Di(2-propylheptyl) Phthalate: A New Plasticizer Choicefor PVC Compounders

Richard R. Kozlowski, Uwe StorzumBASF Corp., 100 Campus Dr., Florham Park, New Jersey 07932

Science and practice have proven that phthalic acidesters are among the most functional plasticizers forpolyvinyl chloride (PVC). The performance properties ofphthalic acid esters can be modified for an advanta-geous cost/benefit position by varying the alcohol moi-ety of the ester molecule in the practical range of C4–C13 and by specifying the linearity of the alcohol mainchain. The C8, C9, and C10 alcohols produce esters ofmost value as PVC plasticizers. Most plasticizer alcoholsare produced by the oxonation process from primaryolefins, of which ethylene, propylene, and butene are themajor refinery products available on a world scale atcosts acceptable to the application. This article intro-duces a C10 phthalate produced from butene rather thanby the current route from propylene. J. VINYL ADDIT.TECHNOL., 11:155–159, 2005. © 2005 Society of Plastics Engi-neers

ALCOHOL PROCESSES

Two major processes dominate the route to the C8, C9,and C10 alcohols: 1) The oligomerization of primary olefinsto the desired molecular weight with subsequent oxonationand hydrogenation to alcohol. This is the classic Oxo pro-cess for alcohols. 2) The oxonation of primary olefins toaldehydes with subsequent dimerization, dehydration, andhydrogenation to alcohol. This subsequent reaction is oftenreferred to as the Oxo-Aldol process. Figures 1–4 diagramthe chemistry of these processes.

The Oxo process has been the major route to plasticizergrade alcohols since its commercialization in the 1940s [1].Figure 2 illustrates the process using propylene as the feedto obtain the highly useful C10 alcohol, commonly knownas isodecanol. The introduction of a mixed olefin feed to theprocess is also practiced in the trade, with the result ofproducing a mixture of useful alcohols. The process chem-istry and catalyst result in a statistical distribution of highlybranched alcohols (predominately trimethyl branched).

The ALDOL process, illustrated in Figure 3 starting withpropylene, has been used for the manufacture of 2-ethyl

hexanol. This process operates at significantly lower pres-sures and temperatures than the oligomerization/oxo pro-cess. This advantage is recognized in the industry by thecontinued demand for 2-ethyl hexanol in the plasticizermarket, maintaining DEHP as one of the most widely usedplasticizers in the flexible polyvinyl chloride (PVC) indus-try.

The ALDOL process when applied to 1-butene, as shownin Fig. 4, produces a similar singly branched, C10 alcohol,2-propylheptanol.

PHTHALIC ACID ESTERS

A major consumption of C10 alcohol is for esterificationwith phthalic anhydride to produce a phthalate ester used inthe production of flexible PVC compounds. Trimethylhep-tanol, commonly known as isodecanol, is esterified to di-isodecyl phthalate, whose acronym is DIDP. The 2-propylheptanol ester has been assigned the acronym DPHP.

Molecular weight is the major determinant of plasticizerperformance in PVC compounds. Within the same molec-ular weight, differences in plasticizer performance can typ-ically be accounted for by the structure of the alcoholchosen. This is referred to as the linearity effect. The alco-hol produced from ethylene as shown in Fig. 1 is normalnonanol and is unarguably linear. The trimethylheptanolproduced from propylene trimer in Fig. 2 has three methylgroups on a heptanol main chain and is known in the tradeas a branched alcohol. The propylheptanol shown in Fig. 4has a single branch on a heptanol main chain and should beconsidered a less-branched alcohol. The size of the sidechain can be expected to affect the plasticizer performancecharacteristics. In the following discussion of PVC perfor-mance parameters, these differences in structure account forthe minor differences observed in PVC compound perfor-mance. In Fig. 5, the gas chromatograms for these plasti-cizers show the isomer differences between the commercialplasticizers.

