Solid solvents for improved processing of vinyl chloride polymers

The processibility during fabrication of a vinyl chloride polymer is improved by reducing the melt viscosity of the polymer when heated for shaping, by the admixture therewith of a small amount up to about 5% by wt of at least one melt-compatible normally crystalline solid solvent as a processing aid. At temperatures below the melting point of the solid solvent and the glass transition temperature of the polymer, the solid solvent deposits within the solid polymer as microdispersed solid microparticles which do not significantly adversely affect the mechanical properties of the solid polymer, whereas upon heating above the melt temperature of the mixture, the molten solid solvent is compatible with the molten polymer thereby effectively reducing its melt temperatures and melt viscosity during processing. Solid solvents found suitabe for vinyl chloride polymers are saturated carboxylic acids and their low molecular weight derivatives which have a crystalline melting point of at least about 80.degree. C., preferably about 100.degree. C., but not higher than about 180.degree. C. and do not exert substantial permanent solvent effect on PVC. The solid solvent can be mixed directly with the polymer or via a carrier polymer containing the same.

BACKGROUND OF THE INVENTION 
Polyvinyl chloride (PVC), by virtue of a particularly desirable combination 
of properties, including non-flammability, high chemical resistance, high 
strength and good weather resistance is one of the largest volume 
polymers, and it is probably the most versatile in the scope of its 
utility. PVC is essentially a glassy or amorphous polymer with a high 
glass transition temperature (T.sub.g) of about 80.degree. C., and thus 
alone it is rigid and brittle at use temperatures up to about 80.degree. 
C., and particularly at the usual ambient temperatures. However, PVC can 
be made flexible and soft by adding a plasticizer typical of which is 
dioctyl phthalate (DOP). DOP, as with most plasticizers, and especially 
carboxylic acid esters, is a liquid at room temperature and acts as a weak 
solvent for PVC. The thus plasticized and flexible PVC, to be called SPVC, 
is used for vinyl seat covers, artificial leather, clothing items, etc. 
But for certain important purposes, PVC must be used without plasticizers. 
For instance, unplasticized rigid PVC, to be called RPVC, is used for 
vinyl siding, drainage and sewer pipe, water and electrical conduits and 
fittings, outdoor furniture, etc. 
PVC is indeed an extremely versatile polymer and serves as an major 
industrial and consumer material for modern man. However, as is well known 
PVC presents serious difficulties in processing during fabrication, 
shaping, etc., and RPVC is peculiarly difficult to process. For 
manipulation during fabrication, RPVC because of a very high melt 
viscosity must be heated to at least about 210.degree. C. to achieve 
complete fusion as is needed to impart optimum properties to the final 
product especially when produced by extrusion or injection molding. 
Unfortunately, PVC is thermally very unstable at about 210.degree. C. and 
even considerably lower temperatures. Sarvetnick in "Polyvinyl Chloride", 
Van Nostrand Co., 1969, Page 90, states that the thermal degradation of 
PVC begins at about 93.3.degree. C. and increases sharply with increasing 
temperature. At practical processing temperature, it quickly undergoes 
degradation, giving off very toxic and corrosive hydrochloric acid (HCl) 
fumes with serious loss in mechanical properties. Excessive scrap due to 
degradation, low production rates and poor product properties are 
notorious problems encountered in PVC processing. 
As one way of improving the thermal stability as well as other properties 
of PVC, copolymers containing a major fraction of vinyl chloride units and 
a minor fraction of other monomeric units, such as vinyl acetate, serving 
to soften and flexibilize the ultimate polymer and reduce its glass 
transition or brittle temperature, have been developed and are widely 
used. Also, many processing aids, e.g. stabilizers, lubricants, impact 
modifiers, and the like also have been developed for PVC in order to 
facilitate processing at lower temperatures or to reduce the tendency of 
the polymer to undergo heat decomposition or impart more favorable 
properties. These aids must often be used in large amounts, up to 30-40% 
by wt of the polymer in the case of impact modifiers, and while these 
levels are tolerable or even desirable for SPVC, they are not acceptable 
for RPVC because they usually result in losses in essential properties. 
