Toughness and Brittleness of Plastics

22 Transparent, Impact-Resistant, Styrene/ Methyl Methacrylate Copolymer Grafted Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch022

onto Polybutadiene BERNARD BAUM, WILLIAM H. HOLLEY, HAROLD STISKIN, ROY A. WHITE, and PAUL B. WILLIS DeBell & Richardson, Inc., Enfield, Conn. 06082 ANTHONY F. WILDE Army Materials and Mechanics Research Center, Watertown, Mass. 02172

A series of impact- and ballistic-resistant, rubber-modified, transparent polymers based on styrene/methyl methacrylate copolymer grafted onto polybutadiene were developed for Army Materials and Mechanics Research Center (AMMRC). The refractice index of the copolymer matched that of polybutadiene. Variables that were investigated included the particle size of the polybutadiene latex, the degree of grafting, the particle content, and the degree of dispersion of the graft constituent. The highest ballistic impact was for a graft polymer based on 300 A rubber (polybutadiene) latex containing 15% rubber and a high degree of grafting and dispersion.

T * h e objective of this research program was to produce a series of transparent, rubber-modified polymers designed for investigating ballistic resistance and possessing good optical properties over as maximum a temperature range as possible. A M M R C had previously specified the types and general ranges of the rubber particle variations to be investigated. Based on laboratory impact testing we determined what clear thermoplastics to use, even though ballistic resistance cannot be correlated to standard impact testing. The size, shape, and speed of a projectile and the projectiles effect on ballistic resistance of a thermoplastic are factors that cannot be accurately predicted.

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263 Deanin and Crugnola; Toughness and Brittleness of Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

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TOUGHNESS A N D BRITTLENESS OF PLASTICS

We used a matrix copolymer system consisting of methyl methacrylate (MMA) and styrene (St) grafted on polybutadiene rubber. The variables investigated were latex particle size ( 3 6 0 , 2 0 0 0 , and 5 0 0 0 A ) , degree of grafting, rubber content, and the degree of particle dispersion. The following variables must be considered when a transparent impact polymer is prepared.

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Impact Properties. C H E M I C A L N A T U R E O F T H E R U B B E R . If the rubber

is too compatible with the matrix, it will dissolve in the rigid material and disperse on a molecular scale. Little or no reinforcement will occur since the rubber particles become smaller than the radius of the tip of a stress-induced propagating crack. However if it is highly incompatible, good adhesion between rubber and matrix cannot be obtained. For example polybutadiene rubber adheres poorly to a styrene/acrylonitrile copolymer, but a nitrile rubber adheres well to the SAN copolymer. If grafting techniques are used however, compatibility is less of a problem since the rubber is chemically bonded to the matrix. ENERGY ABSORPTION BY T H E RUBBER.

For the rubber phase to work

effectively it must be able to absorb energy by elongating in the face of a propagating crack and redistribute the stress to the surrounding matrix, thus dissipating the energy that causes the crack to propagate ( I ) . In general a rubber with the lowest possible glass transition temperature is required for good impact ( 2 , 3 ) . As with commercial elastomers the degree of crosslinking in the rubber particles (i.e., the amount of macrogel crosslinked rubber and microgel highly branched chains) greatly affects the strength, toughness, and resiliency of the rubber, with an optimum degree of crosslinking necessary for maximum toughness in the final product. A M O U N T O F R U B B E R . Generally the impact strength of rubber-modified plastics increases with an increase in rubber content. Although the impact improves, it is usually at the sacrifice of other properties such as strength, modulus, heat distortion, weather resistance, and creep. R U B B E R P A R T I C L E SIZE A N D DISPERSION.

Particle size is important for

impact. If the size distribution is wide—i.e., 1 - 2 0 microns—the large particles represent a less efficient use of the toughness of the rubber and tend to reduce tensile strength and to give poorer surface finish when compared with a narrow particle size distribution—i.e., 1-5 microns (4). If the rubber is evenly dispersed throughout the polystyrene matrix, cracks which form under stress soon encounter a rubber particle during propagation through the material; thus further progress of the crack is hindered by the energy absorbing properties of the rubber. The degree of dispersion depends on the degree of grafting, where the graft acts almost as a surface-active agent to keep the rubber in a colloidal dispersion in the matrix. The effectiveness of the graft appears

Deanin and Crugnola; Toughness and Brittleness of Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

22.

BAUM ET AL.

Transparent, Impact-Resistant Copolymer

265

to depend more on the thickness of the graft layer than on the weight percent used. Below a certain thickness of graft on the particles (e.g. 100 A ) , the rubber tends to aggregate badly (5).