Table 1 compares some selected physical properties ofthese two C10 PVC plasticizers. The typical structuraltrends are apparent in the data. Compared to the plasticizerdifferences that are expected with a difference in molecular

Correspondence to: Richard R. Kozlowski; e-mail: rkozlowski1@woh.rr.comDOI 10.1002/vnl.20055Published online in Wiley InterScience (www.interscience.wiley.com).© 2005 Society of Plastics Engineers

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weight, DIDP/DPHP differences are small. The large singlybranched DPHP is expected to have a lower specific gravityand a lower freeze point. The highly branched DIDP showsthe expected lower vapor pressure and slightly better elec-trical properties. These minor differences in neat plasticizerproperties will be reflected in PVC compound properties.Comparing PVC part performance to common industrialstandards indicates that both plasticizers produce com-pounds that will meet commercial specifications.

Table 2 shows equivalent mechanical properties for anunfilled basic PVC compound. Knowing that a flexible PVCcompound is a two-phase physical blend of resin and plas-ticizer, the properties of the compound reflect the relativeproperties of the plasticizer. The volatility of the DPHPcompound is slightly higher, while the low temperatureflexibility is slightly better than that of the DIDP compound.The higher branching in DIDP also provides a slight supe-riority in oil extraction resistance. PVC has been called themost versatile modifiable polymer in commercial use. It ishighly compatible with a variety of additives to produceapplication-specific performance profiles. A sampling ofapplication-specific C10 phthalate formulations is exhibitedand comparison data between DIDP and DPHP provided.

POWDER AND MELT COMPOUNDING

Flexible PVC compounding begins with the addition ofplasticizer to the powder resin in either a low-speed orhigh-speed mixer followed by melt compounding. Calcu-

FIG. 2. Oxo process C10 alcohols (isodecanols) from propylene.

FIG. 3. ALDOL process C8 alcohol (2-ethylhexanol) from propylene.

FIG. 4. ALDOL process C10 alcohol (2-propylheptanol) from butene.

FIG. 1. Oxo process C9 alcohol (n-nonanol) from ethylene.

FIG. 5. Gas chromatograms of commercial DIDP and DPHP.

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lated from measured plasticizer specific gravities, a DPHPcompound has a lower melted density than one based onDIDP.

Comparative mixing studies revealed another advantagefor DPHP that is not readily expected from a study ofplasticizer properties. Experience with a variety of formu-lations has shown that DPHP powder compounds have asignificantly higher powder bulk density than the identicalDIDP. The high powder bulk density is translated to higheroutputs in subsequent screw processing.

Reference Formula 1 is a typical filled general purpose orwire jacket compound. This formulation produced powdercompounds with measurable and significant differences inpowder bulk density. The 3.3% increase in powder densitycan, along with another melting characteristic, lead to po-tential melt processing advantages.

Table 3 provides some extrusion data gathered on aninstrumented laboratory extruder with 25D 19.1 mm (3/4inch) single screw extruder equipped with a force feeder.Extruder motor torque and output are easily measured vari-ables to compare extrusion performance. At similar extruderconditions, the DPHP compound extrudes with lower torqueand a higher output. The higher output is a direct conse-quence of the higher powder density feed, while the lowertorque implies that the melt viscosity and the melt density ofthe DPDP compound are lower. Processors skilled in the art

will be able to adjust formulations and extrusion conditionsto maximize these effects.

EXTERIOR APPLICATIONS

The suitability of a flexible PVC compound for exteriorservice is a function of all the ingredients in the formulation.“Weathering” is an effect resulting from the exposure of asubstance to a complex combination of ultraviolet radiationabsorption, temperature, moisture, and atmospheric contam-ination. The performance of a plasticizer has been shown tobe related to the structure of the alcohol chosen for the ester[1]. The tertiary carbon structure is known to have pooreroxidative stability than the secondary carbon structure in themain chain. As a consequence of this fact, linear alcoholphthalic esters that show excellent UV color hold and ex-cellent PVC compound property retention are the standardof comparison for exterior applications. Two measures ofexterior performance for a PVC compound are color stabil-ity and weight loss during aging. Weight loss is directlyrelated to mechanical property retention.