The objective of this invention is to provide an improved processing aid 
and methods of utilizing such aids for vinyl chloride polymers which 
enable the same to be processed at lower temperatures without major 
adverse side effects. Selective low molecular weight crystalline chemical 
compounds have been found to behave as solid solvents to PVC when they are 
mixed therewith, qualifying as effective processing aids for PVC. A solid 
solvent is different in function from any of a plasticizer, lubricant or 
impact modifier. Thus, a solid solvent in homogenous admixture with PVC 
acts as solvent for the PVC, with the capacity for greatly reducing the 
melt viscosity of the PVC, only when the admixture is at high temperatures 
where the mixture is fused during processing. But when the mixture is 
cooled to room temperatures below the melting point (T.sub.m) of the solid 
solvent and the T.sub.g of the polymer, the former precipitates out of the 
polymer as microdispersed solid micro-particles. Therefore, if the amount 
of solid solvent is controlled within proper limits, its presence does not 
adversely reduce the strength, rigidity or useful temperature range of 
solid PVC, especially RPVC. A solid solvent must possess thermally 
reversible compatibility or solubility with PVC. A plasticizer, on the 
other hand dissolves permanently into PVC, while a lubricant is a 
mechanically entrained friction-reducing additive that is basically 
incompatible with the polymer and does not act as solvent to it. Both 
therefore tend to impair desirable mechanical properties of the polymer in 
contrast to a solid solvent. 
In a paper "A Solid Solvent as Processing Aid for Polystyrene," J. Applied 
Polymer Research, Vol. 37, 1339-1349 (1989), Chung et al. describe their 
investigation for an effective solid solvent for polystyrene (PS) which 
resulted in the identification of benzenesulfonamide (BSA) as an 
apparently ideal material for that purpose. BSA was found when 
incorporated at the 5% by wt level to reduce the melt viscosity of a 
sample of a typical commercial PS with normal molecular weight (Number 
average=87,000; Weight average=230,000; T.sub.g =379.degree. K.) by about 
60%, i.e. from about 5,000 Pa-s for PS alone to about 2,000 Pa-s when BSA 
was present when tested at 490.degree. K. at a low shear rate of about 
4/S. A heat capacity measurement of powdered BSA in association with a 
finely divided low molecular weight PS fraction (MW=10,000; T.sub.g 
=367.degree. K.) was carried out in a differential scanning calorimeter 
(DSC) wherein transitions at either of the melting or glass transition 
points appear as deflections, e.g. peaks or shoulder-like increases in 
slope of the heat curve output of the DSC. Upon reheating of the DSC after 
once undergoing homogeneous melt mixing with the PS fraction and 
subsequent quenching, i.e. rapid cooling, the BSA caused roughly a 
10.degree. C. downshifting or drop in the T.sub.g of the PS fraction 
while retaining intact to a major degree the area of its own melting peak 
at its distinctive crystalline melting point as was developed during the 
initial melt mixing. The latter behavior signifies the presence of BSA in 
the quenched mixture as a distinct crystalline dispersed phase. In 
contrast for comparison, the same amount of mineral oil, which acts as a 
plasticizer for PS, while imparting during DSC analysis an even greater 
reduction in the T.sub.g of the low molecular wt. PS, i.e. of about 
20.degree. C., was able to reduce the viscosity of the high molecular wt. 
PS sample under the same viscosity test conditions only about half as 
much, i.e., from about 5,000 Pa-s to about 3,500 Pa-s. Mineral oil, being 
liquid over the test temperature range, of course showed no melting peak 
by DSC analysis, either during initial melt mixing or subsequent 
re-heating. 
Also as a comparison, acetanilide (AA), with a crystalline T.sub.m of 
115.degree. C. only a little higher than the T.sub.g of PS, which had 
earlier being thought as a promising solid solvent candidate for PS, as 
reported by Chung in J. Applied Polymer Sciences, 31, 2739 (1986), was 
found inferior to BSA but superior to mineral oil in reducing melt 
viscosity of the same high molecular weight sample of PS to about 3,000 
Pa-s. Upon DSC analysis with the low molecular PS sample, AA while 
achieving the greatest downshift of the three additives in the T.sub.g of 
that sample of at least 30.degree. C., suffered during re-heating a major 
but not complete loss in the area of its melting peak, denoting 
substantial lasting solubility in the polymer after quenching of the 
latter to its solid state. AA by DSC testing alone was found to exhibit a 
wide hysteresis effect in its heat flow curves upon melting and subsequent 
rapid cooling alone in the absence of annealing, which effect was much 
greater than that of BSA when similarly treated. This hysteresis effect 
depressed the apparent recrystallization temperature (T.sub.c) of AA below 
the T.sub.g of the PS sample, suggesting that recrystallizing ability of 
AA within the solidified PS was being hindered by its low apparent 
T.sub.c. 
