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Rubber Toughened and Optically Transparent Blends of Cyclic Olefin Copolymers.

G. KHANARIAN [*]

A study is presented of increasing the toughness of Cyclic Olefin Copolymer (COC) while maintaining its optical transparency. The COC consists of a random copolymer of ethylene and norbornene, and the impact modifiers consist of thermoplastic elastomers. It is shown that several requirements must be satisfied, namely: for toughening there is an optimum finite particle size, but in order to minimize light scattering the particles have to be as small as possible. In addition, the refractive index of the elastomer must be closely matched to COC over the visible wavelength range and use temperature. Also for toughening, there are additional requirements of the adhesion of the elastomer to COC and high molecular weight. It is found that styrene butadiene styrene (SBS) is the most effective elastomer in toughening COC while maintaining optical transparency. It is also found that the addition of an index matched styrene-ethylene-butylene-styrene (SEBS) copolymer as a compatibilizer to the SBS elastomer is beneficial in increasing the toughness and lowering the optical haze. Finally, light scattering calculations are presented based on the Rayleigh Debye model to calculate the optical haze and transmission of these blends. These calculations take into account the refractive index of COC and the elastomer, the particle size distribution and volume fraction of the elastomer. It is shown that there is reasonable agreement between calculation and experimental results. It is possible to increase the toughness of COC to greater than 50 J/m (Notched Izod) while keeping the optical haze to below 5% with an elastomer loading of 5% (w/w). We also identify opaque blends of COC with a toughness of greater than 500 J/m with an elastomer loading of 20% (w/w).

INTRODUCTION

Transparent plastics are an important article of commerce and are widely used in diverse applications such as glazing, sheet, optical storage and imaging applications. Some of these plastics, Including polymethylmethacrylate (PMMA) and polystyrene (PS), are also mechanically brittle. Increasing the toughness of these plastics means increased utility. Therefore, there have been many efforts towards increasing the toughness of plastics while maintaining their high optical transparency (1, 2). During the last ten years, a new optical thermoplastic--Cyclic Olefin Copolymers (COC)--has been introduced, which has number of attractive features, namely low moisture uptake, high water barrier, low birefringence, high optical transmission, high Abbe number (low dispersion of refractive index], chemical resistance to common solvents such as acetone and high heat deflection temperature. Their composition consists of ethylene and a cyclic olefin such as norbornene or cyclopentene (3). The different types of COCs available commercially are Zeonex[TM] from Nippon Zeon, Arton[TM] from Japan Synthetic Rubber, Apel[TM] from Mitsui Petrochemical, and, most recently, Topas[R] (4) from Ticona. Topas is a copolymer of ethylene and norbornene that is available in a range of heat deflection temperatures from 80[degrees]C to 170[degrees]C. It is polymerized by metallocene catalysts (3) in contrast to the more conventional catalysts used to make other COC plastics; this results in a narrower molecular weight distribution, higher optical clarity and less catalyst residue.

The impact strength of Topas with Tg = 140[degrees]C is about 21 J/m (Notched Izod) (52.3 J/m = 1 ft-lb/in), which is adequate for many applications; however, there are certain applications where it is desirable to have an impact strength greater than 50 J/m while maintaining its high optical transparency. This paper describes the technical approach used to increasing the toughness of Topas with Tg = 140[degrees]C while maintaining its optical transparency. Increasing the toughness of optical plastics is technically challenging because many requirements (sometimes contradictory) must be satisfied. To increase toughness alone, one needs rubber particles dispersed in a matrix, which act as centers of cavitation when the plastic is deformed. It has been shown (5-7) that every plastic has an optimum particle size, which is a function of the entanglement molecular weight. It is also important that the particles be composed of low modulus rubbers, which have good adhesion to the polymer matrix. On the other hand, to maintain optical transparency one would like the particles to be as small as possible to minimize light scattering. Also, the refractive indices of the rubber particles should be closely matched to the polymer matrix at ambient temperature and at elevated and low temperatures. The conflicting requirements for an optimum particle size for toughening and very small particle size for transparency means that there is a tradeoff between the toughness and transparency that can be achieved. Also, low modulus elastomers have larger change of refractive index (dn/dT) with temperature T than high modulus polymer matrices. This means that even when the refractive indices are matched at ambient temperature they may differ at higher or lower temperatures. These effects (transparency, toughness] will depend on the concentration of elastomer compounded in the polymer matrix. Therefore, there is also a tradeoff between the concentration of elastomer, the toughness that can be achieved and the resultant optical transparenc y.

