Polymeric Plasticizers

 

POLYMERIC PLASTICIZERS FOR HIGHER PERFORMANCE FLEXIBLE PVC

Applications

 RONALD D. SVOBODA

The C.P. Hall Company
Chicago. Illinois 60638
 

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Flexible PVC applications can be categorized as requiring either general or higher performance characteristics. In many application areas. specification requirements are becoming increasingly more severe requiring the use of Polymeric polyester plasticizers in compounding. Polymeric plasticizers provide excellent migration. volatility. fluid chemical extraction. and/or weathering resistance in higher performance applications compared with that obtained from monomeric plasticizers alone.

INTRODUCTION

End uses for flexible PVC compounds are quite diverse, but they can be loosely categorized as being either general performance or higher performance applications. Each of these performance categories requires a different set of considerations in terms of compounding with plasticizers. For general performance applications, compounders require moderate performance in several areas without particular emphasis on any one. In these applications, good plasticizing efficiency is often desired and this is manifested in processing ease. good compound softening. and modulus and tensile reduction. For economic reasons, compound softening and modulus reduction are ideally obtained using the lowest plasticizer loadings possible. Performance properties are of secondary consideration and general performance applications may or may not require moderate volatility resistance. fluid chemical extraction resistance. and low-temperature properties. Some general performance plasticizers used in the market place today include DIDA. DINP. DIDP. DOP. DBP, and other phthalates made from straight-chain alcohols of seven to eleven carbons-in length.

The consumer demand for more durable goods and specifications for higher performance products require many of the same considerations needed in general performance compounding. however. particular emphasis is placed on superior plasticizer permanence in one or more specific areas. Discussed in this paper are how polymeric polyester plasticizers may be used in higher performance applications where superior plasticizer migration. volatility. fluid chemical extraction. and/or weathering resistance are critical.
 

DISCUSSION

Polymeric Plasticizers

Polymeric plasticizers are Produced by reacting a dibasic carboxylic acid with a glycol or a mixture of different dibasic carboxylic acids with one or more glycols. When these molecules are reacted. the chain propagation or building may be terminated by the use of monofunctional carboxylic acids or alcohols. Some polymeric plasticizers are also produced using no terminator. Following is a representation of the production of an acid terminated polymeric polyester which assumes that stoichiometrically correct quantities of dibasic carboxylic acid, glycol, and monofunctional carboxylic acid have been reacted together to yield a polymeric plasticizer of a viscosity (or molecular weight) within a given range.
 

General Synthesis of an Acid Terminated Polyester

Dicarboxylic          Acid Glycol             Fatty Acid

        O O                                                O      O

nHO-C-R-C-OH + (n + I)HO-R'-OH + 2R"C-OH-R"-C- -O-R’-O-C-R-C- -O-R’-OCR"
+ By Product (2n + 2)H2O

For the purposes of this paper, the polymeric plasticizers discussed are identified by dicarboxylic acid type and typical apparent viscosity In centapoises taken at 25° C. Dicarboxylic acids commonly used to manufacture polymeric plasticizers include the following:
 

Common Dicarboxylic Acids Used to Manufacture Polymeric Plasticizer.

Phthalate

Glutarate - 5 Carbons

Adipate - 6 Carbons

Azelate - 9 Carbons

Sebacate - 1 0 Carbons

 

Mixtures

Polymeric plasticizers are manufactured in a wide range of viscosities and provide varied balances of permanence along with handling and processing ease. Generally, as viscosity is increased, processing and handling become more difficult. The polymeric plasticizers we produce range in viscosity from about 900 cps up to 160,000 cps.

The polymerics discussed in this paper are identified by the first letter of the acid type used. In other words. A-indicates an adipate, G - a glutarate, and so on. Azelates would be identified with a prefix of Z, but are not discussed in this paper. Acid mixtures are identified by the letter MA. Following the letter identifying acid type is the typical apparent viscosity for the plasticizer. Therefore, an adipate polymeric with a viscosity of 5,600 is identified as A-5,600.