Some 30 additional low molecular weight crystalline compounds, all with 
T.sub.m significantly higher than the T.sub.g of PS, that were evaluated 
by DSC during this study, and of these, only four were found to have 
promise as solid solvent for PS, including two carboxylic acid amide plus 
sebacic acid and mannitol. As noted, all DSC tests were conducted using 
the low MW fraction of PS having a T.sub.g significantly lower (by 
12.degree. C.) than that of the commercial PS sample. This difference in 
T.sub.g due to the difference in MW of PS would be expected from the phase 
diagram of that polymer reported in "Plasticization and Plasticizer 
Processes," a symposium report No. 48 of The Advances in Chemistry Series, 
American Chemical Society, Washington, D.C., 1965, Page 39. In as much as 
low MW grades of PS are themselves known to function as internal 
plasticizers for normal high MW general purpose grades of PS, acting to 
improve the flow rate while increasing the brittleness thereof (Cf. 
"Polymer Technology" by Miles and Briston, Chemical Publishing Co., Inc., 
New York, 1965, Page 188), the effectiveness of any of these other 
compounds in actually reducing the melt viscosity of PS in the normal MW 
range, is speculative at best. In particular, extrapolation as to such 
effectiveness between chemically very different materials within the usual 
practical MW range based on DSC data is obviously out of the question. 
The solid solvent concept was applied by Chung in U.S. Pat. No. 4,843,117 
to vinylidene chloride-containing polymers. e.g., copolymers of vinylidene 
chloride and a minor amount of vinyl chloride or methyl acetate. Dimethyl 
sulfone at a concentration of 5% was found to be an effective solid 
solvent for such polymers, reducing melt viscosity e.g. by almost 40%, 
i.e, from about 10,000 Poise to about 6,000 Poise, at a low shear rate of 
100/sec and 175.degree. C. 
While PS can under special polymerizing conditions with Ziegler catalysts 
be produced as an isotactic crystalline polymer with a high crystalline 
T.sub.m of about 230.degree. C., (Cf, Miles and Briston, supra, Page 187), 
PS as ordinarily used is an amorphous polymer, like PVC, with a high 
T.sub.g of about 100.degree. C. Its intrinsic brittleness can be readily 
modified for improved toughness. But unlike PVC, it is quite fluid, e.g., 
suitable for good injection molding, at 217.degree. C. (490.degree. K.) as 
used by Chung et al. PS is free of any tendency to suffer decomposition 
during processing. 
Polyvinylidene chloride (PVDC) being highly crystalline with a true T.sub.m 
of 190.degree. C. and a quite low T.sub.g of -19.degree. C., while lacking 
the brittleness of PVC at ordinary use temperatures, is even more 
difficult to process in its pure form and more susceptible to heat 
degradation than PVC, and so in that form is of no commercial importance 
(Cf, "Manufacture of Plastics", by Smith, Reinhold publishing, New York, 
1964, pp. 337-339). For commercial purposes, PVDC is always copolymerized 
with a modifying co-monomer, such as vinyl chloride, vinyl acetate, 
acrylonitrile, etc., and the resultant co-polymers have a reduced melt 
temperature. For example, Park in "Plastics Film Technology", Van 
Nostrand-Reinhold Co., 1969, Pages 35 & 36. describes a bubble process for 
forming a film of a PVDC-PVC copolymer (85/15) in which the copolymer is 
satisfactorily extruded at 170.degree. C. Such film, which is sold 
commercially in large quantities under the trademark "SARAN", has 
excellent optical clarity and high tensile strength. Also, in the Example 
of the above-identified U.S. Pat. No. 4,843,117 to Chung, the PVDC 
copolymer had a melt temperature of about 175.degree. C. At such 
temperatures, the decomposition problem of PVDC copolymers is serious. 
There is a need in practice to develop processing aids for RPVC to address 
the unavoidable characteristics of use temperature rigidity and processing 
temperature decomposition of that polymer. The identification of workable 
solid solvents for RPVC is a more pressing problem than for either of PS 
or of PVDC copolymers because of the large volume of PVC in use. 
