Polymer Durability - American Chemical Society

31

Weathering of Acrylonitrile-

Butadiene-Styrene Plastics:

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Compositional Effects on Impact and

Color

D. M. Kulich and S. K. Gaggar GE Plastics, Technology Center, Washington, WV 26181

The effects of outdoor aging on surface embrittlement were determined polymers

by using model ABS

systematically

(styrene-acrylonitrile)

varying

in rubber

level, grafting,

SAN

puncture

test was used to provide greater sensitivity than that typically

obtained

pendulum

composition.

and

A high-speed

with routine

copolymer

and color changes

(acrylonitrile-butadiene-styrene)

and falling-dart

type measurements.

Color

readings were also obtained to identify the color changes occurring the time period

to surface embrittlement.

phase composition

were shown to be key factors

face embrittlement,

time period to surface embrittlement

are also

type and

in

SAN

affecting time to sur-

whereas grafting and rubber level had much less

or no significant effect. The primary

effects of ABS

Elastomer

composition

color changes occurring

in the

were bleaching and fading.

The

on yellowing with and without added

TiO

2

described.

JALCRYLONITRILE-BUTADIENE-STYRENE tant class o f m u l t i p h a s e p o l y m e r b l e n d s

(ABS)polymers comprise an i m p o r that contain a discrete,

particulate

elastomeric phase dispersed i n a thermoplastic matrix. I n A B S the b l e n d c o m ponents

consist

o f polybutadiene

( B R ) or butadiene

copolymer

particles

grafted w i t h a c o p o l y m e r o f styrene a n d acrylonitrile a n d dispersed i n a matrix of

styrene a n d acrylonitrile c o p o l y m e r ( S A N ) . A B S provides a favorable

bal

ance o f properties that includes h i g h i m p a c t strength, ease o f processing, a n d g o o d d i m e n s i o n a l stability. V a r i o u s grades o f A B S

are offered to m e e t specific

0065-2393/96/0249-0483$12.00/0 © 1996 A m e r i c a n C h e m i c a l Society

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


484

POLYMER DURABILITY

end-user requirements by adjusting the relative proportions of rubber, graft ing, and matrix composition (J). ABS is susceptible to photodegradation resulting in discoloration and loss of toughness. Applications involving outdoor exposure require protective measures such as the use of light stabilizers, pigments, or protective coatings (2-9). Light aging of A B S results i n degradation of the B R phase and a cor responding embrittlement of the surface layer (2, 10-19) by oxidative crosshnking of the rubber particles and graft scission (15). Mechanisms describing the photooxidation of A B S were proposed (20-25). Prior thermal processing can introduce polymer hydroperoxides that can act as catalysts for photodegradation (10). Oxidation studies with singlet oxygen have also shown that initial attack on A B S involves oxidation of t i e B R phase (23). Studies using laminates of brittle polymer films on A B S to simulate environmental surface embrittlement demonstrated that at a critical brittle layer thickness a surface crack is able to propagate across the layer-core interface (26). Weathering studies were conducted by using both outdoor (11-13, 15-17, 24, 27-30) and artificial aging methods (2,11,15,17, 21, 22, 28, 29); the effects of wavelength were recognized (11, 20-22, 25, 31, 32). Photooxidative degradation of the B R phase (vis-à-vis IR degradation) is primarily initiated by wavelengths be low 350 nm (24, 25). Wavelength sensitivity studies have shown that photo chemical yellowing is caused by wavelengths between 300 and 380 nm, and maximum bleaching of yellow-colored species occurs at wavelengths in the 475^85 nm region (31). Test methods previously used to determine the effects of fight aging on embrittlement of ABS include Izod impact (13, 29, 33-35), Charpy impact (3, 14, 29), flexural tests (3, 14, 27, 28), falling weight (15, 19), dynastat (24), and dynamic mechanical measurements (Rheovibron) (19). Because photodegra dation occurs essentially only on the exposed surface and the interior of the sample remains essentially unaffected, a routine pendulum type of notched impact test will not be sensitive to changes in surface embrittlement. Falling dart types of testing, although more sensitive to the surface, still do not have adequate sensitivity, and a large number of samples is required. In this study, a high-speed puncture test was used to determine the effects of outdoor exposure on crack-initiation energy values by using model rubbermodified S A N copolymers systematically varying in structure. Structural var iations included rubber type, rubber level, grafting level, and S A N composition. The samples studied were not fight stabilized, and comparisons were made by using unpigmented and pigmented (4.0 pph T i 0 ) samples. Correlations are also drawn with associated weather-induced color changes. Obtaining color measurements on samples well characterized for surface em brittlement permits an identification of the types of color changes occurring in the time frame during which surface embrittlement occurs. Although dis coloration phenomenon were described previously (15, 21, 31), many of the observations may relate to exposures well beyond the point required for sur2