In this paper we present experimental results on the toughening of Topas with thermoplastic elastomers such as styrene-butadiene-styrene (SBS), styrene-ethylene-butylene-styrene (SEBS) and styrene-ethylene-propylene-styrene (SEPS). We show the relationship between toughness and transparency, elastomer concentration, particle size, elastomer modulus and molecular weight and refractive index mismatch. We also present light scattering calculations for the transparency and optical haze of these blends, thereby delineating the important optical parameters. We demonstrate that It is possible to increase the toughness of COC while maintaining its optical clarity. Some of the results in this paper have been disclosed earlier in a patent (8).

EXPERIMENTAL METHODS

Topas (grade 6013) was obtained from Ticona AG (Frankfurt, Germany) with a glass transition temperature of 140[degrees]C. The physical properties are given in Table 1 (4). The elastomers tested were SBS Dl184, D1101, D1118 and SEBS G1650, G1651, G1654. G1652 (Kraton brand, Shell Chemical Company, Texas, USA) (9), SEPS 2006, 2104 and 1050, (Septon brand, Kuraray Company, Japan) (10) and EPDM 465A (Royaltuf brand, Uniroyal Chemical, Connecticut, USA) (11). Developmental grades of elastomers PAR 2422 and 1721 were obtained from Dr. M. Modic of the Shell Chemical Company. The elastomers were blended in Topas in a Leistritz brand twin screw extruder (Model MC L8GG/GL, Leistritz AG, Germany). The twin screw design consisted of conveying elements, three or five kneading blocks for intense mixing and followed by additional conveying elements. The ratio of the length L to diameter D was 30. The screws were corotating at 200 or 450 rpm and the temperature for mixing was 230[degrees]C. Some compounding was also done on a Brabender single screw extruder. The blends were molded into test tensile bars using an Arburg Allrounder 220M injection molding machine (Arburg, Germany). The temperature of the barrel was 240[degrees]C, the injection pressure was 0.71 GPa (15,000 psi) (1 MPa = 145 psi), the screw rotation speed was 30 rpm and the cycle time was 20 seconds. The impact strength (Notched Izod) of the flexure bars was measured according to ASTM D256 (American Society of Testing Materials, Philadelphia).

The refractive indices of the Topas and elastomers were measured with the Metricon 2010 thin film analyzer (Metricon Corp., Pennington, NJ). Films were compression molded in a TMP press (Technical Machine Products, Cleveland, Ohio) and indices measured at a wavelength of 0.542 [micro]m, 0.633 [micro]m and 0.780 [micro]m. (1 [micro]m = 3.94 X [10.sup.-5] inch). The refractive indices were fitted to a Sellmeir equation

[n.sup.2] = [A.sub.1] + [A.sub.2][[lambda].sup.2]/([[lambda].sup.2] - [[A.sup.2].sub.3] (1)

where [lambda] is the wavelength and [A.sub.1,2,3] are fitting parameters. The results of the refractive indices and the Sellmeir fit parameters are presented in Table 2. The optical properties (transmission/haze) of the injection molded plaque blends at ambient temperature were measured with a Macbeth Color-Eye 7000 (Kollmorgen Instruments) according to ASTM D1003. The haze measured by the Macbeth is the average over a wavelength range from 0.400 [micro]m to 0.700 [micro]m. The optical transmission at elevated temperatures was measured by placing samples in a Mettler hot stage and measuring the transmission of a He-Ne laser (0.633 [micro]m) through the sample as the temperature was increased.