A D H E S I O N O F R U B B E R TO T H E M A T R I X . Another important variable for

impact properties is how well the rubber adheres to the plastic continuum (6). Without sufficient adhesion the energy of a propagating crack can tear a rubber particle from the matrix, and the crack will effectively bypass the particle. The most commonly used method to adhere rubber to a matrix is grafting. Grafting has the advantage of using rubbers which are only moderately compatible with the plastic matrix because the chemical bonding achieves adhesion. Grafting can be performed in bulk or suspension, but for better control of rubber particle size it is usually done in an emulsion. The rubber is usually swollen, at least in the outer layer of the particle, with a monomer, and then the monomer is polymerized with free-radical initiators such as peroxides or persulfate. Grafting is achieved by chain transfer of the growing polymer chains to the rubber molecules. For unsaturated rubbers such as polybutadiene grafting probably occurs at allylic carbons or at pendant double bonds. P L A S T I C M A T R I X . T O ensure good impact properties the plastic matrix should have a medium to high molecular weight. On a commercial basis this generally means a melt flow (ASTM D 1238) of one to five or an inherent viscosity (7) greater than one. Optical Properties.

M A T C H E D R E F R A C T I V E INDEXES.

Of all the vari-

ables, the most important to optical properties is the refractive indexes of the continuous and dispersed phases. The effects of semicompatible polyblending on optical properties are generally undesirable. Since each incompatible phase has its own refractive index, light traveling through such a semicompatible polyblend is refracted and dispersed many times as it enters and leaves the dispersed phase in its journey through the continuous phase. Thus most semicompatible polyblends are translucent to opaque. One way to overcome this problem is to match the refractive index of the plastic matrix to that of the rubbers (8, 9). Most rubbers have refractive indexes of ca. 1.52, and this is closely matched by styrene/ methyl methacrylate copolymers. P A R T I C L E SIZE. Another method to produce transparency is to make the particle size of the rubber phase smaller than the wavelength of light (i.e., less than 4000 A ) . Although the technique is effective, these small particle sizes are difficult to achieve on a commercial basis and may have an adverse effect on the impact properties of the final product. C O M P O S I T I O N O F T H E C O N T I N U O U S P H A S E . T O ensure a haze-free plas-

tic, the continuous phase, if a copolymer, must have a consistent composition. If the monomers react in the same ratio as they are added, there


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TOUGHNESS A N D BRITTLENESS O F PLASTICS

is no problem. However if one monomer reacts preferentially to the other and this is not taken into account during monomer addition, the material can gradually change in composition during the polymerization reaction and can result in a mixture of molecules of different comonomer content.


Experimental Laboratory. Methyl methacrylate/styrene copolymer, polybutadiene latex, and a graft combination of the two were synthesized in returnabletype soda bottles. To produce polybutadiene latex, water, surfactant, and initiator were charged to the bottle followed by butadiene (de-inhibited by distilling through aqueous sodium hydroxide) at specific time intervals using a hypodermic syringe to inject the monomer and/or surfactant through a resealable cap. No mercaptan, chain transfer agent, was used with 360 A latex formulations. Larger particle sizes generally required mercaptan to yield a more linear, less crosslinked, more rubbery polymer structure. For some grafting runs, distilled water, surfactant, polybutadiene latex, styrene/MMA monomers, and initiator were all injected into the bottle. The bottle was then purged with nitrogen, sealed, and rotated in a water bath at 60°C for 16 hr. In other grafting runs, preswelling in the absence of initiator was carried out by adding all the ingredients except the initiator to the bottle. The bottle was purged with nitrogen, sealed, and rotated inside the water bath at 50 °C for 2 hr to swell the rubber slightly in the monomer. The bottle was removed from the bath, cooled, and initiator was added. It was then repurged with nitrogen and was resealed and rotated inside a 60 °C bath for 16 hr. After grafting, the latex was coagulated in and washed thoroughly with methanol and was filtered. During washing Irganox 1010, an antioxidant, was added to prevent degradation of the rubber during subsequent drying. The graft polymer was dried in a circulating air oven at 60 °C overnight and was compounded on a steam-heated, two-roll mill at 315 ° F for 10 min with 0.1% Irganox 1010. Films used to evaluate clarity and impact were prepared by compression molding at 350 ° F using a hydraulic press equipped with electrically heated platens. Pilot Plant Scale-Up. All of the pilot plant polymerizations to produce a rubber latex or a graft polymer were carried out in a 10-gal, glasslined Pfaudler kettle. A 50-gal, glass-lined kettle was used to prepare MMA/St copolymer. E D T A was used in all pilot plant runs to sequester iron. The first pilot plant grafting that was run with the commercial 2000 A latex from B. F. Goodrich coagulated. We switched Run 17B formulation with the pentamine/peroxide initiator system to the persulfate initiator system, Run 26 (Table VII). Based on tensile impact data, No. 26 had almost the same impact resistance as No. 17B. For the 5000 A latex, scale-up in the 10-gal, glass-lined kettle required 64 hr polymerization at 60°C. After each addition of butadiene, initiator, surfactant, and/or mercaptan, an additional 16 hrs of polymerization was needed. Four separate additions were needed to grow the 2000 A polybutadiene latex to 5000 A. The final latex had a solids content of 25.3%


BAUM ET AL.