Table 4 lists the data gathered on DIDP and DPHPcompounds (Formula 2) along with a linear phthalate plas-ticizer as the reference using a QUV accelerated weatheringtest device equipped with UV-B bulbs in an 8-h light at70°C, 4-h dark at 50°C repeat cycle. These data imply adramatic improvement in property retention for a DPHPcompared to DIDP, but not yet reaching the performance ofthe predominately linear C711P.

It is often difficult to match the conditions of naturaloutdoor weather exposure in an accelerated device. Pru-

TABLE 1. Typical properties of neat decyl plasticizer.

DIDP DPHP

Specific gravity @ 25/25°C 0.965 0.959Refractive index @ 25°C 1.484 1.484Vapor pressure mbar @ 350°C 1.1 1.2Direct current resistivity 1.3 E�13 ohm-cm 0.94E�13 ohm-cmDielectric constant 3.9 4.7Heat capacity J/g K 1.901 1.900Fuel value cal/g 8082 8082Viscosity at 25C cp 85 85Pour point °C �40 �48Flash point (COC) °C 232 232Solution temperature °C 135 146Efficiency factor 1.11 1.11

TABLE 2. Typical PVC compound at 50 phr plasticizer.

DIDP DPHP

Hardness shore A 87 87Specific gravity at 25°C 1.255 1.253Tensile strength (Mpa) 1.71 1.70100% Modulus (Mpa) 1.09 1.09Elongation % 350 370Brittle temp °C �27 �28Clash Berg °C �19 �20Weight loss air 24 h @ 70°C % 0.20 0.30Weight loss % 12 weeks,

12 weeks, 100% RH, 120°C 0.39 0.28Weight loss oil 7 days @ 23°C % 4.5 6.0

TABLE 3. Extrusion performances, PVC compound Formula 1.

RpmScrewrpm

(D � g/cm3)FFeed rpm

DIDPTorque

Nm(D � 0.636)

rate kg/h

DPHPTorque

Nm(D � 0.656)

rate kg/h

40 35 12 2371 11.2 242640 36.5 12.2 252250 44 12.1 315560 50 12 3246 11.7 3500

PVC Formula 1. PVC resin 100 phr, tribasic lead sulfate 3 phr, dibasiclead stearate 0.8 phr, calcium carbonate 80 phr, plasticizer 55 phr as noted.

TABLE 4. Accelerated QUV weathering, % weight loss (PVC Formula 2).

DIDP(64 phr)

DPHP(64 phr)

711LP(59 phr)

118 h 0.48 0.39 0.36256 h 1.53 1.07 0.85612 h 4.93 3.26 2.5940 h 8.08 5.60 4.511226 h 10.74 7.60 6.281483 h 12.77 9.42 7.70

PVC Formula 2. PVC resin 100 phr, stabilizer Ba/Zn 3 phr, Unival3039 0.6 phr, plasticizer as noted.

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dence on the part of the processor often requires naturalweather exposures to confirm accelerated studies. Table 5shows the weight loss performance of these same plasticiz-ers exposed for 4 years in central Europe. These data showthe same lower weight loss for DPHP relative to DIDP andin this case a clear indication that property retention may besuperior to linear C711 phthalate in practice. It can beproposed that the bulky side chain of DPHP provides theadditional oxidative stability. Visual observations of thesesamples shows less color change for the DPHP plasticizedfilms.

Wire Insulation

One of the most recognized applications for DIDP hasbeen in primary wire insulation and abrasion jacket appli-cations. Insulated wire safety standards are written to pro-vide safe use constructions for specific product use or ap-plication. Electrical insulation materials must meet a set ofminimum standards as manufactured and maintain a safelevel of properties after application-determined acceleratedaging tests. Underwriters Laboratories [2] categorizes PVCinsulation performance standards in a series of service tem-perature environments for the wide variety of wire construc-tions. DPHP has been screened in a variety of formulationsin both Europe and NAFTA with a great deal of success.The test formulations were developed for a wide variety ofconstructions. The summary of data in Table 6 demonstrates

that DPHP should meet the requirements of these standardswhere DIDP is successfully utilized.

