Use of surface modified recycled rubber particles for toughening of epoxy polymers

Use of Surface Modified Recycled Rubber Particles for Toughening of Epoxy Polymers

R. BAGHERI,'12 M. A. WILLIAMS,' and R. A. PEARSON2

Composite Particles, Inc. Allentown, Pennsylvania 181 03

Materials Research Center Lehigh University

Bethlehem, Pennsylvania 1801 5

The objective of this research is to investigate the feasibility of using surface treated recycled rubber particles for toughening of epoxy polymers. These particles are obtained through grinding of scrap tires followed by oxidizing the surface of the particles in a reactive gas atmosphere. Surface treated recycled rubber particles with a nominal particle size of approximately 75 pm and a commonly used reactive liquid elastomer, CTBN, have been incorporated in a DGEBA epoxy resin. I t has been shown that the recycled rubber particles are not as effective as CTBN in toughening of the epoxy matrix. However, blending of the two modifiers results in a synergistic toughening. Microscopy reveals that, when used alone, recycled rubber particles simply act as large stress concentrators and modestly contribute to toughening via crack deflection and microcracking. In the presence of micron size CTBN particles, which cavitate and induce massive shear yielding in the matrix, however, the recycled particles "stretch" the plastic deformation to distances far from the crack tip. This mechanism causes plastic zone branching and provides an unexpectedly high fracture toughness value. This study, therefore, provides a practical approach for manufacturing engineering polymer blends utilizing the surface modified recycled rubber particles.

INTRODUCTION liquid elastomers used for toughening of epoxies ( 12).

poxies are the most widely used and studied ther- E mosetting materials and are used in a very wide range of industrial applications such as coatings, matrices for composites, adhesives, and encapsulating materials. Neat epoxies, however, have very low crack growth resistance and are among the most brittle polymeric materials (1).

Addition of a second rubbery particulate phase, i.e. rubber modification, has been successfully applied to overcome the inherent brittleness of many epoxies over the past 25 years (2-10). There are two ap- proaches for rubber modification of epoxies. The first approach, which has been in use since 1970 (21, is based on using reactive liquid elastomers. The reactive liquid rubber used in this technique is dissolved in the epoxy first. After adding curing agent, as the resin begins to cure and the molecular weight rises, the rubber precipitates out and forms the second phase particles. Volume fraction and size of the rubber domains are controlled by the degree of compatibility of two phases and the kinetics of gelation (1 1). Butadiene-acrylonitrile base rubbers are the principle

Carboxyl-terminated butadiene-acrylonitrile (CTBN) elastomers with different acrylonitrile content have been extensively used ( 12).

The second approach for rubber modification of epoxies which is a relatively recent development, is based on using pre-formed rubber particles instead of reactive liquid elastomers. The advantage of this approach is that particle size and volume fraction of the second phase are not varied by the resin type and processing conditions. Compatibility with the epoxy resin and adhesion to the matrix can be achieved via the chemistry of the surface of particles. Core-shell latex particles with an acrylic rubber or a butadiene- base rubber core and a thin layer of a glassy shell have been successfully used for toughening of epoxies (13- 15).

The aim of this investigation is to explore the feasibility of using surface treated recycled rubber particles for toughening of epoxy polymers. These particles are obtained through grinding of scrap tires followed by exposing the particles to a reactive gas atmosphere to oxidize their surface (16). Oxidation of surface results in formation of polar functional groups such as

POLYMER ENGINEERING AND SCIENCE, FEBRUARY 1997, Vol. 37, No. 2 245

R. Bagheri, M. A. Williams, and R. A. Pearson

carboxylate and hydroxyl on the outer surface of particles. These functionalities improve compatibility and adhesion to different polymer matrices (16). Recycled rubber is an ideal source of low-cost elastomeric particles.

Our earlier investigation illustrated that surface modified recycled rubber particles toughen a DGEBA epoxy cured with piperidine (1 7). Although the toughening efficiency of these particles was much less than that of CTBN, that study revealed the possibility of synergistic toughening if both kinds of rubber are incorporated simultaneously (1 7). A more recent study by Gerace et al. (18) showed an improved shear-lap strength when surface treated recycled rubber particles were added to a CTBN modified adhesive.