I have now discovered that low molecular weight crystalline carboxylic 
acids and their simple derivatives which have a crystalline melting point 
at least equal to the T.sub.g of about 80.degree. C. for PVC, and 
preferably higher, but not higher than about 180.degree. C. are useful 
solid solvents for rigid PVC polymers consisting essentially of vinyl 
chloride. This is surprising since, in the first place, amorphous 
polymers, as is PVC, have a strong tendency towards permanent solvation 
with compounds which exert a plasticizing action thereon. As stated in the 
ACS symposium report "Advances in Chemistry Series", No. 48, supra, at 
Page 3: "With an amorphous polymer, any plasticizer is a solvent 
plasticizer--i.e., under suitable conditions the polymers would eventually 
dissolve in the plasticizer." This holds true for common closely related 
carboxylic acid esters and diesters which are liquids at room temperature 
and are among the most widely used solvent plasticizers for various 
polymers, notably PVC. In the second place, the compounds found 
advantageous for RPVC are chemically different from those found effective 
for PS and PVDC co-polymers, respectively, namely BSA and dimethyl 
sulfone, as described above, and the latter proved unsuitable as solid 
solvents for RPVC. 
DESCRIPTION OF THE INVENTION 
An effective solid solvent for PVC needs to satisfy several requirements. 
First, the additive compound should be a generally low molecular weight 
crystalline material having a crystalline melting point of at least about 
80.degree. C. up to about 180.degree. C. and preferably at least about 
100.degree. C. If the T.sub.m of the additive is appreciably below 
80.degree. C., the PVC undergoes solidification first. Since the PVC 
becomes rigid below its T.sub.g of about 80.degree. C., the rigidity tends 
to interfere with the ready precipitation and growth of the additive into 
micro-dispersed micro-crystals, forcing the additive to remain in solid 
solution form. The additive needs to be free to undergo micro-dispersion 
in order to assume a benign condition with respect to the mechanical 
properties of the solid polymer and those properties suffer if the polymer 
is the first to solidify. On the other hand, if the T.sub.m of the 
additive is much above 180.degree. C., the mixture must be heated to so 
high a temperature to become completely fused for processing that the 
danger of decomposition of the polymer is increased. 
By utilizing an additive having the preferred minimum T.sub.m of at least 
about 100.degree. C., one can minimize the possibility of encountering a 
hysteresis effect during quenching and re-heating of the mixture resulting 
in an apparent T.sub.c of the additive below the polymer solidification 
temperature notwithstanding a higher rated T.sub.m. In this way, 
solidification of the additive prior to polymer solidification is 
virtually ensured. It is noted that T.sub.c is always lower than T.sub.m. 
Also, a significant difference between the T.sub.g range of the polymer and 
the T.sub.m of the additive is advantageous for accurate DSC evaluation 
since if these values overlap, discrimination in the behavior of the 
polymer and additive during DSC testing is less clear and evaluation more 
difficult. 
With regard to the molecular weight of the additive, the upper limit is not 
particularly critical and generally the molecular weight can range up to 
several hundred, say about 400-500 without difficulty. 
The selected additive compound must exhibit thermally reversible 
compatibility or solubility in the polymer. This means that it is 
compatible/soluble in the polymer when both are in molten condition but is 
substantially incompatible/insoluble in the polymer when both are in solid 
condition. In the latter case, the additive is present as micro-crystals 
homogeneously dispersed within a matrix of the solid polymer. These 
micro-crystals are held in uniform suspension within the polymer as they 
are formed during cooling because they are prevented from settling out by 
the high melt viscosity of the polymer until the polymer itself 
solidifies. 
The requisite differential solubility behavior of the additive vis-a-vis 
the polymer can be identified in various ways. The visual appearance of 
the mixture is a gross but generally reliable indicator. If the additive 
is compatible with the polymer when both are molten, the fused mixture 
will be at least essentially optically clear or water-white. Conversely, 
incompatibility when in the solid state is denoted by a cloudy or 
translucent appearance. This observation naturally must be made in the 
absence of pigments, fillers, etc. which are often included in commercial 
PVC formulations and would be likely to inherently alter the appearance of 
the mixture. Theoretically, the additive could form as micro-crystals of 
such fineness to be below the limits of optical visibility and in that 
event, it could be present as an invisible micro-dispersed phase. But, 
this would be a rare occurrence and as a general rule visual inspection is 
a reliable test. 
