Substrate Particle Size in ABS Graft Polymers

22 Substrate Particle Size in ABS Graft Polymers CHARLES F. PARSONS and EDMOND L. SUCK, JR. Development Division, Chemicals and Plastics Group, Borg-Warner Corp., Washington, W. Va. 26181

Homogeneous particle size latices were prepared as substrates for ABS graft copolymers. Phase composition studies determined that the number of graft chains per unit area of substrate surface was essentially constant over the range of particle sizes and substrate concentration in the graft reaction. The average distance between graft chains was 51 ±9 A. Notched Izod impact strength increased with increasing particle size when measured at a substrate level of 20 and 30 wt %.

T^Vispersion of rubbery polymers i n glassy resins produces materials which combine the useful properties of high impact resistance and high modulus. The early ideas of Merz, Claver, and Baer (4) have been regarded b y most workers as the first attempt to explain the impact resistance of rubber-rigid resin composites. Bragaw has proposed that rubber particles i n rigid polymers greatly increase the energy absorbing volume by causing cracks and/or crazes to branch dynamically at rubber sites ( I ) . H e also suggests that an optimum balance of impact strength and stiffness derives from regular particle size rubber particles. F o r A B S , this size is predicted to be greater than 5000 A . Although the experimental evidence presented b y Bragaw is consistent with both predictions, A B S resins have been prepared using substrate particles with a mean diameter considerably less than 5000 A and impact strengths greater than predicted by theory. These A B S resins were prepared b y grafting styrene-acrylonitrile ( S A N ) to a diene rubber latex. This paper reports results of a study of impact reinforcement by four grafted rubber substrates which had a narrow distribution of latex particle size. 340

22.

PARSONS AND SUCK, JR.

Substrate Particle Size

341

A discussion of structure-to-properties relationships of graft A B S polymers by Frazer (3) emphasized the influence of substrate structure. A t that time it was recognized that the average particle size of the substrate latex influenced the number of grafted chains and that relationships could be obtained between substrate particle size and graft structure with the mechanical properties of the graft polymer. Identical impact strengths were obtained for A B S graft polymers prepared from substrates of different average particle size when different conditions were imposed to alter grafting favorably. B y making certain changes i n grafting conditions (e.g., type of initiator) the average substrate particle size w i l l not only influence impact resistance but also tensile strength, modulus, orientation, photoxidation resistance, creep* clarity, heat distortion, and flexural modulus. As pointed out by Frazer, many workers have noted the interaction among particle size, grafting, and impact strength, but these findings have never been collated adequately. Impact strength of graft A B S polymers can be shown to increase with an increase in substrate level, or, depending upon the level of grafting, impact strength may increase with increased substrate particle population at constant substrate level. In other words, impact strength increases with increasing interfacial area. Dinges and Schuster (2) stated that the probability of a thrust of energy or a fracture crack hitting a substrate particle and the dependent notch impact strength should increase with the decreasing particle size of the grafted particles. Their initial experimental data showed exactly the opposite effect: notched Izod impact strength decreased with decreasing particle size i n their particular A B S systems. Reasoning that their model must be i n complete, they investigated grafting variations and demonstrated an i n teraction of particle size and graft structure on impact strength which permitted them to conclude that their original model and mechanism was correct. However, to have the same quantity of substrate in the path of the fracture crack, the total level of the small particle size substrate had to be increased much higher than the large particle size substrate. In our opinion, the increase of rubber content of the small particle size substrate is misleading. For our work we measured Izod impact strength of A B S resin blends at two levels (20 and 3 0 % ) of substrate and four substrate particle sizes. Graft structure was maintained so that the molecular weights of the graft and free copolymer were reasonably constant, and the area per grafted chain on the surface of the substrate particle was relatively equal regardless of substrate particle size. Consequently, at this particular graft level, impact strength should depend on substrate particle size and rubber level.