The current study focuses on the toughening mechanisms in epoxies modified by CTBN and a surface treated recycled rubber. I t is intended to explore the synergistic toughening in a systematic fashion when these two types of rubber are incorporated simultaneously. Linear elastic fracture mechanics (LEFM) approach is used for fracture toughness assessment. Different microscopic techniques are employed to investigate the toughening mechanisms.

EXPERIMENTAL

Materials

The epoxy resin used in this study is a solid digly- cidyl ether of bisphenol A (DGEBA) epoxy with a n epoxy equivalent weight of about 310 g/eq (EPONTM 836 from Shell Chemical Co). Curing agent used is piperidine (PIP). Rubber modifiers used are surface treated recycled rubber particles (VISTAMERTM R-4200) from Composite Particles, Inc., and carboxyl terminated random copolymer of butadiene and acrylonitrile (HYCARTM CTBN 1300x8) produced by B. F. Goodrich Co. VISTAMERTM R-4200 particles have a nominal particle size of 200 mesh (approximately 75 microns). CTBN domains formed in these epoxy matrices are usually on the order of a few microns. Throughout the paper, these two modifiers are called R4200 and CTBN, respectively.

Processing

Curing of the neat epoxy was accomplished by the following procedure. First, the epoxy resin was melted and degassed at 90°C. The curing agent was then injected gently into the epoxy at the same temperature and mixed for about 20 min under vacuum. The so- lution was then poured into a six millimeter-thick vertical aluminum mold which was preheated at 150°C. The material was finally cured at 150°C for 16 h in a circulating air oven.

Similar procedure was employed for curing of rubber-toughened epoxies. The rubber modifiers were blended with epoxy for about 2 h prior to addition of the curing agent. Concentration of the rubber in modified blends was kept constant at 10 phr, while the

ratio between the two modifiers was varied. Formula- tions used are shown in Table 1.

Characterization

Single edge notched (SEN) specimens loaded in three point bending (3PB) geometry were used for fracture toughness measurement (Fig. 1 ) . The ASTM D5045 (19) guideline was followed to obtain the plane strain fracture toughness (KJc) in this test. Pre-cracks were introduced at the bottom of two-millimeter deep notches by hammering a razor blade which was chilled in liquid nitrogen. These tests were performed using a screw-driven Instron testing frame at a cross- head speed of 5 mm/min. KJc values reported repre- sent averages of a minimum of five tests. The following equations were used to calculate K,c:

and

[1.99 -X(1 -X)(2.15 - 3.93X+ 2.7X2)] = 3x112

2(1 + 2X)(l - x)3/2 ( 2 )

where

P = critical load for crack propagation (in kilo- Newton),

S = span length (in mm), t = specimen thickness (in mm),

w = specimen width (in mm), f (X) = non-dimensional shape factor,

X = a / w , which is the crack length to specimen width ratio,

a = crack length measured after the specimen breaks (in mm).

Fracture surfaces of the SEN-3PB specimens were examined using a JEOL 6300F scanning electron microscope at an accelerating voltage of 5 KV. Samples were coated with a thin layer of gold-palladium before examination to protect the fracture surfaces from the beam damage and the charge build-up.

In order to observe the crack tip damage zone of modified epoxies, double-notched four-point bending (DN-4PB) method in conjunction with transmission

Table 1. Description of the Formulations Used.

EPOXY Curing R4200 CTBN Designation Resin (phr) Agent (phr) (phr) (phr)

~

Neat 100 3 0 0 R4200(10) 100 3 10 0 R4200(7.5)/CTBN(2.5) 100 3 7.5 2.5 R4200(5)/CTBN(5) 100 3 5 5 R4200(2.5)/CTBN(7.5) 1 00 3 2.5 7.5 CTBN(1O) 100 3 0 10

246 POLYMER ENGINEERING AND SCIENCE, FEBRUARY 1997, Vol. 37, No. 2

Use of Surface Modifled Recycled Rubber Particles

a - I

50.8

76

Fig. 1. Schematic of the SENSPB specimens used for fracture toughness testing. All dimensions are in millimeters.

optical microscopy (20) was employed. Details of this technique are as follows:

First two edge cracks of equal length are introduced to a bending sample (Fig. 2). A screw-driven Instron testing frame at a cross-head speed of 5 mm/min is employed for breaking the samples. The specimen is loaded in a four-point bending fixture until damage zones form at the crack tips. Finally, one of the cracks which first reaches to the instability point propagates and sample fractures. The other crack which is un- loaded, therefore, contains a well developed damage zone that represents the conditions prior to the failure of the material. This damage zone can be observed using a transmission optical microscope after thin- ning via petrographic polishing (20). Thin specimens (-50 pm) taken from the mid-planes of four-point bending samples are viewed using an Olympus BH-2 transmission-light microscope.