The particle size (diameter) of the elastomeric particles in Topas was determined by electron microscopy. The molded samples were notched, freeze dried and then cracked. The free surface was examined with an electron microscope. The number of particles for a given size range were counted using automatic imaging techniques and then their number and volume average were calculated. The volume average of the particle diameter is given by [d.sub.v] = [sigma][N.sub.i][[d.sup.4].sub.i]/[sigma][N.sub.i][[d.sup.3].sub.i] where [N.sub.i]is the number of particles with diameter [d.sub.i]. The adhesion between elastomers and Topas was measured according to ASTM D903-93. Sheets of elastomers and Topas were first prepared by compression molding in a TMP press [Technical Machine Products, Cleveland, Ohio). Then the two sheets were adhered to each other over a 1" wide strip by overlapping the two sheets and compressing together in the TMP press at an elevated temperatures. In this manner one obtained a strongly adhering bond betw een the elastomers and Topas. Finally, narrow 1" wide strips were cut out such that the overlapping region between the elastomer and Topas was 1 [in.sup.2] in area. The samples were placed in an Instron tester (Instron Corp., Canton, MA) and pulled to measure the force needed to delaminate the adhered region.

RESULTS ON TOUGHNESS AND OPTICAL PROPERTIES

The process of identifying elastomers for toughening COC while maintaining optical transparency is complicated because many requirements must be satisfied simultaneously. Figure 1 is a schematic of the key requirements and parameters that were optimized during this process. First, we focused on identifying elastomers that toughened Topas. Then we measured the refractive indices of those elastomers and chose those that were closely matched to Topas. A large number of elastomers and core shell modifiers were screened to find the optimum combination of greatest toughness and highest optical clarity. The results of these investigations are outlined below.

Improving Toughness

Optimum Elastomer Particle Size

It is well known (5-7) that there is an optimum particle size to toughen plastics. It has been shown by studying the micromechanics of deformation and cavitation of rubber particles, that both the particle size and the interparticle distance are important. In order to study this phenomenon for Topas, we compounded several different elastomers of different molecular weights using mild and intense mixing extruders. EPDM 465A, and Kratons D1184, D1101 and D1118 were compounded in Topas using a Brabender single screw extruder. The concentration was 20% by weight for the mild mixing experiment.

The size of particles was varied by more intense mixing using a twin screw Leistritz extruder. The concentration was 10% w/w. In addition to D1184 and EPDM 465, we also used Kraton G1651, G1652 and G1654, and Septon 2104 and 1050. The compounded resin was molded into test bars and their impact strength and particle size were measured. Figure 2 is a plot of the impact strength versus particle size (volume average) for the two sets of experiments in the Brabender and Leistritz extruders.

The results of Fig. 2 show that there is an optimum particle size for toughening Topas. The optimum particle diameter is near 1 [micro]m (volume average. 1 p.m = 3.94 X [10.sup.-5] inch) and it appears to be independent of the type of elastomer and concentration. The elastomers tested varied in their chemical composition. i.e saturated carbon-carbon bonds (SEBS) and unsaturated carbon-carbon bond (SBS), their polymer structure (linear and branched) and morphology. It is possible to increase the toughness of Topas to 156 J/m (Notched Izod) with a concentration of elastomer of 10% w/w.

Molecular Weight

It is well known from polymer processing studies that the optimum particle size is obtained during mixing when the viscosity of the elastomer and the resin are matched. Viscosity is directly related to the molecular weight of the elastomers. Therefore, we measured the impact strength of Topas/elastomer blends (10% w/w) as a function of the molecular weight of the elastomer. Figure 3 shows the relationship between impact strength and molecular weight of the elastomers. It is clear from this Figure that higher molecular weight results in higher toughness. This is probably because higher molecular weight results in higher viscosity, which in turn results in larger particles in Topas. Figure 2 shows clearly the need for larger ([greater than] 0.5 [micro]m) in order to effectively impact modify Topas.

Adhesion

An important characteristic of impact modifiers is their adhesion to the polymer matrix. The adhesion of various elastomers to Topas was measured according the Lap Joint Shear test (ASTM D903-93). The adhesion was plotted versus the impact strength of blends of Topas containing those elastomers with a concentration of 10% w/w. Figure 4 shows a plot of the impact strength (notched Izod) of Topas blends versus the adhesion of those elastomers to Topas.