342

MULTICOMPONENT POLYMER SYSTEMS

Experimental Diene substrate latices were prepared by seeding techniques following the scheme shown i n Figure 1. Persulfate anion was the initiator, and an alkyl aryl sulfonate was the emulsifier. Reaction time varied between 13 and 33 hours. Since all the substrates were prepared b y similar techniques, the volumetric swell index and gel gravimetric data (toluene) varied little with particle size. Small differences i n swell and gel characteristics should not be a variable i n this study because of the polarity of the grafting monomer mixture and rapidity of the graft reactions. Figures 2, 3, and 4 are electron photomicrographs of three of the substrate latices. Because of softening of the particles, it was difficult to obtain more representative pictures. The effects of the double seed can be observed in Figure 2. Graft A B S polymers were prepared b y the reaction of an approximate azeotropic mole ratio of styrene-acrylonitrile monomer mixture i n the presence of different weights of diene substrate latex. Initiator, chain transfer agent, and soap levels were constant. Grafting reactions were LATEX A ICLE SMAL^L ATEX

KV_ I A T T V A K)% LATEX A 10% LATEX B

•

LATEX fi SEED 10% LATEX A

LATEX D SEED 2 0 % LATEX A

LATEX E SEED 3 0 % LATEX D

,

£ED 4 0 % LATEX A K

X

)

A

T

E

X

1450 ^ £ T E X

1950 ^ T E X

2950^LATEX

4 0 % LATEX C

2 0 % LATEX B

2 0 % LATEX E

Figure 1.

Preparation of substrate latices

22.

PARSONS A N D SUCK, J R .


Figure 2.

1450 A substrate

Figure 3.

1950 A substrate

343

344

MULTICOMPONENT

Figure 4. STYRENE ACRYLONITWLE MONOMERS PER

POLYMER SYSTEMS

2950 A substrate

WW.

CENT

2 0 — * 80

70

+

+

30-

60

40-

50

5 0 — *

40

6 0 — *

GRAFT REACTIONS

DILUTION

MEIICHANICAL TESTS

SUB^%ATE

Figure 5.

Preparation of graft polymers

carried out b y standard techniques of polymerization i n the presence of substrate latex. Monomers, substrate latex, redox initiator system, etc. were charged to the reaction vessel. After a sufficient reaction time at 60 °C, a portion of the graft latex was removed and treated with an antioxidant system. The latex was coagulated with calcium chloride solution. Resin powders were dried i n an air-circulating oven at 50° to 60 °C for approximately 24 hours. Samples for A B S phase composition studies

22.


345


were dried an additional 16 hours at reduced pressure. Procedures described by Moore et al. (5) were followed for phase separation and structure analysis. The graft polymers were diluted with an S A N copolymer to 20 and 30 wt % substrate. The dry blends were mixed on a two-roll m i l l for a maximum of 5 minutes ( 1 6 5 ° - 1 7 5 ° C ) without added lubricants. Compression-molded samples from the milled slabs were evaluated for impact and tensile strength. Figure 5 is a schematization of this procedure. Additional electron photomicrographic efforts have indicated no aggregation of substrate particles during grafting. Grafting apparently permits the substrate particles to retain their dimensions during milling and molding. Discussion Phase separation of the resins from for determining molecular weight and copolymer. The S A N number average Table I were obtained from viscosity

the graft reactions gave material the weight percent of the graft molecular weights contained i n measurements on the separated

Table I. Number Average Molecular Weight of Styrene—Acrylonitrile Copolymer, X 10" 5

Substrate in Graft Reaction, Wt. % 20 30 40 50 60 Table II.

Particle Size, A 1100

1.10

1450

1950

2950

1.10 1.20 1.10 1.15

1.00 0.93 1.20 0.76 1.10

1.20 2.00 1.15 1.47 1.20

Weight Percent of Grafted Styrene—Acrylonitrile Copolymer

Substrate in Graft Reaction, Wt. % 20 30 40 50 60


1450

1950

2950

22.7 21.8

11.2 13.9 13.7 16.3

5.6 8.2 5.7 12.7 14.3

1.9 6.0 5.0 10.6 11.9

solubilized copolymer and previously prepared correlation curves. The equality of the molecular weight of the S A N in the graft phase and i n the free solubilized copolymer has been shown by Dinges and Schuster and assumed in our study. The weight percent of S A N copolymer grafted to the diene substrate during each graft reaction is given in Table II.

