Rubber-Toughened Plastics - American Chemical Society

15 Stabilizer Partitioning in RubberModified Systems Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0222.ch015

Donald M . Kulich and Michael D . Wolkowicz Technology Center, G E Plastics, Washington, WV 26181

Additives in rubber-modified systems partition between the elastomeric phase and the thermoplastic phase. Because the rate and mechanism of oxidation of each phase differ, the actual concentration of stabilizer in the elastomeric and thermoplastic phases can be an important factor influencing overall polymer stability. The partitioning behavior of various antioxidants in model two-phase systems was studied by electron microscopy by using energy-dispersive X-ray analysis. Antioxidants used in the study included thiodipropionates, phosphites, and a phenolic antioxidant. Highly alkylated additives partitioned preferentially into the polybutadiene phase when the elastomer was dispersed in styrene-acrylonitrile copolymer. Effects of composition and relative solubility measurements of additives in polybutadiene and thermoplastics are described. The effects of matrix-phase composition on oxidative stability as determined by oxygen uptake and dynamic scanning calorimetry are shown to correlate with partitioning behavior.

R U B B E R - M O D l F I E D SYSTEMS such as A B S (aerylonitrile-butadiene--styrene) and HIPS (high-impact polystyrene) are composed of an elastomeric component dispersed as a discrete particulate phase in a thermoplastic con tinuous phase. Rubber-modified polymers are susceptible to oxidative deg radation, and studies have been conducted to determine which structural feature is most sensitive to oxidation, the effects of oxidation on property changes, and the effects of additives on stability (1-6). The rate and mech anism of oxidation of the rubber phase, usually polybutadiene (PB) or b u tadiene copolymers, will differ significantly from that of the thermoplastic 0065-2393/89/0222-0329$06.00/0 © 1989 American Chemical Society

Riew; Rubber-Toughened Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

330

R U B B E R - T O U G H E N E D PLASTICS

phase, usually polystyrene (PS) or styrene-acrylonitrile (SAN) copolymer. Because additives can partition between phases, the antioxidant concentra tion in the elastomeric and thermoplastic phase can differ significantly from the average concentration. Partitioning of the additive within the polymer may, therefore, exert a controlling influence on the resulting effectiveness of the additive. This chapter describes the partitioning behavior of various antioxidants as determined by scanning electron microscopy (SEM) by using X-ray en ergy-dispersive spectrometry (XEDS). Antioxidants used in the study i n cluded thiodipropionates, phosphites, and a phenolic antioxidant. Relative additive concentrations were determined in two-phase model systems by

Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1989-0222.ch015

using X E D S , and results are compared with a determination of the relative solubility of the additives in PB and various thermoplastics. Additive concentrations in polymers have been previously determined by microscopic, radiographic, thermogravimetric, spectroscopic, and ex traction techniques (7-12). The use of S E M - X E D S to observe the redis tribution

of

additives

during

polypropylene

crystallization

has

been

described (8). This chapter applies S E M - X E D S to analysis of additives in rubber-modified systems.