Polymeric Plasticizer Identification

                Acid Type                                         Viscosity, cps

                Adipate                                                  5600

                                                A-5600

                                                G-12,000

                Glutarate                                             12,000

 

Factors Influencing Plasticizer Permanence

The permanence of a polymeric plasticizer in a flexible PVC compound depends upon three major factors which include structure, molecular weight/ viscosity, and polarity. Polymerics composed of branched structures are more permanent than those based upon linear structures. Branching tends to hinder movement or entangle the plasticizer within the polymer matrix making it more difficult to migrate or be removed by volatilization or extraction. Although linear structures provide less permanence, they do yield better low temperature properties.

The greater the viscosity/molecular weight of a plasticizer, the greater will be its permanence. Simply stated, the longer and bulkier the molecule is, the more difficult for it to be removed.

Polarity can be roughly viewed as the ratio of oxygen to carbon atoms in a plasticizer. The greater the ratio of oxygen to carbon atoms, the greater the polarity. It is vital that the polarity of the plasticizer be properly matched to that of the polymer, in this case PVC. If the polarity of the plasticizer is not sufficiently similar to that of PVC, varying degrees of plasticizer incompatibility may result. Plasticizers which are somewhat incompatible are more prone to migration, volatilization, and extraction.
 

Migration Resistance

Migration is the movement of a plasticizer within and from a PVC compound into or onto a substrate to which it is held in intimate contact. Plasticizers which are migratory can have negative effects upon substrates including marring, crazing, discoloring, weakening, and dissolving. Several application areas come to mind where little or no plasticizer migration can be tolerated and where polymeric plasticizers are currently used. Plasticizer used in PVC refrigerator door gaskets cannot mar or discolor the ABS and polystyrene inner cabinets they are held in contact with when the door is closed. Plasticizer used in PVC to construct automotive instrument panels, head rests, and arm rests cannot migrate to the polyurethane backing used in these constructions. If migration occurs, the PVC covering could become brittle and crack or adhesion could be lost at the PVC covering, polyurethane interface. For vinyl electrical tapes, plasticizer cannot migrate through the tape and dissolve the adhesive backing. Furthermore, significant work has been done examining the use of polymeric plasticizers in vinyl medical tubing. In this application area, it has been shown that non-migrating plasticizers can be used so that polycarbonate couplings, unto which the tubings are connected, do not stress crack (1).

Using our own laboratory test procedure, we have tested several polymeric plasticizers and a general performance monomeric (DOP) for migration to ABS, polystyrene, and nitrocellulose substrates (2). In Table 1. DOP, and polymerics A-3,300, G-3,700, and G-12,000 are compared for migration resistance to the three aforementioned substrates. Like most all other general purpose monomerics, DOP migrates readily to all three substrates. Polymeric plasticizer A-3,300 provides a respectable balance of migration resistance properties, while G-3,700 and G-12,000 provide excellent resistance to migration. Glutarate polymerics are especially known for their non-migration characteristics (3).
 

Table 1. Migration Resistance of Selected Plasticizers in PVC.
	DOP	A-3,300	G-3,700	G-12,000
ABS	P	G	G	E
Polystyrene	P	G	E	E
Nitrocellulose	P	G	G	G
E = Excellent, G = Good, P = Poor
Recipe: PVC-100, BaCd-1, Plasticizer-67.

Volatility Resistance

Excellent volatility resistance is particularly important in PVC wire and cable jacketing where specifications (i.e. UL 62 105° C Class 12 Thermoplastic Insulation and Jacket) require minimal losses of elongation and tensile after air aging (4). Newer specifications for interior automotive trim, particularly instrument panels, also require stringent requirements for minimal losses of elongation.

Ideally, most compounders for applications requiring low volatility will be in search of the lowest losses of elongation and weight and the least increases in modulus, tensile, and compound hardness. Compared in Table 2. are DOP, and polymerics MA-5,500 and A-6,800 in unfilled PVC compounds which have been air aged for three days at 136° C. As can be seen, much of the DOP has been volatilized from its compound under these test conditions and as a result elongation and tensile changes are excessive. Both polymerics enjoy lower volatility weight losses and, as a result, changes in stress-strain properties are of lesser magnitude (5).