346


Usually the ratio of the weight of grafted copolymer to the weight of diene substrate is calculated for each graft reaction. Variations of grafting w i l l change this ratio and permit the tailoring of polymer systems. Variation of this ratio in these experiments was less than ± 0 . 0 5 for each particle size and decreased as particle size increased. It was also evident (Table II) that the amount of copolymer grafted decreased as the particle size increased at any one particular substrate level. This is a further indication that grafting is proportional to available substrate surface area. T o prove that uniform grafting had occurred, the relative substrate surface area per grafted copolymer chain was calculated for each graft reaction. The moles of grafted copolymer, Avogadros number, the particles' diameter, and the weight and density of the diene substrate are the basis for the calculations. The density is important since actual surface area per grafted copolymer chain w i l l depend on the number of particles per gram of rubber which depends on the estimation of substrate particle density. Accuracy of measuring the particle size and monomer swelling of the particle during the graft reaction may affect the density. W e used 0.9 gram/cc for these calculations. Table III shows the range of areas Table III.

Average Substrate Surface Area Per Graft Chain, A X 10" 2

, . . . „ substrate in Graft Reaction, Wt. % 20 30 40 50 60 0

fA

Table IV.

2950 4.74 3.74 3.44 2.59 2.26

Average Distance Between Graft Chains, A

Substrate in Graft Reaction, Wt. % 20 30 40 50 60

1100 — — — 2.41 —

Particle Size, A : 1450 1950 — 2.02 2.25 1.93 2.63 4.77 3.05 1.70 3.23 2.62

3


1450

1950

2950

— —

47 51 55 57

—

45 44 69 41 51

69 61 59 51 48

49

—

per chain as determined from the molecular weight and weight of graft copolymer given in Tables I and II. The square root of these areas or the average distance between chains for each graft reaction is given i n Table IV. Of the examples presented, three sets of experiments definitely are outside the regular order of data. T w o are attributed to the low weight of grafted copolymer (2950 A particle size at 20 wt % substrate

22.


347


in the graft reaction and 1950 A particle size at 40 wt % substrate i n the graft reaction), the third graft polymer had a much lower molecular weight graft chain than any of the other graft reactions (1950 A particle size at 50 wt % substrate i n the graft reaction). The first two had a mean distance of 69 A between graft chains while the remainder average 51 ± 9 A . This value approaches the limits of experimental accuracy at the present state of technique and equipment. W i t h i n limits of experimental measurements of impact and tensile strength at two substrate levels (Tables V and VI), this variation i n interchain distance is too small to affect significantly impact or tensile strength when the number average molecular weight of the grafted chain is approximately 100,000. The present study was not undertaken to determine any preferred molecular weight, distance per graft chain, optimum impact strength, or combiTable V .

Notched Izod Impact Strength (ft-lbs/inch) of Polyblends at 20 and 30% Substrate Levels Particle Size, A

Substrate in Graft Reaction, Wt. %

20 30 40 50 60 30 40 50 60

3.0 2.4 2.4 2.4

3.3 3.2 2.8 2.2 2.8

4.1 4.6 4.2 4.0 5.4

30% Substrate Level — 6.8 — 6.5 4.5 6.0 5.3 6.3

6.2 6.5 6.7 6.1

7.5 7.5 7.0 7.5

—

— — 1.8 1.4

30 40 50 60

Substrate in Graft Reaction, Wt. %

2950

1450

20% Substrate Level

20 30 40 50 60

Table VI.