RESULTS AND DISCUSSION

Mechanical Testing

The result of the fracture toughness measurements are shown in Table 2. While, the incorporation of CTBN improves fracture toughness of the epoxy resin by 220%, R4200 alone enhances only by 17%. This result is consistent with the finding of Pearson and Yee (13) who compared the toughening efficiency of 1.5 and 150 pm size rubber particles in a DGEBA

101.6

I I

I IL 50.8 1 127

Fig. 2. schematic of the DN-4PB specimens used for obsema- tion of the crack tip damage zones. All dimensions are in millimeters.

Table 2. Fracture Toughness of the Neat and Modified Epoxy Blends.

Designation K,, IMPa.m'.')

Neat R4200(10) R4200(7.5)/CTBN(2.5) R4200(5)/CTBN(5) R4200(2.5)/CTBN(7.5) CTBN(10)

0.98 1.15 1.41 2.05 3.30 3.14

epoxy matrix. These researchers observed that while large particles do not alter the fracture toughness con- siderably, small particles are capable of increasing the crack growth resistance dramatically (13). Pearson and Yee (13) explained their observation by means of the difference in toughening mechanisms observed, i.e. rubber bridging in case of large particles and particle cavitation and shear banding of the matrix when fine rubber particles were employed. It is generally agreed that rubber particle cavitation and concomi- tant shear yielding of the matrix is the major toughening mechanism in rubber-modified epoxies (7-10,

Table 2 shows a dramatic 236% increase in the fracture toughness value for the R4200(2.5)/ CTBN(7.5). Interestingly, the fracture toughness value of this material is higher than that of CTBN(l0). Therefore, synergistic toughening occurs when 2.5 (phr) R4200 and 7.5 (phr) CTBN are incorporated. This synergistic toughening is further illustrated in Fig. 3 where the fracture toughness is shown vs. the modifier composition. R4200(7.5)/CTBN(2.5) and R4200(5)/CTBN(5) almost follow the rule of mixtures. However, R4200(2.5)/CTBN(7.5), shows a positive deviation from the rule of mixtures.

Positive deviation from the rule of mixtures in rubber-toughened polymers when bimodal size particles are incorporated has been reported by some investi- gators (13, 2 1-23). Synergistic toughening, however, is not necessarily found (13). In addition, the researchers who claimed synergistic toughening with

13-15).

E

t

3 0 m c

I . O T ' . ' . ' . ' . . ' . ' ' ' ' . . ' 0 2.5 5 7.5 10 10 7.5 5 2.5 0

Modifier Composition (phr)

CTBN R4200

Fig. 3. Fracture toughness us. blend composition. Synergistic toughening is evident in case ofR4200(2.5)lCTBN[7.5) where the maximum toughness is obtained.


R. Bagheri, M. A. Williams, a n d R. A. Pearson

twin population of rubber particle sizes (21-231, did not investigate the toughening mechanism(s1 responsible for their observation. Therefore, it is believed that the use of a twin population of rubber particle sizes does not guarantee synergistic toughening.

Scanning Electron Microscopy (SEM)

Figure 4 presents the SEM micrographs taken from the fracture surfaces of R4200( lo), R4200(2.5)/ CTBN(7.5). and CTBN( 10) at low magnification. As seen in Fig. 4a, R4200 particles are uniformly distrib- uted throughout the matrix which could be attributed to some extent to the surface modification of particles (16). Figure 4 also illustrates a significant difference in the surface roughness among these three materials. While R4200( 101 has the smoothest surface, R4200(2.5)/CTBN(7.5) has the roughest surface.

The roughness of the fracture surface in rubber- toughened epoxies can be attributed to crack path deflection and plastic deformation of the matrix. SEM technique, however, does not reveal the toughening mechanism(s) occurring far from the plane of fracture. Therefore, one needs to investigate the sub-surface damage to explore the exact toughening mechanism(s) (20). This has been carried out in this study using transmission optical microscopy (TOM). Before shar- ing the TOM results, however, some additional points found in SEM micrographs are discussed.