Often times a higher performance application may require superior permanence with regards to two or more performance parameters. In Table 3. PVC compounds have been air-aged for up to 500 h at 120° C while in contact with polyurethane (PU) foam. For this type of aging, we will have the combined effects of volatilization and migration. Compared are a compound plasticized entirely with a linear phthalate and another with a blend of polymeric A-1,200 and the same phthalate in a 3:1 ratio. Blending plasticizers such as this is a popular way of getting the best of both worlds for cost and performance. Using the polymeric plasticizer, we have greatly reduced 50% modulus and elongation loss and slightly reduced tensile increase values.
 

Table 2. Air Oven Volatility Resistance of Select Plasticizers in PVC.
Air Oven Aging, 3 days at 136ºC
	DOP	MA-S,500	A-6,800
Percent-
Weight change	-28	-4.3	-1.6
Elongation change	>100	-14	-9
Tensile change	>100	2	-2
Recipe: PVC-100, BaCd-1, Plasticizer-67.

 

Table 3. Long Term Air Oven Volatility of Plasticized PVC in Contact With PU Foam.
		A-1,200
	Linear	(75)/Linear
	Phthalate	Phthalate (25)
After 250 h
50% modulus change. %	69	50
Tensile change. %	-18	-5
Elongation change. %	-36	-17
After 500 h
50% modulus change, %	258	189
Tensile change. %	28	24
Elongation change. %	-67	-28

Fluid Chemical Extraction Resistance

When studying plasticizers for fluid chemical extraction resistance, we would obviously look for the least PVC compound weight loss. Low plasticizer extraction usually results in the preservation of a PVC part's dimensional stability and surface appearance. Failures due to plasticizer extraction can be as severe as part ripping, disintegration, or cracking. Applications requiring excellent extraction resistance include: hoses and tubing: printing rollers: belting, sheeting and film: and upholstery.

Polymeric plasticizers have been successfully used in these applications and blending with monomerics is common to optimize properties. In Table 4 is a comparison of the extraction resistance properties for DOP and polymerics A-1,200, A-20,000, and S-160,000 in unfilled PVC compounds. It can be seen that as viscosity increases, so does resistance to extraction with S- 160,000 having superior permanence in all three test fluids.

We consider hexane and cotton seed oil to be relatively low in polarity, while 1% soapy water solution is relatively high. Lower polarity fluids generally have a greater ability to extract lower polarity plasticizers and vice versa. For example, since polymeric plasticizer A-20,000 is higher in polarity compared with A-1,200 and S-160,000, it was found to be more readily extracted in the 1% soapy water solution.
 

Weathering Resistance

Weathering resistance is another application area which requires superior permanence with respect to several performance parameters. Flexible PVC products which are weathered are exposed to a combination of: elevated ambient and surface temperatures: wet-dryout cycles due to rain and dew. humidity exposure: UV exposure: contact with dirt and pollutants: and fungal attack.

In weathering applications, we look for several performance properties. Probably the primarv property would be resistance to surface degradation. This could mean least color change if the flexible PVC was pigmented or little yellowing if the compound was clear. Other ideal properties would include little or no microbial attack, surface tack development, and surface dirt pick-up. Compounds which are made containing higher performance polymerics in order to weather well, will generally provide better tensile, modulus, elongation, and hardness retention. Specific application areas, where these properties would be of concern include PVC decals, film and sheeting, and a variety of interior automotive trim applications.
 

Table 4. Fluid Chemical Extraction Resistance of Select Plasticizers in PVC.
	DOP	A-1,200	A-20,000	S-160,000
Percent Weight Loss
n-Hexane, 24 h	-31	-1.9	-0.65	-0.39
	@ 23ºC
Cottonseed oil, 24 h	-16	-2.9	-1.6	-0.08
	@ 60ºC
1% soapy water, 7 d	-19	-5.2	-11	-5.3
	@ 90ºC
Recipe: PVC-100, BaCd-1, Plasticizer-67.