31.

KULICH & GAGGAR

Weathering of ABS Plastics

485

face embrittlement or were determined using artificial light sources raising concerns of relevance to natural weathering conditions.

Experimental Laboratory prepared ABS samples were used to provide better control over compositional variations. Elastomers grafted with SAN were prepared in an emul sion process (I) and compounded using a BR (Banbury) with SANs having various styrene-to-acrylonitrile ratios or with a SAN at different rubber levels. Pigmented samples contained 4 pph rutile T i 0 . After compounding, samples were prepared for fight aging by injection molding into 75 X 126 X 3.2-mm plaques by using an 8-oz injection molding machine at 260 °C stock temperature. Samples for outdoor aging were mounted on south-facing racks inclined at 45° to the horizon located at a site adjacent to the Technology Center in Wash ington, WV; sample exposures were in the month of July. Color readings were on a Macbeth 1500+, specular component included, illuminant D65, 10° observer. Color changes were recorded [1976 Commission International de l'Eclairage (CIE) L*a*b*] as L * (lightness-darkness axis in color space with lower L * indicating a darker sample), a* (red-green axis with a larger value indicating a more red sample), and b* (yellow-blue axis with a larger value indicating a more yellow sample) values by using the Macbeth spectrophotometer. Puncture impact testing on aged and unaged (control) samples was performed on a Material Test System high-rate tester at a crosshead speed of about 0.21 m/s (500 in./min.). The test specimen was positioned such that the punch with a 0.635cm (0.25-in.) diameter hemispherical tip impacted the sample on the unexposed surface and caused tensile failure of the aged surface (Figure 1). The punch pen etration depth in the specimen was controlled to determine the crack initiation energy values by recording the load-displacement curve on a Nicolet oscilloscope. The energy values (divide in lb/in. by 22.4 to get J/cm) were calculated by meas uring the area under the load-displacement curve and normalizing for the sample thickness. At each punch-penetration depth, the specimen surface was visually


2

Figure 1. Puncture impact apparatus.


POLYMER DURABILITY


486

examined for the appearance of any surface cracks, and the punch depth was adjusted accordingly for the next impact on an untested portion of the specimen surface until the minimum depth for crack initiation was identified. A maximum of four impact points on each test specimen was used to establish the crack ini tiation energy value. The total fracture energy values were also established for the samples by driving the punch clear through the specimens and recording the loaddisplacement curves corresponding to total fracture. A typical load-displacement curve to total fracture is shown in Figure 2 for unaged. and aged samples. The area under the entire load-displacement curve normalized for the specimen thick ness is defined as the total fracture energy. Initial energy values can be reproduced within 5% of reported averages. Measurement accuracy will decrease as the meas ured values decrease due to aging; as energy values approach 50 units, differen tiation becomes difficult.