Alternatively, and perhaps more reliably, one can resort to heat capacity 
measurement using a DSC as described previously. In this procedure, the 
additive and polymer must first be homogeneously blended and melted 
together--a mixture of solid particles of additive and polymer separately 
will always upon initial melting in a DSC reflect as deflections in the 
heat flow curve the separate melting/transition points of each of the 
components. After the initial melting followed by cooling to solidify the 
mixture, re-melting in the DSC will indicate whether the additive is 
present as a solution, i.e. in solvated form, or as a dispersed phase 
within the solid polymer. In the former case, only the melting point/range 
of the polymer will upon re-melting appear as a deflection in the scanning 
curve of the DSC; the original melting peak at the T.sub.m of the additive 
at least essentially disappears upon re-melting. In the latter case, a 
peak-like deflection will appear in the vicinity of the T.sub.m of the 
additive, reflecting the effect of the heat of fusion thereof. It follows 
that the area under the melting peak for the additive during the second 
melting compared to the area of the peak for the additive during the 
initial melt mixings is an indication generally of the degree of 
incompatibility of the additive and the polymer under solid state 
conditions. For 100% incompatibility, the melting peak area of the 
additive will approximate the area under the peak shown by the additive 
during the original melt mixing of the mixture of separate particles, 
while for 100% compatibility, the additive peak upon re-melting 
disappears. Complete or 100% incompatibility is not desirable but the 
additive should retain at least about 20-25% of its original peak area, 
and preferably at least about 50% and most preferably 75-80%, to avoid an 
undesirable plasticizing/solubilizing effect of the additive upon the 
solid polymer, which adversely influences the solid state properties of 
the polymer-additive blend. 
DSC evaluation preferably includes duplicating the re-melting following 
different cooling conditions, i.e. re-melting after rapid cooling or 
quenching as well as after gradual cooling to ensure an accurate 
observation by allowing for any differences in the rate of crystallization 
at substantially different cooling rates, as usually occur. 
Next, as a measure of the melt compatibility of the polymer and the 
additive, the presence of the additive during DSC scanning should upon 
re-melting cause a small downward displacement or drop in the glass 
transition temperature range of the polymer component of the mixture, that 
is, a change in the location of the T.sub.g deflection of the polymer in 
the DSC scanning curve from the original location of that range. To 
explain further, during the original melt mixing of the separate solid 
particles of polymer and additive, a deflection in the heat flow curve of 
the DSC appears at a temperature range corresponding approximately to the 
T.sub.g of the polymer. Then, during re-melting after cooling, the 
deflection range for the polymer should appear at a lower temperature 
differing from the original one by a small amount. 
The extent of this drop is affected by the amount of the additive and the 
cooling rate but will usually be about 1.degree.-2.degree. C. up to about 
10.degree. C. and occasionally somewhat higher. This T.sub.g down-shifting 
effect indicates the degree of compatibility between the polymer and the 
additive, and thus the ultimate effectiveness of the additive in reducing 
the melt viscosity of the polymer. 
Upon re-melting after rapid cooling or quenching, the drop in T.sub.g of 
the polymer will usually be greater than after slow cooling, say about 
5.degree. C. or higher. This indicates that the additive possessed a 
desirable compatibility with the polymer in the molten state and that this 
compatibility persisted when the blend was rapidly cooled and 
re-crystallization of the additive was consequently hindered. After slow 
cooling affording the additive the opportunity to undergo 
re-crystallization freely, the T.sub.g drop should preferably be toward 
the lower limit. This reflects only a small solvent action by the additive 
on the polymer in the solid state, maximizing the useful physical 
properties contributed by the polymer to the solid blend. Theoretically, a 
negligible drop in polymer T.sub.g would be ideal from the standpoint of 
solid state properties but this would be offset by a loss in melt 
compatibility. Hence, a balance between the two extremes should be the 
objective, achieving good melt compatibility needed to reduce melt 
viscosity without a serious loss in the physical properties of the solid 
blend for product purposes. 
Because PVC is amorphous, its T.sub.g during DSC analysis is more likely to 
be manifested by significant upward change or shoulder in the slope of the 
scanning curve extending over a limited temperature range rather than as a 
true peak or vortex as generally holds for the crystalline additive. 
The most important contribution of the additive is a significant reduction 
in the melt viscosity of the polymer under actual melt flow conditions. 
For a practical evaluation of this effect, one should employ a continuous 
rheometer, such as a Haake or Brabender extruder-based rheometer because 
of the thermal instability of PVC. A reduction of at least about 20% in 
polymer melt viscosity is desirable for minimum effectiveness as a solid 
solvent and a greater reduction is better if possible. 
By reducing the melt viscosity of PVC by the use of a solid solvent, 
processing of the polymer can take place at a significantly lower 
temperature with important benefits of less scrap, higher production rate, 
enhanced product properties and less energy consumption in processing. 
Even a relatively small reduction in processing temperature can be 
definitely advantageous. This is because the thermal degradation rate of 
PVC decreases exponentially with decreasing temperature and thus even a 
small reduction of the melt temperature during processing can 
significantly improve the thermal stability of PVC. 