Experimental Details M o d e l systems were prepared for partitioning studies by mixing P B emulsion crumb with additive and the selected thermoplastic with a mixing bowl (Brabender) oper ating at 190 °C for 5 min at 60 rpm. A cross section of the compounded material was then prepared for S E M - X E D S analysis. T h e selected mixing time was sufficient to achieve the desired particle size and dispersion. Designated samples were thermally equilibrated under inert atmospheres at elevated temperatures. Solubility measurements were carried out by placing weighed amounts of poly mer together with excess additive in an ampoule, followed by evacuation and sealing. Samples were then equilibrated at the specified temperature for extended times to ensure that equilibrium was reached. The samples were quenched and additive concentrations were determined by S E M - X E D S analysis by using calibration curves. Measured concentrations are considered to be representative of additive concentra tions attained after equilibration at the specified elevated temperature. The cross-linked P B was prepared by emulsion polymerization to >90% con version. The linear P B was Firestone Diene 35. The S A N , PS, and linear low-density polyethylene ( L L D P E ) were commercially available materials. T h e following addi tives were commercially available: dilauryl thiodipropionate ( D L T D P ) ; distearyl thiodipropionate ( D S T D P ) ; triphenyl phosphite; tris(2,4-di-i-butylphenyl) phosphite; and 2,4-bis(N-octylthio)-6-(4-hydroxy-3,5-di-f-butylanilino)-l,3,5-triazine (Irganox 565, Structure 1). T h e bis(4-nonylphenyl) thiodipropionate and bis(4-cyanophenyl) thiodipropionate were synthesized. Oxidative stability measurements were determined on a dynamic scanning cal orimeter ( D S C , D u Pont). Samples analyzed were prepared by compounding 20% PB with experimental bulk SANs with styrene/acrylonitrile (S/A) ratios of 63:37, 75:25, and 80:20, and also with commercial PS. Stabilized samples were prepared


15.

K U L I C H & WOLKOWICZ

Partitioning in Rubber-Modified Systems

331

HO

1


by adding 0.5 parts compound 1 during compounding. The samples were equilibrated at 190 °C in evacuated sealed ampoules for 60 h prior to analysis. Oxygen-uptake measurements were conducted by using a previously described computerized method (13) on 40-mesh samples that had been compounded with a mixing bowl (Brabender).

Discussion Thiodipropionates, phosphites, and hindered phenolic compounds are classes of compounds widely used as stabilizers against the thermal oxidation of rubber-modified systems such as HIPS and A B S . Sulfur and phosphorus stabilizers and some hindered phenolic compounds contain atoms of sufficient atomic number to permit detection by X E D S . Using model rubber-modified systems, relative antioxidant concentrations were thus determined from X ray signal intensities in rubber and rigid areas. Rubber-particle sizes ranged from 10 to 100 μπι, as shown in Figure 1. As illustrated in Figure 2 for the system D L T D P in PB and S A N , the signal intensity emitted from the additive in the rubber phase is considerably greater than that from the additive in the S A N phase. This signal intensity indicates greater additive concentration in the rubber phase. The data in Table I demonstrate the significance of substituent effects on partitioning. A n increase in substituent polarity reduces partitioning of the thiodipro pionate into the rubber phase. The effects of the polymer matrix on parti tioning are illustrated in Table II. Partitioning is strongly dependent on the nature of the matrix phase; increasing matrix polarity promotes concentration of the additive in the rubber phase. Significant differences exist between P B - P S and P B - S A N . Differences in partitioning behavior are consistent with calculated sol ubility parameter differences. Solubility parameters (δ) were calculated from Small's equation (14)

δ =

dXG M

(1)

where d is the density of the sample, M is the molecular weight of the additive or repeat unit in the polymer, and is the sum of the group molar




332

Figure 1. Cross section of PB-SAN blend, prepared by compounding on a mixing bowl Key: a, xl20; b, X2500.


15.

KULICH &


WOLKOWICZ

333

(a)


CO

(b)

1.0

4.0 2.0 3.0 ENERGY, eV X I0"

5.0

3

Figure 2. X-ray energy-dispersive spectra ofDLTDP in PB-SAN. Key: a, sulfur (DLTDP) concentration in PB phase; b, sulfur (DLTDP) concentration in SAN phase.

attraction constants of all the chemical groups in the additive or polymer repeat unit. The solubility parameter of the S A N was calculated according to Krause (15)

δ = Χδ,φ,

(2)

where φ is the volume fraction of each component. Density values were (

calculated with equation 3

+ v

(3)

r

where M is the molecular weight of additive or polymer repeat unit, V is the group contribution to the molar volume, and V is a residual volume factor (16). t

r

As can be seen in Tables III and IV, the solubility parameters for the alkyl-substituted thiodipropionates are similar to the value for the PB com ponent. However, polar substitution results in solubility parameter values of the additive closer to that of the PS or S A N phase. The solubility parameter