There are a number of interesting results in Fig. 4. One can obtain good adhesion to Topas with a variety of different materials: SEBS elastomers such PAB 1721, SEBS elastomers G165 1, and SBS elastomer such as D1 101 or Dl 184. These elastomers have very different chemical structures; for example, SEBS have saturated carbon-carbon bonds. SBS have unsaturated carbon-carbon bonds. They also have different amounts of styrene; for example, SEBS G1 651 has 32%, whereas PAB 1721 has 66%. Also, D1184 is radial copolymer, whereas G1651 is a linear triblock terpolymer.

However, good adhesion is a necessary but not sufficient condition for obtaining high impact strength. PAB 1721 has excellent adhesion to Topas but has poor ability to impact modify Topas. The key additional requirement for impact modification is the particle size in the range 0.7-1 [micro]m. Blends containing elastomers with particle size in that range gave excellent impact modification, which is consistent with the results of Fig. 2. Thus G1651, D1101 and D1184 were all effective impact modifiers of Topas. D1101 had very high adhesion, whereas D1184 had moderate adhesion. What seems to matter is the particle size of the blended elastomer in Topas. On the basis of these results and the optical properties discussed below, we chose D1184 as the preferred elastomer for impact modification.

Figure 5 reports the impact strength of various elastomers tested versus the concentration in Topas. We identified several elastomers including Royaltuf EPDM 465A, Kraton Dl184, G1654 and Septon 2006, which toughened Topas very effectively. It is possible to attain very tough blends of Topas with a Notched Izod of 260 J/m with only a moderate amount of elastomers present (10-15% w/w). However, only Dl184 had a refractive index close to that of Topas and gave transparent blends, as we shall see below.

Improving Optical Properties of Blends

Optical Properties

A key parameter in identifying elastomers that will give transparent blends is the refractive index measured over a wide wavelength range. Table 2 lists the refractive indices at 633 nm of the elastomers tested. One observes that Dl184 has the closest refractive index to Topas. It is also one of the most effective impact modifiers. The other elastomers were not index matched and so could not be used in making transparent blends.

It is also important to match refractive indices over a wavelength and temperature range. Figure 6 shows the dispersion of the refractive index for Topas. D1184, PAB 2422 and 1721. One notes that Topas has a lower dispersion than D1184 because it contains only saturated C-C bonds, whereas D1184 has double C-C bonds. This mismatch in dispersion can cause light scattering, resulting in a slight blue haze when viewed in reflection.

Figure 7 shows the change in refractive index with temperature of Topas and the elastomer D1184. One finds that the index decreases more rapidly (about a factor of 3) than for Topas. The reason is that the glass transition temperature of the rubber is lower than 25-100[degrees]C, whereas in the case of Topas, it is higher. It is well known that the coefficient of thermal expansion changes greatly going from below to above a material's glass transition. This means that even if the indices are matched at ambient temperature, they will become mismatched at elevated temperatures, thereby causing increase in light scattering and haze. Figure 8 shows the optical transmission through blends with a He-Ne laser (633 nm). The sample (#3 in Table 3, to be described in greater detail below) was mounted in a Mettler hot stage and the temperature raised to 100[degrees]C. Figure 8 shows that up to 50[degrees]C the transmission stays fairly constant but drops to near 50% at elevated temperatures (100[degrees]C) because of i ncreased light scattering. The origin of this effect is the mismatch of refractive indices between the elastomer and the matrix polymer and other effects that are not well understood at this time. Figure 8 also shows the calculated optical transmission of the sample based on the Rayleigh-Debye theory presented in the next section.

Figure 9 shows the tradeoff between impact strength and optical haze of the blend Topas/D1184. Three curves are shown. The first curve is with no compatibilizer and mixed at 200 rpm in the Leistritz. The second curve is for the blend with compatibilizer mixed at 200 rpm. The third curve is for the blend with compatibilizer mixed with high intensity at 450 rpm and additional two kneading blocks in the screw of the extruder.