Finally, the additive must meet any obvious practical standards applicable 
to a particular contemplated use such as freedom from toxicity, 
carcinogenic action, adverse environmental consequences, etc. 
The carboxylic acids and their low MW derivatives as solid solvent of the 
invention should be saturated; unsaturated compounds are susceptible to 
oxidation and can lead to degradation of polymers with which they are in 
association. Except for those having a long carbon chain, simple 
monocarboxylic acids below about capric acid (decanoic acid; T.sub.m 
=31.degree. C.) are not suitable because they are not solid even at room 
temperature. For behenic acid (docosanoic acid; T.sub.m =80.degree. C.) 
and above, a crystalline melting point above about 80.degree. C. usually 
is present (e.g., tricosanoic acid, T.sub.m =79.degree.-80.degree. C.; 
octacosanoic acid, T.sub.m =92.degree.-94.degree. C.; etc. listed in Table 
1). These higher monocarboxylic acids should be workable here. In 
particular, tricosanoic acid, hexacosanoic acid and octacosanoic acid show 
the desirable behavior by DSC analysis. 
TABLE 1 
______________________________________ 
Long-Chain Monocarboxylic Acids as Potential Solid 
Solvent for PVC 
Chemical Structure Melting Point .degree.C. 
______________________________________ 
Docosanoic Acid 
CH.sub.3 (CH.sub.2).sub.20 COOH 
80 
Tricosanoic Acid 
CH.sub.3 (CH.sub.2).sub.21 COOH 
79-80 
Tetracosanoic Acid 
CH.sub.3 (CH.sub.2).sub.22 COOH 
75-83 
Hexacosanoic Acid 
CH.sub.3 (CH.sub.2).sub.24 COOH 
87-89 
Octacosanoic Acid 
CH.sub.3 (CH.sub.2).sub.26 COOH 
92-94 
______________________________________ 
Saturated dicarboxylic acids are as a group suitable in principle provided 
of course that the carbon chain is not so long that the T.sub.m falls 
outside the above-specified range of about 80.degree.-180.degree. C. These 
acids illustrated by those listed in Table 2 all have a crystalline 
structure below their T.sub.m. For example, the lowest member, oxalic acid 
has T.sub.m of 101.degree. C. in dihydrate form (although it is poisonous 
and better avoided unless suitable safeguards are available) while at 
least up to tetradecanedioic acid (T.sub.m =122.degree.-125.degree. C.), 
the T.sub.m is within the desired limits. For instance, adipic acid 
(T.sub.m =152.degree.-154.degree. C.), suberic acid (T.sub.m 
=142.degree.-144.degree. C.), azelaic acid (T.sub.m 
=109.degree.-110.degree. C.), sebacic acid (T.sub.m 
=135.degree.-137.degree. C.), dodecanedioic acid (T.sub.m 
=128.degree.-130.degree. C.), and tetradecanedioic acid (T.sub.m 
=126.degree.-128.degree. C.) are all useful at least to some degree 
although some may be better than others. Succinic acid (T.sub.m 
=185.degree. C.) with a high melting point a little over 180.degree. C. is 
an exception from the melting point pattern of this group and is generally 
at the upper threshold of usefulness as regards melting point. All of the 
above saturated dibasic acids possess the characteristics required as a 
solid solvent for PVC. In particular, sebacic acid, suberic acid, adipic 
acid, dodecanedioic acid and tetradecanedioic acid show the desirable 
behavior by DSC analysis. Azelaic acid retains a somewhat smaller 
proportion, say about 20-25%, of the area of its melting peak upon DSC 
re-melting for reasons not presently understood and may represent 
approximately the lower threshold of usefulness. 
TABLE 2 
______________________________________ 
Dicarboxylic Acids as Potential Solid Solvent for PVC 
Chemical Structure Melting Point .degree.C. 