334

Table I. Substituent Effects on Partitioning for R in RQ CCH CH SCH CH CQ R 2

2

2

2

Additive, pph

R —C18H37 —C12H25

(C H ) - C 9 H 19 6

4

-C H5

(C H )-CN 6

4

2

PSAN Phase

PB Phase I00 60-80 80 70 12

2,3,5 2,3,5 3 3 3

a

not detected 20-40 20 30 88

b

d

d

6

2

Thiodipropionate, %

d

c

"Suspension with S / A = 2.2:1, w / w . Same results were obtained with mixing bowl. M o r e heterogeneity was found with dilauryl vs. distearyl thiodipropionate. Thermally equilibrated samples at 190 ° C .

6


c

d

Table II. Effect of Matrix on Partitioning for D S T D P Matrix

Matrix Phase

PB Phase

a

Styrene-acrylonitrile copolymer Polystyrene Polyethylene

not detected 50 67

100 50 33

6

c

NOTE: All results are percent DSTDP, with 5 parts per hundred of additive in 80:20 matrixrubber system equilibrated at 190 °C after mixing bowl compounding. "Emulsion rubber. S/A = 2.2:1, w/w. "Linear low-density polyethylene. b

Table III. Substituent Effect on Calculated Solubility Parameter for R in R Q C C H C H S C H C H C Q R 2

2

R

2

2

2

2

δ (cal/cm f 3

8.4 8.5 8.9 10.4 11.7

—C18H37

—C12H25 ( C e H ^ - C g H 19 —C H5 6

(C H )-CN 6

/2

4

Table IV. Calculated Solubility Parameters for Various Polymers Polymer Polybutadiene Polyethylene Polystyrene Styrene-acrylonitrile copolymer

b

a

6

(cal/cmf

2

8.1 8.2 9.0 10.7

Calculated from group molar attraction constants according to Small. S/A = 2.2:1, w/w. fc



15.



335

rankings in Table III correlate with the partitioning trends in Table I. How ever, a relatively higher concentration of the diphenyl derivative was found in the PB phase than expected on the basis of the solubility parameter values calculated with Small's equation. These values do not take into account the reduction in the polar component's contribution to the solubility parameter caused by identical polar groups present in a symmetrical position. This change would increase the expected concentration of the additive in the less-polar phase. S E M - X E D S analyses were also used to determine relative antioxidant solubilities in P B alone and in various thermoplastics. Solubility meas urements were conducted to provide comparisons with previous partitioning data and to provide comparative data on phosphites and a hindered phenolic antioxidant. Concentrations of D L T D P and D S T D P in quenched samples of various thermoplastics equilibrated with excess additives are shown in Table V. D L T D P and D S T D P were found to be miscible with PB at elevated temperatures. Differences in behavior between D L T D P and D S T D P were observed, with D L T D P exhibiting slightly greater solubility in S A N and PS. Increasing alkyl chain length in D S T D P (C ) versus D L T D P (C ) re duces solubility in S A N and PS versus PB. Solubility behavior thus correlates with partitioning behavior. Figure 3 illustrates that these solubility rankings remain unchanged over a wide temperature range. Phosphite esters represent another class of widely used antioxidants. Most commercially available phosphite antioxidants are isomeric mixtures. Triphenyl phosphite and tris(2,4-di-i-butylphenyl) phosphite were selected for investigation to avoid the presence of structural isomers. The triaryl phosphite was shown to exhibit high solubility in S A N , as well as in PS and PB (see Table VI). Alkyl substitution again significantly reduces solubility in S A N and favors solubility in P B . Most hindered phenolic antioxidants do not contain atoms sufficiently high in atomic number to permit detection by X-ray analysis. However, 18

12

Table V. Solubility of Thiodipropionates in Various Polymers at 157 °C Polymer P B (linear) P B (cross-linked) Polyethylene Polystyrene Styrene-acrylonitrile copolymer* 6

c

d

DLTDP*

DSTDP"