As the concentration of elastomer is increased, it results in Increased impact strength but also increased haze. The initial results with no compatibilizer showed that it is possible to obtain a Notched Izod of 80 J/m with an associated optical haze of 22% in a standard thickness of 3.2 mm (1/8 in). This is too high for most applications, and so we investigated further different methods of reducing haze while increasing toughness.

Effect of Compatibilizer and Blending Process

It was discovered during this study that there was a beneficial effect of adding small amounts of SEBS or SEPS thermoplastic elastomers to D1184. The benefit was increased impact strength and lowering of haze resulting in optically clearer and tougher Topas blends. However, the commercially available SEBS or SEPS had refractive indices that were either too low or too high so that they could not be used in Topas. For example, PAB 1721 (or Septon 2104) has a styrene content of 66% and PAB 2422 (or Septon 1050) had a styrene content of 50% and their refractive indices were either too high or too low, as reported in Table 2. Through blending experiments of PAB 1721, PAB 2422, G1650 together with Topas, It was discovered that a styrene content of 57% gave the closest refractive index match to Topas and lowest haze in blends. We blended various SEBS or SEPS together with Topas to vary the effective styrene content and the optical haze was measured. Figure 10 shows the results of the optical haze of the various SEB S/Topas blends plotted versus the average styrene content of the SEBS blends. One notes that the lowest haze was obtained with a styrene content of 57%.

A number of experiments were carried out to increase the toughness of Topas while improving its haze. We used SEBS 1:1 blends of PAB 1721 and PAB 2422 (or their equivalent SEPS 1:1 blends of Septon 2104 and 1050) corresponding to an effective styrene content of 57% as a compatibilizer to the Dl184/Topas blend. We found that the optimum ratio of Dl184 to the compatibilizer was 2:1. Figure 9 shows a curve (denoted by compatibilizer) of improved toughness versus optical haze. One can see clearly the improvement over the Dl184/Topas with no compatibilizer.

It is worth noting that the SEBS and SEPS with high styrene content (greater than 50%) are not effective as impact modifiers of Topas by themselves, apparently for two reasons. The polymers are not very rubbery because of the high styrene content (i.e., they have higher moduli than D1184 or G1651), and they seem to have lower molecular weight, which causes their viscosity to be lower than that of Topas, resulting in small particle size (volume average particle size is 0.15 [micro]m) (see also results in Figs. 2, 3 and 4). They appear to be effective as compatibilizers for D1184 by increasing impact strength and reducing optical haze.

We also studied the effect of compounding conditions on the impact strength/optical haze tradeoff. As we have seen in Fig. 1, there is tradeoff between particle size, impact strength and optical transparency. Since Dl184 is a thermoplastic elastomer, it is possible to "adjust" the particle size by processing conditions. We increased the intensity of mixing by adding five kneading blocks in the screw of the Leistritz instead of the usual three, and increased the revolutions per minute from the usual 200 to 450 rpm. The blend consisted of D1184/PAB (2422/1721) (2:1) in Topas. The concentration of the elastomer mix was in the range 5-14% w/w. The improved results are also shown in Fig. 9 (labeled by intense mixing and compatibilizer). The results clearly show the improvement in the tradeoff between toughness and optical transparency. It was possible to obtain an impact strength of 57 J/m with an optical haze less than 5%, or 104 J/m with an optical haze of 6.5%.

Table 3 summarizes the influence of processing and compatibiizers on the particle size distribution, optical haze and Notched Izod for a 5% D1184/Topas blend. Figure 11 shows the experimental normalized particle size distributions for the three cases in Table 3 and the best log normal fits to the data. The fitting parameters are given in Table 4 for the three cases of Table 3. These fits are used in the next section to calculate the optical properties of the samples. One notes that as processing conditions (i.e., screw speed) are increased, the particle size gets smaller, which results in lower haze, while the compatibilizer improves both impact strength and lowers haze. The ratio (NI/H) of Notched Izod (NI) divided by haze H (i.e the slope) is a measure of the improvement in blend system. This is also seen in the increased slope of the curves in Fig. 9. It appears that as the particle size decreases as a result of intense mixing, the toughness decreases, as expected from the results of Fig. 2, but there is an improvement in the optical properties. The improvement in optical properties is greater than the decrease in toughness, and so there is an overall improvement in the tradeoff between toughness and optical haze.