Results and Discussion Figure 3 shows the effect of exposure time on crack initiation energy values for rubber modified S A N copolymers differing in elastomer type as follows: polybutadiene in A B S , E P D M (ethylene-propylene-diene monomer) rubber in A E S (acrylonitrile-EPDM-styrene), and polybutylacrylate rubber in

37.0

φ ο Ο

25.0

u. 12.5

0.2

0.4

0.6

Deformation

Figure 2. Effect of outdoor aging on the load-displacement curve to total fracture. The total fracture energy value (lb-in./in.) is determined from the area under the curve normalized for sample thickness.



31.

KULICH & GAGGAR

0


200

400

487

600

800

Figure 3. Effect of outdoor exposure time (hours) on crack initiation energy valu (lb-in./in.) for unpigmented samples varying in elastomer type: ABS; ·, AES; A, ASA. ASA (acrylate-styrene-acrylonitrile). In each case, the sample was prepared by using the same S A N as the continuous phase. Note that embrittlement occurred in all samples, and stability increased in the order: A B S < A E S < A S A . After 400 h, all samples showed a decrease to very low impact values. The reduced stability of ABS is consistent with the increased lability of polybutadiene toward oxidation. Rather interestingly, in the absence of light stabilizers, even A E S and A S A showed relatively rapid surface embritdement ensuing after only 200 h of outdoor exposure for A E S and 400 h for ASA. The bulk-failure mode (the energy required to punch through the sample) is shown in Figure 4. Comparison of the percent energy change in Figure 3 with Figure 4 illustrates the greater sensitivity obtained through crack-initia tion energy value measurements. The results in Figure 4 indicate that impact was reduced to almost a constant value and that a constant brittle layer thick ness was formed due to light screening and a reduction in oxygen permeability by the oxidized layer. Because brittle failure does not occur in the bulk-failure mode, the brittle layer formed is not sufficiendy thick for a crack to propagate through the brittle layer-core interface. Previous studies on the thermal oxi dation of ABS indicated that the critical layer thickness at which surface cracks will propagate through the sample is between 0.07 and 0.2 mm for impact tests at ambient temperatures (35). The critical layer thickness will vary with the impact test temperature and test rate.


488

POLYMER DURABILITY


1100

0

200

400

600

800

Hours

Figure 4. Effect of outdoor exposure time (hours) on total energy value (lb-in. in.) for unpigmented samples varying in elastomer type: ABS; ·, AES; Δ , ASA. The effect of pigmentation with T i 0 on impact retention is shown in Figure 5. Note that the addition of T i 0 has a stabihzing effect, but the same relative ranking of elastomer type is obtained. The protective effect by pig mentation is consistent with the ability of rutile T i 0 to strongly absorb U V below 400 nm (36). Figures 6 and 7 illustrate the effects of rubber level for pigmented and unpigmented A B S , respectively. Although initial energy values are significantly affected, complete brittle failure occurs after 72 h. Similarly, varying of the extent of grafting (Figures 8 and 9) primarily influences initial toughness. In sharp contrast to the effects of grafting and rubber level, S A N com position does not significantly affect initial energy values but does significantly affect time to embritdement. The effect was consistently seen both in the unpigmented and pigmented samples (Figures 10 and 11). We proposed that varying the percent acrylonitrile in S A N affects oxygen permeability and thereby affects time to embrittlement. The effect of S A N composition on oxygen permeability in S A N is shown in Figure 12 (37). The addition of T i 0 is not expected to have a significant effect on permeability. The effect on permeability due to simply an increase in path length with added filler can be calculated from an expression by Nielson (38) and is estimated to be »Φ c

LU

C Ο

Ο CO Xm

ο

200

Figure 9. Effect of outdoor exposure (hours) on crack-initiation energy value in./in.) for unpigmented ABS samples at constant rubber hut differing in gra rubber ratio: •, graft/rubber = 0.43; · , graft/rubber = 0.50


492

POLYMER DURABILITY


1200

0

100

200

300

400

Hours

Figure 10. Effect of outdoor exposure (hours) on crack-initiation energy val (lb-in./ίη.) for ABS samples pigmented with Ti0 and differing in SAN composition: •, 28% (w/w) acrylonitrile; · , 36% (w/w) acrylo 2

1200

0

50

100

150

200

Hours

Figure 11. Effect of outdoor exposure (hours) on crack-initiation energy val (lh-in./in.) for unpigmented ABS samples differing in SAN composition: Ώ, 28% (w/w) acrylonitrile; · , 36% (w/w) acrylonitrile.