>95 48 >95 >95 4

>95 59 >95 32 2

"Additive concentration, w/w %. ^Solution polymerized with 35:55 cis-trans and 10% vinyl. Prepared by emulsion polymerization. ^Linear low-density. S / A = 2.2:1, w/w. c

e


336


695 >95

>95 95 94 8.5

"Additive concentration, w/w %. ^Solution polymerized with 35:55 cis-trans and 10% vinyl. Prepared by emulsion polymerization. S/A = 2.2:1, w/w. c

rf


15.

337



compound 1 is soluble in various polymers (191 °C) at the following w/w additive concentrations: • cross-linked P B , prepared by emulsion polymerization, 60 • PS,

>95

• S A N (3:1 w/w),

20

• S A N (2.2:1 w/w),

11

The ratio of styrene to acrylonitrile (S/A) of the matrix phase was changed in the P B - S A N model system, and the effects on partitioning and oxidative stability were determined by oxygen uptake and also by D S C . The relative stabilities of the unstabilized polymeric components are shown in Figure 4. The PB component oxidizes at a significantly higher rate than the S A N matrix components. No significant differences in stabilities were evident in the S A N components alone under the test conditions. Thus, stability measurements on the P B - S A N model system reflect primarily the stability of the rubber component.

12.04

IO.O- km

\

·-

?

2 3ν

?

1 ο

PRESSURE,


Compound 1 shows low solubility in S A N , despite the presence of polar functional groups. Consistent with previous results, decreasing acrylonitrile comonomer content significantly increases additive solubility.

I

ι

40

80

120

TIME, min.

1

160

I

ι

200

1

240

Figure 4. Oxygen absorption at 161 °C of unstabilized blend components. Key: 1, SAN with S/A of 63:37; 2, SAN with S/A of 75:25; 3, SAN with S/A of 80:20; 4, PS; 5, PB.



338


No significant differences between unstabilized P R - S A N blends were detected, as indicated by the negligible induction times obtained at the test conditions selected (see Figure 5). However, in the presence of 0.5 parts of compound 1, induction times increase with increasing acrylonitrile content of the matrix phase (127 min at 100:0; 179.5 min at 80:20; 206.4 min at 75:25; and 241.5 min at 63:37) (see Figure 6). Stability differences determined by D S C correlate with oxygen absorp tion measurements. Thus, time to onset of exotherm using programmed temperature rise or under isothermal conditions increases with increasing acrylonitrile content in the matrix phase (see Table VII). Figure 7 shows that increasing percent antioxidant in the rubber phase (due to differences in additive partitioning caused by changing S / A in the matrix phase) increases time to onset of exotherm. Each sample contained the same total loading of antioxidant.

Summary The partitioning of additives between the rubber and rigid phases of rubbermodified systems was studied by S E M - X E D S . The effects of partitioning on additive concentrations were determined by direct measurements in model systems and assessed indirectly through solubility measurements.

°· ~Γ υ

0

1

1

40

1

1

80

1

1

120

1

T I M E , min

1

160

1

1

1

200

1

240

Figure 5. Oxygen absorption at 161 °C of unstabilized 20% PB blends with various rigid-phase compositions. Key: 1, SAN with SI A of63:37; 2, SAN with S/A of 75:25; 3, SAN with S/A of 80:20; 4, PS.


15.

KULICH & WOLKOWICZ

339



ii.OH

6

·° 1 Ο

I

I

40

1

ι ι ι ι ι ι ι ι ι

80

120

I

I

I

I

I

I

I

1 I

I

160 200 240 280 320 B60 400 4 4 0

I

1 I

480

TIME, min. Figure 6. Oxygen absorption at 161 °C of stabilized 20% PB blends with various rigid-phase compositions. Key: 1, SAN with SI A of 63:37; 2, SAN with SI A of 75:25; 3, SAN with SI A of 80:20; 4, PS.