We measured the optical and mechanical properties of the toughened sample with 5% D1184+ 2% PAB (2422/1721) and the results are shown in Tables 1 and 3 and Fig. 9. It was possible to increase the toughness of Topas from 25 to 52 J/m while maintaining high optical transmission at room temperature and moderate transmission at elevated temperatures. It was also possible to increase the elongation at break from 2% to 13% and go from a brittle to ductile failure.

THEORETICAL CALCULATION OF OPTICAL HAZE

The optical haze or scattering was calculated theoretically based on the following model. It was assumed that the elastomer dispersed in the polymer matrix were spherical in shape and scattered independently of one another. The well known Rayleigh-Debye model (12) was used to calculate the scattering cross sections. The expression for total optical transmission T and haze H has been derived by Willmouth (13) and is given by,

T = 16[n.sup.2] exp (-[less than] NC [greater than] t)/[(n + 1).sup.4] (2)

H = [(n + 1).sup.2] [less than] [[C.sup.90].sub.2.5] [greater than] [1 - exp(- [less than] NC [greater than] t)]/4n[less than]C[greater than]exp(- [less than] NC [greater than] t) + [(n + 1).sup.2] [less than] C [greater than] [1 - exp(- [less than]NC[greater than] t)] (3)

In the above expression [less than]C[greater than] is the average total scattering cross section for a distribution of particles, while [less than][[C.sub.2.5].sup.90][greater than] is the scattering cross section integrated from 2.5[degrees] to 90[degrees]. This is the angular range for the measurement of optical haze in the Macbeth instrument used in this work. t is the thickness of the sample (1/8 inch or 3000 [micro]m) and n is the average refractive index of the sample. [less than]NC[greater than] is the average of the product of the number of particles per unit volume times the total scattering cross section. The expressions for [[C.sub.2.5].sup.90] and NC are given below:

[less than][[C.sup.90].sub.2.5][greater than] = 4/9 [[[integral].sup.[infinity]].sub.a=0] [[[integral].sup.90].sub.[theta]=2.5] n(a)[pi][a.sup.2] [(m - 1).sup.2][(2[pi]a/[lambda]).sup.4]

P([theta], a, [lambda])(1 + [cos.sup.2][theta] sin [theta]d [theta]da (4)

[less than][[NC.sup.90].sub.0][greater than]= (4/0)[phi]/(4/3)[[[integral].sup.[infinity]].sub.a=0] n(a)[a.sup.3]da [[[integral].sup.[infinity]].sub.a=0][[[integral].sup.90].sub.[theta] =0 n(a)[pi][a.sup.2][(m - 1).sup.2]

[(2[pi]a/[lambda]).sup.4] P([theta], a, [lambda])(1 + [cos.sup.2] [theta])sin [theta]d [theta]da (5)

In the above expressions [phi] is the volume fraction of particles, a is the radius of the particle, m is the ratio [n.sub.1]/[n.sub.2] of the indices of the rubber particle [n.sub.1] and the matrix [n.sub.2], respectively, n(a) is the normalized distribution function of the particles sizes, and P is the form factor of a sphere given by

P([theta]) = [[(3/[u.sup.3])(sin u - u cos u)].sup.2]

u = 4[pi]a/lambda] sin([theta]/2 (6)

The normalized distribution function is approximated by the log-normal function (12), which is often used to model particles broken up by shearing processes:

n(a) = 1/[square root]2[pi][sigma]a exp[ -[(log a - log [a.sub.m]).sup.2/2[[sigma].sup.2]] (7)

In Eq 1, a is the radius of the particle, [a.sub.m] is a measure of the averag particle size, and [sigma] is a measure of the width of the distribution. A typical plot of the particle size distribution encountered in this work for examples 1-3 in Table 3 are shown in Fig. 11 below.