1950

1100

'

—

nsile Strengths (psig) of Polyblends Particle Size, A 1950

2950

6075 6300 6325 6400

6200 6225 6350 6500 6325

6400 6300 6225 6325 5975

30% Substrate Level — 4375 — 4450 5000 4400 4725 4325

4500 4600 4325 4550

4525 4575 4600 4400

1100

1450

20% Substrate Level

—

— — 6800 6600

—

348


nations of specific properties. Both smaller and larger areas per grafted chain and different molecular weights of grafted copolymer can be achieved. Additional effort is required to determine if grafting can be controlled within even narrower limits over a similar range of particle sizes and a wider range of molecular weights. It is our belief that Table III or I V should be the preferred method to describe graft structures when molecular weight and particle size data are available. Impact Strength. Figure 6 shows the dependence of the average Izod impact strength on substrate particle size at 20 and 30 wt % substrate of the polyblends. A t the 20 wt % rubber substrate level the extra-

1000

2000

3000

Portide Diameter, Angstroms

Figure 6. 'Notched Izod impact strength vs. particle diameter

22.

349



12.0

•

•

1,

1

*

, II 0

2 3

5 7 ,

0

I 2

2 3

5

7

^3

2 3

5

7

^

2 3

57,QI5

Number of Substrate Particles per Gram of Blend •20

Weight per cent substrate,

•

3 0 Weight per cent substrate

O and • Calculated number of 5 0 0 0 A particles.

Figure 7.

Notched Izod impact strength vs. number of substrate particles per gram of blended resin

polated intercept of 0.4 ft-lb/inch is the equivalent of the impact strength of unmodified styrene-acrylonitrile copolymer. At 30 wt % of rubber substrate the increase of notched Izod impact strength with increasing substrate particle size is less linear at these individual particle sizes. Extrapolation of the 30% impact values to the copolymer impact value requires a substantial increase in impact strength when substrate particles are less than 1000 A in diameter. The shape of the impact-particle size curve in this region is subject to conjecture and may depend heavily on graft structure. For a prediction of the impact strength at a particle diameter of 5000 A (Bragaws hypothesis) at this level of grafting and molecular weight, impact strength was plotted vs. the number of substrate particles in the polyblend. Figure 7 is a semilogarithmic plot of this data. Notched Izod impact strengths of 9.5 and 11.0 ft-lb/inch for 20 and 30 wt % substrate are predicted. The level of grafting (area or molecular weight per grafted chain) could change the location or slope of these lines. As for lower rubber substrate levels, both Figures 6 and 7 suggest that large particle size substrates are preferred. Tensile Strengths. Table V I contains the tensile strengths of the polyblends corresponding to the impact values of Table V . Fluctuations are small since the weight ratio of grafted copolymer to substrate is reasonably constant. Tensile strengths of the 1100 A substrate polyblends fc

350


were higher than any of the other three particle sizes. These observations are consistent with the inverse relationship of impact and tensile strength. Conclusions Impact strength of A B S graft resins w i l l increase with increasing particle size of the substrate latex when the substrate surface area per grafted copolymer chain is maintained at equivalent values. A t 20 w t % substrate levels the increase i n impact strength appears linear with i n creasing particle size, but at 30 wt % substrate, nonlinearity is indicated wjien the substrate particle is less than 1100 A . The inverse relationship of tensile and impact strength is preserved. Acknowledgments W e are pleased to recognize the technical assistance of D . O . Conley. Helpful discussions were held with W . J. Frazer, V . E . Malpass, P. J. Fenelon, P. L . Wineman, and E . Baer. W e thank E . Lanterman and M . Draginis of the Roy C . Ingersoll Research Center, Borg-Warner Corp. for the electron photomicrographs. Literature Cited (1) Bragaw, C. E., ADVAN. C H E M . SER. 99, 86 (1971).

(2) (3) (4) (5)

Dinges, E., Schuster, H.,Makromol.Chem. 101, 200 (1967). Frazer, W. J., Chem. Ind. 1399 (1966). Merz, E. H., Claver, G. C., Baer, M.,J.Polymer Sci. 22, 325 (1956). Moore, L. D., Moyer, W. W., Frazer, W. J., Appl. Polymer Symp. 7, 67 (1968).

RECEIVED June 17, 1970.

Substrate Particle Size in ABS Graft Polymers

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