Table VII. Effect of Matrix S/A on Stability by DSC Matrix, SI A

Isothermal DSC, min

0

Dynamic DSC, °C

13

209

100:0

5.4

80:20

6.3

208.5

75:25

9.9

217

63:37

17.3

226

"At 190 °C. At 10 °C/min. fc

Significant differences in additive concentration exist in each phase, de pending upon the composition of the additive and that of the polymeric components. Solubility measurements correlate with partitioning experi ments. The behavior of various thiodipropionates, phosphites, and a phenolic antioxidant is described. In general, with PB-modified styrenics, increasing alkyl substitution in the additive strongly favors partitioning of the additive into the PB phase. Increasing acrylonitrile content in the sty renie portion also significantly promotes the concentration of these additives into the PB component. Oxidative stability differences correlate with partitioning be havior in model systems.


RUBBER-TOUGHENED

340

PLASTICS

230-

225H ) (63/37 S/A) Ο

ο

£220


IAJ

(75/25 S/A)

yj • / Ί 8 0 / 2 0 S/A)

2I0H

(100/0 S/A) 50

T

T

60

70

1

!

80

90

100

WEIGHT % ANTIOXIDANT

Figure 7. Effect of weight percent antioxidant in the PB phase of 20% PB blends with various rigid-phase compositions on temperature to exotherm in air by DSC. Each blend contains 0.5 parts total antioxidant loading.

Acknowledgment The authors thank the technical staff of G E Plastics for their support.

References 1. Shimada, J.; Kabuki, K.; Ando, M. Rev. Electr. Commun. Lab. 1972, 20, 564.

2. Gesner, B. D. J. Appl. Polym. Sci. 1965, 9, 3701. 3. Kelleher, P. G. J. Appl. Polym. Sci. 1966, 10, 843. 4. Wolkowicz, M. D.; Gaggar, S. K . Polym. Eng. Sci. 1981, 21, 571.

5. Developments in Polymer Stabilization—1; Scott, G., Ed.; Applied Science: London, 1979; Chapter 9.

6. Ghaemy, M.; Scott, G . Polym. Degrad. Stab. 1981, 3, 233. 7. Roe,R.;Blair, H . E.; Gieniewski, C. J. Appl. Polym. Sci. 1974, 18, 843. 8. Billingham, N. C.; Calvert, P. D.; Manke, A . S. J. Appl. Polym. Sci. 1981, 26,

3543. 9. Klein, J.; Briscoe, B . J . Polymer 1976, 17, 481.

10. Kwei, T. K . ; Zupko, H . M. J. Polym. Sci. Part A2 1969, 7, 867.


15.



11. Cicchetti, O.; D y b i n i , M.; Parrini, P.; Vicario, G . P.; Bua, E. Eur. 1968, 4, 419. 12. 13. 14. 15.


16.

341

Polym. J.

Stabilization and Degradation of Polymers; Allara, D . L.; Hawkins, W. L.; E d s . ; Advances in Chemistry 169; American Chemical Society: Washington, D C , 1978. Wozny, J. In Polymer Additives; Kresta, J.; Ed.; Polymer Science and E n g i neering, Vol. 26; Plenum: New York, 1984; pp 111-126. Polymer Handbook, 2nd e d . ; Brandrup, J.; Immergut, Ε. H., E d s . ; Wiley: New York, 1975; pp IV-339. Krause, S. In Polymer-Polymer Compatibility in Polymer Blends; Paul, D.; Newman, S., Eds.; Academic: New York, 1978; Vol. 1, Chapter 2. Van Krelvelen, D . V.; Hoftyzer, P. J. Properties of Polymers, 2nd e d . ; Elsevier: New York, 1976; Chapter 4.

R E C E I V E D for review February 11, 1988. A C C E P T E D revised manuscript September 15, 1988.


Rubber-Toughened Plastics - American Chemical Society

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