The scattering equations (Eq 3-6) were used to calculate the optical haze for the three samples in Table 3. The wavelength dependence of refractive indices is given by Eq 1 and the fitting parameters for the sellmeier equation are in Table 2. Figure 12 shows the calculated haze as a function of the wavelength. Also shown on Fig. 12 are the experimental average values of the haze for the three samples. The average haze is measured over a wavelength range 0.400 [micro]m to 0.700 [micro]m. The calculated results show a strong dependence on wavelength, as expected. Sample 3 shows good agreement between theory and experiment and much lower haze than samples 1 and 2 because the particle size is smaller and also because the compatibilizer narrows the refractive index mismatch between D1184 and the matrix polymer Topas.

Equation 2 was used to calculate the transmission through the samples versus temperature. The sample chosen was #3 in Table 3. The temperature dependence of the refractive index of Topas and D1184 is given in Fig. 8. The agreement between theory and experiment is quite good up to 50[degrees]C Above that temperature, there is a growing discrepancy between theory and experiment. It seems that a new mechanism for scattering is occurring at the higher temperatures, i.e., it is not due simply to the difference in refractive indices of Topas and D1184. We did not investigate further the mechanisms for light scattering at elevated temperatures.

CONCLUSIONS

The toughening of transparent plastics with elastomers while maintaining their optical transparency is challenging because many requirements must be satisfied. The key requirement hinges around the particle size of the elastomer and its distribution. From the point of view of transparency, one would like particles as small as possible with a narrow particle distribution. However, the toughening of plastics requires a finite optimum particle size that acts as a cavitation center for absorbing energy. Other requirements that are almost as important are that the refractive indices be matched over a wide wavelength and temperature range. The elastomer must also have good adhesion to the matrix and have a high molecular weight so that the viscosities are matched to the matrix at processing temperatures.

The investigation presented here shows that it is possible to meet all these requirements for cyclic olefin copolymers with an elastomer made from styrene-butadiene styrene. It is possible to increase toughness to greater than 50 J/m while maintaining haze levels below 5% with an elastomer loading of 5% (w/w). It is also possible to make opaque blends of COC that have a toughness of 500 J/m with an elastomer loading of 20% (w/w).

We have also shown that it is possible to model theoretically the optical properties of transparent blends using the Rayleigh-Debye model of light scattering with no adjustable parameters. The calculation of haze is reasonably accurate, but there are some discrepancies in calculating the optical transmission at elevated temperatures.

ACKNOWLEDGMENTS

I would like to thank Dr. T. Dolce for useful discussions on toughening of plastics and Dr. M. Modic of the Shell Chemical company for sampling elastomers. I would like to thank Dr. R. Chen for measuring the particle size of elastomers in Topas using electron microscopy and Dr. D. Karim for measuring the temperature dependence of the refractive index of elastomers. I acknowledge help from V. Astone for compounding the elastomers in Topas, J. Clawson for injection molding test samples, and B. Kelly and F. Ayotte for optical haze measurements. The scattering calculations were performed with the computer program Mathematica[TM].

(*.) Present address: Rohm and Haas, 727 Norristown Rd. Spring House. PA 19477

REFERENCES

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(2.) M. Colella and B. Marcoulier, U.S. Patent 5,344,878.

(3.) W. Kaminsky and M. Arndt, "Metallocenes for Polymer Catalysis," in Advances in Polymer Science, Vol. 127, Springer Verlag (1997).

(4.) Topas[R]: Thermoplastic Olefin Polymer of Amorphous Structure, brochure from Ticona, Frankfurt, Germany.

(5.) S. Wu. Polymer Internationol, 29, 229 (1992).

(6.) J. A. Sauer and C. C. Chen in Adv. Polym. Sci., 52/53, p.169, H. H. Kausch, ed., Springer Verlag (1983).

(7.) G. H. Michler, Acta Polymer, 44, 113 (1993).

(8.) G. Khanarian and T. Dolce, PCT Int. Appl., WO 9746617 Al 971211, U.S. Patent 6,090,888.