Figure 5 contains the same SEM micrographs shown in Fig. 4 taken at a higher magnification. Figure 5a illustrates very strong adhesion at the interface of R4200 particles. Figure 5b also shows proper bonding of R4200 particles to the matrix, despite the very con- siderable dilation of the matrix around these particles. The reason for the significant amount of dilation around the R4200 particles in this case is that the surrounding matrix is already toughened by CTBN rubber. This causes a change in the crack tip stress field such that the R4200 particles are fully engulfed in the process zone. Strong adhesion of R4200 particles to the matrix is caused by the surface treatment of particles and is not seen with the untreated particles

Another noteworthy point shown in Figs. 5b and c is the difference in the size of CTBN domains. Although the same processing conditions were applied, CTBN domains have larger average size and broader size distribution in the presence of R4200 particles. While FJ&, 5c illustrates a particle size range of 1 to 4 pm, CTBN particles up to 8 pm are found in Fig. 5b. This result was also found in other formulations containing both types of rubbers (not shown here). The cause of this effect is not clear and might be attributed to the change in the polarity of the blend when R4200 particles are introduced. As mentioned earlier, uncontrol- lable particle size is a common problem when liquid rubbers are employed for toughening purposes (1 1).

Transmission Optical Microscopy (TOM) TOM micrographs taken from the crack tip dam-

age zone of R4200(10), R4200(2.5)/CTBN(7.5), and

(1 7).

FYg. 4. SEM micrographs taken from the damage zone of SEN- 3PB samples of (a) R4200(10), (b) CTBN(7.5)/R4200(Z2.5), and (c) CTBN(1 0) materials.

CTBN(10) are shown in Figs. 6a to c, respectively. These Figures show a major difference in toughening mechanism. The crack tip damage zone of CTBN( lo), seen in Fig. 6c. is typical for rubber-toughened epoxies where the cavitated particles induce massive shear deformation (9, 10, 13, 15). Internal cavitation of rub-


Use of Surface Modified Recycled Rubber Particles

Fig. 5. Same as Fig. 4 but at higher magnijlcation.

ber particles is caused by the hydrostatic stress field at the crack tip. This is essential for the relief of plane strain constraint and massive shear yielding of the matrix (9, 13, 15, 24).

Figure 6a shows no evidence of shear deformation at the crack tip of R4200( 10). Instead, this Figure illustrates crack path deflection caused by the large rubber particles (arrows show the path of the crack from

Fig. 6. TOM micrographs taken from the midplane of the crack tip damage zone of [a) R4200[10), [b] CTBN[7.5]/R4200(2.5), and (c] CTBN(l0) materials. Crack growth direction is from left to right in these Figures. Arrows in (a) illustrate the crackpath deflection caused by R4200 particles.

left to right). Additionally, this Figure shows limited microcracking in front of the crack tip. Higher magnification in Fig. 7 shows this more clearly. Crack deflection and microcracking in this material are caused by the stress concentration effect of R4200 particles. As expected, these mechanisms are not as effective as massive shear yielding in a ductile epoxy matrix where


R. Bagheri, M. A. Williams, and R. A. Pearson

Fg. 7. Same as Fg. 6a but at higher magn@cation. Arrows show microcracks forming perpendicular to the loading direction ahead of the crack tip.

shear deformation contributes very significantly to toughening (25). This accounts for the higher fracture toughness of CTBN( 10) vs. R4200(10).

Another possible contribution of R4200 is particle bridging. These particles are large enough to bridge the crack wake (131. However, it is not clear that surface treatment of R4200 particles improves their bridging capability since there may be an optimum interfacial adhesion for the maximum bridging efficiency (26). Our TOM study showed a few bridging particles in the wake of the crack in R4200( 10). but the majority of the particles were found cut (Fig. 8). This observation illustrates that crack bridging may not contribute at all to the toughening of this system. It is noteworthy that, in general, particle bridging is not considered an important toughening mechanism in rubber-modified epoxies, owing to the very low modulus of elastomeric phase that prohibits efficient load bearing (13, 25, 27). Therefore, the modest im-

provement in the fracture toughness gained by em- ploying 10 phr R4200 (Table 2) can be primarily attributed to the crack deflection and microcracking mechanisms. This is provided by the stress concentration effect of R4200 particles. These particles are too large to interact with the crack tip and promote shear deformation in the matrix (13).