To examine the weatherability of polymeric plasticizers we prepared unfilled PVC compounds containing several polymeric plasticizers and two monomeric diesters, DOP and DIDG (Diisodecyl Glutarate). The compounds prepared were based upon 100 pphr PVC resin. 1 pphr Barium/Cadmium stabilizer, 0.5 pphr of a UV stabilizer, 3 pphr of ESO, and 67 pphr of the plasticizer variable. Both "direct" and "underglass" agings were performed. For direct agings, specimens are affixed to a wooden panel held at a 45° angle facing south in Miami, Florida. For this type of weathering, specimens are in direct contact with the elements. For underglass agings, specimens are exposed in a similar manner, however, the samples are encased in a glass box, thereby being protected from rain, dew, etc. Underglass weathering is performed mainly for interior automotive trim applications. As would be expected, samples aged under glass experience significantly greater surface temperatures than do those which are direct aged.

As mentioned earlier,color retention of pigmented or yellowing resistance of clear PVC films can be vital. Table 5 compares general performance monomerics DIDG and DOP to polymerics G-4,000 and G-12,000 for yellowness index change after two, four, and six months of direct aging. Both monomerics show relatively high initial and longer term tendencies to yellow. G-4,000 and G-12,000 provide excellent short term and longer term resistance to discoloration by yellowing. Glutarate polymerics in general have a proven history of providing good resistance to weathering for PVC compounds.
 

Table 5. Direct Weathering of Select Plasticizers in PVC
(450 South, with backing, South Miami).
	DIDG	DOP	G-4,000	G-12,000
Yellow Index Change
2 months	8.25	4.93	0.17	0.5
4 months	9.25	8.93	3.17	1.5
6 months	14.9	9.85	4.55	5.35
Recipe: PVC-100, BaCd-1, UV Stabilizer-0.5, Plasticizer-67, ESO-3.

 

Table 6. Underglass Weathering of Select Plasticizers in PVC
(450 South, with backing, South Miami).
	DOP	DIDG	A-20,000	G-12,000
Yellow Index Change
2 months	2.36	-1.9	-1.08	-2.83
4 months	6.05	2.94	0.57	-0.28
6 months	8.38	6.05	2.72	1.34
Recipe: PVC-100, BaCd-1, UV Stabilizer-0.5, Plasticizer-67, ESO-3.

Table 6 provides a similar comparison, however, for underglass weathering. Compared are net changes from initial yellowness values for monomerics DOP and DIDG, and polymerics A-20,000 and G-12,000. The magnitude of short term and longer term yellowness index change values is less overall for the underglass weathered compounds compared with those which were direct aged. Polymeric polyesters A-20,000 and G-12,000 provide about a threefold reduction in yellowness index change compared with the general performance monomerics. 
 

SUMMARY AND CONCLUSIONS

In this paper, higher performance parameters for flexible PVC applications including plasticizer performance with respect to migration, volatility, fluid chemical extraction, and weather resistance have been discussed. Ideal compound physical property retentions, application areas, and PVC compound data have been given for each of the four higher performance areas. We have seen that higher performance polymeric polyester plasticizers provide superior permanence properties compared with general performance monomerics in all four of these demanding areas. General purpose monomerics provide greater processing and handling ease, greater compound softening efficiency, and are lower in cost than polymeric plasticizers. Optimization of permanence properties, processing, efficiency, and cost can be obtained by judicious blending of higher performance polymerics with monomeric plasticizers. 
 

ACKNOWLEDGEMENT

The author would like to express gratitude to his colleagues in the Technical Service Laboratory for generating much data and information contained in this paper. Thanks also goes to W. H. Whittington for his guidance and useful suggestions.

 

REFERENCES

1. S.E. O'Rourke, J. Vinyl Tech., 9, 147 (1987).

2. "Surface Mar-CPH Method 01-80B" (1989). The C.P. Hall Co., 5851 West 73rd St., Chicago, IL 60638.

3. J.L. O'Brien, W.H. Whittington, and G. Chalfant, Plast. Compounding, May/June 1981.

4. Standard for flexible Cord and Fixture Wire, UL-62. 12th ed., pp. 53-56, Underwriters Laboratories, lnc., Northbrook, IN., September 1981.

5. Polymeric Plasticizers for Flexible PVC (brochure), The C.P. Hall Co., 5851 West 73rd St., Chicago, IL 60638.

 

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