(9.) Shell Kraton Polymers for Modification of Thermoplastics, product brochure from Shell Chemical Company, Houston, Texas.

(10.) High Performance thermoplastic elastomer: SEP/SEPS, product brochure from Kuraray Co, Japan.

(11.) Royaltuf Modified EPDM, product brochure from Uniroyal Chemical, Middlebury, Conn.

(12.) M. Kerker, The Scattering of Light, p. 353 and p. 414, Academic Press, New York (1969).

(13.) F. M. Willmouth, chapter 5, p. 265, in Optical Properties of Polymers, G. H. Meeten, ed., Elsevier (1986).
 Properties of Neat Topas 6013 (4) and
 Toughened/Optically Transparent Topas.
Property Topas Tough and
 Transparent Topas [R][*]
Mechanical
Notched Izod (J/m) 21 57.2
modulus (MPa) 3103 2690
elongation @ break (%) 2 13
Optical [**]
haze(%)at 25[degrees]C [less than]1% 5
Transmission (%) at 25[degrees]C [greater than]93 89
Transmission (%) at 90[degrees]C [greater than]93 50
(*.)5% D1184+ 2% PAB (2422/1721) in Topas 6013.
(**.)3.2 mm (1/8 inch) thick specimen.
 Refractive Indices of Elastomers and Topas[R]
 at Wavelength [lambda] = 0.633 [micro]m,
 Fitting Parameters to Sellmeier Equation
 1 and Chemical Composition (9, 10).
Elastomer Chemical Type Styrene Refractive Index at A1
 Content (%) [lambda] = 0.633 [micro]m
Topas na 1.5319 2.076
D1184 SBS (radial) 30 1.5359 2.011
D1118 SBS(diblock) 30 1.5391 2.012
D1101 SBS(triblock) 31 1.5375 2.007
G1651 SEBS(triblock) 32 1.5083 2.0139
G1654 SEBS(triblock) 31 1.5060 2.009
G1650 SEBS(triblock) 29 1.5053 2.003
PAB 2422 SEBS 50 1.5252 2.005
PAB 1721 SEBS 66 1.5422 2.086
Septon 1050 SEPS 50 1.5240 2.085
Septon 2104 SEPS 66 1.5412 2.029
Elastomer A2 A3
Topas 0.2353 0.2165
D1184 0.2994 0.2368
D1118 0.307 0.2317
D1101 0.3048 0.2372
G1651 0.222 0.2407
G1654 0.2187 0.2447
G1650 0.2238 0.2373
PAB 2422 0.2753 0.2355
PAB 1721 0.2506 0.2409
Septon 1050 0.1933 0.2694
Septon 2104 0.2967 0.2353
 Influence of Compatibilizer and Processing
 Conditions on Particle Size, Haze, and
 Impact Strength.
# Sample Avg. Particle Std Dev. Volume Notched
 Diameter ([micro]m) Average Particle Izod (NI)
 ([micro]m) Diameter ([micro]m) (J/m)
1 5% D1184 0.54 0.30 1.06 67.6
2 5% D1184 0.41 0.16 0.61 53
3 5% D1184 + 2% PAB 0.24 0.11 0.42 57.2
 (2422:1721 1:1)
# Haze (%) Notched Blending
 Izod/Haze Conditions [*]
 (Screw Rotation)
1 20 3.4 200 rpm
2 9.6 5.5 450 rpm
3 6 9.5 450 rpm
(*.)Leistritz, barrel temp. 230[degrees]C.
 Fitting Parameters for the Log Normal
 Particle Distribution Equation 7 for Sample
 1-3 in Table 3.
 #1 #2 #3
[a.sub.m] ([micro]m) 0.254 0.196 0.102
[sigma]([micro][m.sup.-1]) 0.385 0.371 0.388
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Article Details
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Author:KHANARIAN, G.
Publication:Polymer Engineering and Science
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Dec 1, 2000
Words:5735
Previous Article:A Study of the Effect of PP-g-MA and SEBS-g-MA on the Mechanical and Morphological Properties of Polypropylene/Nylon 6 Blends.
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