The TOM micrograph shown in Fig. 6b illustrates massive shear yielding at the crack tip of R4200(2.5)/ CTBN(7.5). The shape and size of the damage zone in this Figure, however, is significantly different from that of CTBN( 10) seen in Fig. 6c. The damage zone has a uniform and elliptical shape in Fig. 6c that is associated with the stress field at the crack tip in this material. This is similar to that predicted by the Dug- dale model (28). Figure 6b, however, illustrates a highly branched damage zone where the shear deformation is stretched out from the crack tip vicinity towards the large rubber particles. The plastic zone size in CTBN( 10) is on the order of 100 pm (half of the width of the damage zone in Fig. 6c). Arrays of plastic deformation in Fig. 6b, however, are stretched up to 300 pm from the crack tip.

Plastic zone branching (Fig. 6b) can be explained through the stress concentration effect of R4200 particles. The high stress field around these particles interacts with the crack tip stress field causing ease of cavitation of CTBN particles and thus, shear yielding of the matrix in the vicinity of the R4200 particles. As a result, the shear yielding zone is stretched out from the crack tip vicinity towards the R4200 particles. This phenomenon results in enlargement of the plastic zone size and synergistic toughening. By this defi- nition, one may expect to see the synergistic toughening at a certain ratio of modifiers and also when the two modifiers have a given size ratio. This study sug- gests a 1:3 ratio of coarse to fine particles for synergistic toughening to occur. No attempt was made to optimize the size ratio between the modifiers.

Synergistic toughening caused by plastic zone branching was also observed in a hybrid-epoxy composite where CTBN rubber and hollow glass spheres were incorporated simultaneously (29, 30). Interest- ingly, the optimum toughness in this system was found at a 1:3 ratio of hollow glass spheres to CTBN (29). The hollow glass spheres in the hybrid-epoxy composite act as large stress concentrators, similar to R4200 particles, and cause plastic zone branching (30). This mechanism of synergistic toughening is different from that predicted by Evans et al. (31). Their model predicts a synergistic toughening in rubber- modified polymers if crack tip and crack wake mechanisms operate simultaneously (31). In the case of plastic zone branching induced synergistic toughening, however, the primary contribution for toughening evolves from the crack tip mechanisms.

Fig. 8. TOM micrograph taken from the crack wake of CONCLUSION R4200(10) in the midplane of DN-4PB specimen. Arrows show the crack propagating from Zeft to right. As seen, rubber par- A DGEBA epoxy is toughened by two types of rub- tides are mostly cut in the crack wake. bers, a commonly used reactive liquid elastomer


Use of Surface Modijied Recycled Rubber Particles

(CTBN) and a surface modified recycled rubber (R4200). While the total rubber content in toughened blends is kept constant at 10 phr, the ratio between the two modifiers is varied. The blends are subjected to fracture toughness testing and different microscopy techniques.

The results illustrate that while the recycled rubber particles improve fracture resistance of the epoxy resin by less than 20%, the CTBN rubber enhances the fracture toughness to more than 200%. The reason for this significant difference in crack growth resistance lies in different toughening mechanisms, i.e. crack deflection and microcracking with R4200 vs. particle cavitation and massive shear yielding of the matrix with CTBN.

The results also reveal synergistic toughening when 2.5 phr R4200 and 7.5 phr CTBN are incorporated. The mechanism responsible for the unexpectedly high fracture toughness in this blend is the plastic zone branching caused by interaction of the stress field of R4200 particles with that of the crack tip. The stress field associated with R4200 particles enhances cavitation of neighboring CTBN domains and stretches the shear deformation from the crack tip towards the large particles. Plastic zone branching mechanism enlarges the effective damage zone size and further enhances the crack growth resistance.

In summary, this study introduces a new approach for rubber-toughening of engineering plastics by blending traditional modifiers and surface treated recycled particles. In addition to cost savings, this approach results in improved mechanical properties such as higher fracture toughness.

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Received May 15, 1996 Revised Sept. 15, 1996

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Use of surface modified recycled rubber particles for toughening of epoxy polymers

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