______________________________________ 
Oxalic acid dihydrate 
(COOH).sub.2.2H.sub.2 O 
101 
Malonic acid CH.sub.2 (COOH).sub.2 
136 
Succinic acid (CH.sub.2).sub.2 (COOH).sub.2 
184 
Glutaric acid (CH.sub.2).sub.3 (COOH).sub.2 
98 
Adipic acid (CH.sub.2).sub.4 (COOH).sub.2 
152-154 
Pimelic acid (CH.sub.2).sub.5 (COOH).sub.2 
106 
Suberic acid (CH.sub.2).sub.6 (COOH).sub.2 
142-144 
Azelaic acid (CH.sub.2).sub.7 (COOH).sub.2 
109-110 
Sebacic acid (CH.sub.2).sub.8 (COOH).sub.2 
135-137 
Hendecanedioic acid 
(CH.sub.2).sub.9 (COOH).sub.2 
111 
Dodecanedioic acid 
(CH.sub.2).sub.10 (COOH).sub.2 
128-130 
Brassylic acid 
(CH.sub.2).sub.11 (COOH).sub.2 
114 
Tetradecanedoic acid 
(CH.sub.2).sub.12 (COOH).sub.2 
126-128 
______________________________________ 
In addition to the acids themselves, their simple derivative should also 
prove useful where they otherwise satisfy the criteria imposed above. This 
includes the amides, imides, anhydrides, aldehydes, hydroxyl substituted, 
lower alkyl-substituted, say up to about C.sub.4-5 and the like. The 
amides of monocarboxylic acids have somewhat higher T.sub.m than the free 
acids themselves and hence may qualify when the free acids do not. The 
cyclic imides of the dicarboxylic acids should be suitable; succinimide 
(T.sub.m =124.degree. C.), for example, retains a rather weak peak upon 
re-melting by DSC and is also at or near the lower threshold of 
usefulness. As a rule, the T.sub.m of dibasic acid diamides exceed the 
stated limits and are excluded as illustrated by succinic acid diamide 
(T.sub.m =268.degree. C.) and adipic acid diamide (adipamide, T.sub.m 
=226.degree. C.). However, the monoamides may have the necessary lower 
T.sub.m such as adipic acid monoamide (T.sub.m =161.degree. C.) and 
succinamic acid (T.sub.m =153-156.degree. C.). Cyclic anhydrides of the 
dibasic acids can fit within the parameters such as succinic anhydride 
(T.sub.m =119.degree. C.) but the monobasic anhydrides tend to be low in 
T.sub.m such as stearic anhydride (T.sub.m =72.degree. C.) and palmitic 
acid anhydride (T.sub.m =64.degree. C.). Succinic aldehyde occurs in 
several different isomeric forms of which at least one has a T.sub.m 
within the present range, e.g. the delta isomer (T.sub.m =135.degree. C.). 
Adipaldehyde, on the other hand, is normally liquid and thus unsuitable. 
Examples of substituted acids include DL-12 hydroxystearic acid (T.sub.m 
=81.degree.-82.degree. C.) and 2,2'-dimethylsuccinic acid (T.sub.m 
=142.degree. C.). The latter has a considerably reduced re-melting peak 
indicating substantial permanent solvation in PVC and is among the least 
preferred compounds. Other analogous and homologous substituted 
derivatives having the necessary combination of properties could certainly 
be substituted. A variation in the structure of the dibasic acids is 
represented by ketopimelic acid of the formula CO(CH.sub.2 CH.sub.2 
COOH).sub.2 with T.sub.m =143.degree. C., which shows strong melting peak 
retention with only a minimum melting peak drop of a little less than 
5.degree. C. Another variation is thiodiglycolic acid of the formula 
S(CH.sub.2 COOH).sub.2 with T.sub.m =128.degree.-131.degree. C., which 
also shows the desirable behavior in DSC analysis. 
For purposes of test evaluation to determine the existence of the 
fundamental qualifying properties, the solid solvent candidate should be 
combined with PVC in the absence of other constituents. After suitability 
has been established, however, the solid solvent can be added via other 
ingredients that may be present in a working formulation, such as a 
carrier polymer or other additive. While the invention is concerned 
especially with PVC homopolymers, the PVC can be co-polymerized with a 
minor amount of a co-monomer imparting some advantageous modification in 
polymer properties. Up to, say about 15% by wt. of the total monomer 
content can be supplied by the co-monomer and the phrase "consisting 
essentially of PVC" as used herein is intended to cover such co-polymers. 
An illustrative co-monomer is vinyl acetate.

EXAMPLES 
One crystalline chemical selected as a solid solvent for PVC was sebacic 
acid with the chemical structure, HOOC(CH.sub.2).sub.8 COOH, and T.sub.m 
at about 135.degree. C. Five percent (by weight) sebacic acid powder was 
mixed with unplasticized PVC powder used as RPVC, and the mixture was 
tested on a Perkin-Elmer DSC obtaining the DSC heat flow curves presented 
in FIG. 1. Curve obtained during the initial heating, i.e. melt mixing, of 
the mixture of separate powders at a heating rate of 20.degree. C./min, 
shows the midpoint (T.sub.g) of the glass transition range of the PVC 
powder around 77.degree. C. (350.degree. K.) and the measured melting peak 
(T.sub.m) of sebacic acid at about 122.degree. C. (395.degree. K.). At the 
end of the initial heating, the mixture was rapidly cooled or quenched in 
the DSC at about 320.degree. C./min and then re-heated. Curve 2 was 
obtained during the second heating of the mixture at a rate of 20.degree. 