Automotive Interior Applications

Automotive applications for flexible PVC vary widely informulation and design for both interior and exterior parts.Often, two performance specifications are the determinantof plasticizer choice: low temperature flexibility and fogresistance. Fog resistance is a function of molecular weightand structure, while low temperature flexibility is a functionof plasticizer level and structure. Linear C9 phthalate is aplasticizer that has acceptable fog performance for a varietyof interior automotive parts and provides a comfortablelevel of low-temperature flexibility for many suppliers.

“Fogging” has come to be defined as the diffusion ofplasticizer and other additive volatiles from the compoundthat may condense on the automobile windshield. “FogIndex” is measured by exposing the specimen to a producer-specified set of temperatures and times. Volatiles are con-densed on a glass plate for measurement either gravimetri-cally or photometrically as reflectance. SAE J 1756 setsforth some of the test methods and conditions.

Table 7 records the results from testing of both neatplasticizer and a simple PVC screening compound. It isobserved that DPHP and DIDP produce equivalent resultsand can be used in many established applications.

Linear alcohol esters confer better low-temperature per-formance characteristics to PVC than branched esters of thesame molecular weight. Both DIDP and DPHP are at adisadvantage to CL9P plasticizers for this attribute. C10branched phthalates are 11% less efficient than CL9 phtha-

TABLE 8. Low temperature flexibility examples.

DIDP DPHP L9P

Tf (°C) at 54 phr �22 �23 �31Tf (°C) at 80A �25 �27 �31Compound SG at 80A 1.241 1.238 1.248

TABLE 5. Natural weathering central Europe, % weight loss (PVCFormula 2).

DIDP(64 phr)

DPHP(64 phr)

711LP(59 phr)

6 months 0.32 0.23 0.3612 months 0.81 0.46 0.8918 months 1.51 0.74 1.4124 months 1.35 0.73 1.5536 months 2.90 1.07 2.3448 months 11.90 2.70 3.26

TABLE 6. Wire insulation performance matrix.

DIDP DPHP DPHP/DUP

60C Service 7 days at 100°C0.5 mm Pass Pass1.0 mm Pass Pass

75C Service 10 days at 100°C Pass Pass0.5 mm Pass Pass1.0 mm Pass Pass

80C service 7 days at 113°C Pass Pass Pass0.5 mm Varies Fail Pass1.0 mm Pass Pass Pass

90C Service 7 days at 121°C0.5 mm Fail Fail Pass1.0 mm Fail Fail Pass

TABLE 7. Typical fog results.

DIDP DPHP L9P

Neat plasticizer % reflectance3 h at 100°C/1 h condition 68 70 69

Neat plasticizer mg collected16 h at 100°C 4 h condition 0.74 0.72 0.60

PVC compound % reflectance3 h at 100°C/1 h condition

67 phr plasticizer 95 9154 phr plasticizer 93 9143 phr plasticizer 93 99PVC compound mg collected

16 h at 100°C/4 h condition67 phr plasticizer 0.67 0.7554 phr plasticizer 0.68 0.6843 phr plasticizer 0.63 0.67

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lates, an advantage of the resultant lowering of compoundspecific gravity. Table 8 provides some guidance for thecompounder.

CONCLUSION

When 2PH is esterified with phthalic anhydride it pro-duces a plasticizer with performance in PVC equivalent tothe widely known, highly branched DIDP.

Each of these C10 phthalates has some application-spe-cific advantages, but in general both are capable of produc-ing a PVC compound to meet a performance specificationneeding this molecular weight. DPHP should be of partic-ular interest to fabricators specializing in exterior applica-

tions such as roofing, geomembrane, or tarpaulins. Wireinsulation manufacturers will find satisfactory performancewith only minor compound changes necessary in recognizedformulations. All compounders should be able to take ad-vantage of the lower pound volume costs and higher extru-sion rates.

REFERENCES

1. L.P. Hatch, Higher Oxo Alcohols, Enjay Laboratories, NewYork (1957).

2. Reference Standards for Electrical Wires, Cables and FlexibleCords UL 1581, Underwriters Laboratories, Northbrook, IL.

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