelaic acid.

14. A method according to claim 11 or 12, wherein the molar ratio of mono-basic acid to monofunctional alcohol is 1:1.

15. A method according to claim 11 or 12, wherein (c) is a mixture of pelargonic acid and 2-ethylhexanol.

16. A method according to claim 11 or 12, wherein (c) is a mixture of lauric acid and 2-ethylhexanol.

17. A method according to claim 11 or 12, wherein the reaction is carried out in the presence of about 0.01 to 1.0 percent by weight, based on the total reactants, of a catalyst selected from phosphoric acid, sulfuric acid, p-tol-uenesulfonic acid, methane-sulfonic acid, dibutyl tin oxide, tetrabutyl titan-ite and zinc acetate.