22.

Transparent, Impact-Resistant Copolymer

267


and was calculated to be 80% converted. This latex was used to make the 5000 A graft. The ballistic performance of these materials was determined by a standard projectile impact test. This performance is characterized by the average impact velocity at which the projectile barely penetrates the test specimen. Accordingly the higher the characteristic velocity, the better is the ballistic performance or the ballistic impact resistance of the specimen. The data are presented in arbitrary units. Discussion Graft Polymer Studies on "Model" System. Methyl methacrylate/ styrene ratio, chain transfer agent level, and type of initiator system were evaluated in graft polymer using Firestone 2004 polybutadiene latex as a model. CLARITY OF GRAFT POLYMERS WITH VARYING M E T H Y L METHACRYLATE/

S T Y R E N E RATIOS. Refractive index considerations (7,8) and general state of the art considerations (8) favor methyl methacrylate ( M M A ) over styrene (St) as the major monomer portion of the matrix. With this in mind, we selected M M A/St ratios from 62/38 to 80/20 (wt %). Using a commercial Firestone butadiene latex (2004) to supply the 15% rubber graft, polymerization runs were made as outlined in Table I. After the resultant polymers had been worked up, an evaluation was made based on clarity. At 73° and 140 ° F the 80/20 M M A/St polymer (Run 5) had the best clarity, but at - 4 ° F the 67/33 MMA/St (Run 2) Table I.

Clarity of Graft Polymers with Varying MMA/St Ratios Run No.

Formulation

1

2

3

4

5

Distilled water, total

180.0

180.0

180.0

180.0

180.0

15.0

15.0

15.0

15.0

15.0

52.8 32.2 5.0 0.5 0.3 99.5 5 5 5

56.9 28.1 5.0 0.5 0.3 98.5 4 4 1

59.5 25.5 5.0 0.5 0.3 98.8 3 3 2

63.8 21.2 5.0 0.5 0.3 97.8 2 2 3

68.0 17.0 5.0 0.5 0.3 99.5 1 1 4

62/38

67/33

70/30

75/25

80/20

0

2 0 0 4 butadiene latex, ( 2 5 0 0 A) solids basis

Methyl methacrylate MMA Styrene Sodium stearate Na P0 -12 H 0 K2S2O8 3

4

2

Yield, % Clarity - @ 140°F Clarity @ 73°F Clarity @ - 4 ° F M M A / S t based upon 100 Monomer 1

All formulations are based on 85 parts by weight monomer (MMA/St) and 15 parts by weight rubber solids. Rated from 1-5 (1 is best and 5 is worst). 0

6


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TOUGHNESS A N D BRITTLENESS O F PLASTICS

Table II.

Effect of Level of Chain Transfer Agent on Impact of Graft Polymer Run No.

Formulation

6

6A"

9

10

tert-Dodecyl mercaptan Melt index, g/10 min Tensile impact, ft-lbs/in. Clarity

0.3 1.4 6 sit. haze

0.5

0.7 1.7 7 clear

0.9 4.3 brittle clear


0

2

—

6 clear

° H 2 O - I 8 O parts, butadiene latex-15, MMA-65.5, styrene-19.5, sodium stearate-5, Na P04-12 H2O-O.6, and K S 08-0.3. Pre-swelled for 2 hrs @ 50°C without K S 0 ; K S 0 added and polymerized for 16 hrs. @ 60°C. 3

2

2

b

2

2

8

2

2

8

ratio was superior, and the 80/20 was rated next to the worst. As a compromise the 77/23 MMA/St was chosen for the remaining experiments. EFFECT OF C H A I N TRANSFER A G E N T CONCENTRATION O N I M P A C T OF

G R A F T P O L Y M E R . With the MMA/St ratio at 77/23, 15% rubber grafts were made using Firestone butadiene latex and different levels of the chain transfer agent (terf-dodecyl mercaptan) to lower the molecular weight of the polymer so that it can be readily processed. Our first attempt, without mercaptan, yielded a difficult to process, almost intractable material. At low mercaptan levels the high molecular weight polymer caused haze (Table I I ) . Run 10 with 0.9 part mercaptan lowered the molecular weight to a point where the specimen produced was brittle. We choose 0.5 part mercaptan as the working level for the remainder of the project. Table III.

Effect of Initiator Systems on Impact of Graft Polymers Run No.

Formulation

6A

13

Distilled water, total 2004 butadiene latex, solids basis MMA Styrene Sodium stearate Na P0 -12 H 0 fert-Dodecyl mercaptan

180.0 15.0 65.5 19.5 5.0 0.6 0.5 0.3 — — 0 — 6

180.0 15.0 65.5 19.5 5.0 0.6 0.5 — 0.170 0.085 — 0.017 14

3

4

2

K2S2O8

Cumene hydroperoxide Tetraethylene pentamine

Toughness and Brittleness of Plastics

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