31.

KuLiCH

& GAGGAR


493


5 Τ

2+

τ -τ

·

20

1

1

1

1

25

30

35

40

% AN

Figure 12. Effect of SAN composition (percent acrylonitrile [AN], w/w) on oxyg permeability (cm · cm)/(cm · s · cm Eg) X 10 at 25 °C. Data from reference 37. 3

0

2

200

n

400

600

800

Hours

Figure 13. Effect of outdoor exposure (hours) on the yellowing (b* value) unpigmented samples differing in elastomer type: •, ABS; ·, AE


POLYMER DURABILITY

494

Φ

D CO Downloaded by UNIV OF PITTSBURGH on October 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch031

>

800

Figure 14. Effect of outdoor exposure (hours) on the darkening (L* value) o unpigmented samples differing in elastomer type: •, ABS; ·, AE

φ 3

CO

>

-1.5

800

Figure 15. Effect of outdoor exposure (hours) on the red-green color shift value) of unpigmented samples differing in elastomer type: •, ABS; ·, ASA.


KuLiCH


31.


& GAGGAR

495

creasing B R stability by increasing acrylonitrile content (lower permeability) should have a small effect on overall discoloration. This result was observed (Figure 16). A comparison of Figure 3 with Figure 13 shows that impact loss and initial bleaching and fading were independent phenomenon; the time scale for bleaching and fading was relatively unaffected by rubber type and stability. Yellowing that ensued subsequent to initial bleaching and fading does appear to correlate with the onset of impact loss. However, surface embrittlement precedes the point at which yellowness surpasses initial unaged values. A comparison of the weathering-induced discoloration of the components of A B S (Figure 17) confirms that the S A N phase alone significantly fades and the B R phase alone yellows. As determined by Fourier transform IR (FTIR), B R films underwent photooxidation within the time frame studied; hence, some discoloration is not unexpected. Because the percent B R in the A B S samples was 25%, the contribution of B R i n A B S to yellowing will be pro portionately reduced. The A B S sample after 72 h showed evidence of — O H formation at 3300 c m , carbonyl formation at 1640-1720 c m , ether for mation at 1050-1200 c m " , and loss of trans-butadiene absorption at 960 c m . A determination of the subambient glass-transition temperature via dif ferential scanning calorimetry analysis of an A B S sample aged 400 h shows complete loss of the polybutadiene transition to a depth of at least 2 mils. G e l Permeation chromatography (GPC) area data also indicate the presence of more of the soluble polymer fraction after aging (more ungrafted SAN) and suggest degrafting may occur. Conversely, F T I R analysis indicated no change in the composition of the S A N sample after 240 h of exposure. F o r S A N alone, G P C analysis showed no change in molecular weight after 400 h, even in the top 1-mil layer. - 1

- 1

1

- 1

With T i 0 pigmented samples, rapid initial fading (decrease in b * value) was observed with all samples as with unpigmented samples, and the magni tude of fading showed little dependence on elastomer type (Figure 18). Pig mented S A N alone similarly underwent fading (Figure 19). The sources of yellow chromophores in S A N include conjugated species derived from se quences of adjacent acrylonitrile units as described by Grassie (41). More fading was observed with S A N versus polystyrene (PS), and this result is con sistent with greater initial yellowness of S A N versus PS (PS, unaged b * value = 3.0; S A N , unaged b * value = 4.7). However, subsequent discoloration of pigmented ABS was considerably more pronounced relative to A E S and ASA. The result is unexpected based on the screening of wavelengths

Polymer Durability - American Chemical Society

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