C./min. Curve 2 shows a drop in the T.sub.g of the PVC powder to about 
72.degree. C. (345.degree. K.) and the melting peak of sebacic acid 
dropped slightly to about 120.5.degree. C. The area under the re-melting 
peak of sebacic acid of Curve 2 is slightly less than that of the original 
peak of Curve 1, indicating that a small amount of sebacic acid was 
permanently dissolved in the PVC powder. The glass transition of the PVC 
powder was shifted to a lower temperature by about 5.degree. C., due 
apparently to the plasticizing effect of the dissolved sebacic acid. The 
melting peak of sebacic acid also was shifted on re-melting to a slightly 
lower temperature due to the interaction between sebacic acid and PVC. At 
the end of the second heating, the mixture was gradually cooled at the 
rate of 20.degree. C./min and Curve 3 was obtained during the third 
heating of the mixture at 20.degree. C./min. Curve 3 is identical to Curve 
2. The DSC results revealed that sebacic acid, after being mixed and 
melted with the PVC powder, recrystallized upon cooling leaving only a 
small amount dissolved in PVC, and that the small amount of sebacic acid 
dissolved in PVC effectively solvated PVC reducing its glass transition 
point. Thus, sebacic acid clearly showed the desired solid solvent 
behavior for PVC in the evaluation. To confirm that the ability of an 
additive to effectively reduce the T.sub.g of PVC could be taken as an 
indication for the ability of the additive to effectively reduce the melt 
viscosity of PVC, the solubility/compatibility of sebacic acid in PVC at 
melt temperatures was studied by measuring the melt viscosity of PVC using 
a Haake continuous rheometer. FIG. 2 presents the results of the viscosity 
measurements at 200.degree. C., which show that the viscosity of the PVC 
sample was reduced by as much as 30% by the addition of 5% sebacic acid, 
clearly establishing that sebacic acid acted as a solvent to PVC at 
200.degree. C. Thus, sebacic acid was found to be a good solid solvent for 
PVC. Succinimide shown in FIG. 2 also was found to be a good solid solvent 
for PVC. 
Various other compounds were tested by DSC analysis and were found to 
possess generally the desired characteristics for this invention. These 
compounds and the test results are summarized in Table 3. The data in 
Table 3 were read during re-melting after gradual or slow cooling. 
TABLE 3 
______________________________________ 
Results of DSC Evaluation 
Re-Melting 
Drop in PVC 
Peak 
T.sub.m, .degree.C.*.sup.1 
T.sub.g, .degree.C. 
Retention 
______________________________________ 
Tricosanoic acid 
81 --*.sup.2 very strong 
Hexacosanoic acid 
90 --*.sup.2 very strong 
Octacosanoic acid 
94 --*.sup.2 very strong 
Adipic acid 152 4 very strong 
Suberic acid 146 6 very strong 
Azelaic acid 109 11 weak 
Sebacic acid 122 5 very strong 
Dodecanedioic acid 
126 5 very strong 
Tetradecanedioic acid 
134 5 very strong 
Ketopimelic acid 
143 4 very strong 
Thiodiglicolic acid 
130 4 moderate 
DL-12 Hydroxystearic acid 
82 --*.sup.2 moderate 
2,2'-Dimethyl succinic acid 
145 11 weak 
Succinimide 119 18 weak 
Octadecanamide 107 6 weak 
______________________________________ 
*.sup.1 As actually measured during the initial heating 
*.sup.2 Unable to detect due to T.sub.g and T.sub.m overlapping 
The amount of the solid solvent can be adjusted within fairly broad limits 
depending upon the applicable circumstances, from a fraction of 1%, say 
about 0.3%, up to 10% or so based on the weight of the polymer present. 
For a solid solvent having a strong down-shifting effect on the T.sub.g of 
the polymer, lower amounts might be sufficient. 
The concept of this invention was developed especially for use with 
unplasticized PVC polymers to make RPVC products but it could also be 
useful for plasticized PVC polymers in special situations to permit a 
reduction in the amount of plasticizer or other additives and better 
control over the ultimate properties of the polymer formulation. 
Variations other than those described above could of course be devised by 
those skilled in the use of additives in polymer chemistry and are 
intended to be within the scope of this invention.