Processing and Thermo-Oxidative Aging of High-Impact Polystyrene

Chapter 22 Processing a n d Thermo-Oxidative

Aging

Downloaded by STANFORD UNIV GREEN LIBR on September 17, 2012 | http://pubs.acs.org Publication Date: September 15, 2001 | doi: 10.1021/bk-2001-0797.ch022

of High-Impact Polystyrene-Containing Fire Retardant Kenneth Moller, Jukka Lausmaa, and Antal Boldizar Department of Chemistry and Materials Technology, SP Swedish National Testing and Research Institute, Box 857, S-501 15 Boras, Sweden

High Impact Polystyrene (HIPS) with and without a fire retardant, deca-bromodiphenylether (deca-BDE), has been subjected to an accelerated thermo-oxidative aging corresponding to a normal in-door service time of about ten years. The purpose of the study was to examine if HIPS with deca-BDE can be recycled without loss of important properties. Test specimens of HIPS were aged at 80 °C for 1400 hours. Changes in mechanical properties, e.g. elongation at break, were measured. The material with deca-BDE added was found to retain its mechanical properties. Inductive Coupled Plasma Mass Spectrometry (ICP-MS) was used to examine if deca-BDE leaves the HIPS material during processing and aging. The ICP investigation showed that no significant amount of deca-BDE has left the HIPS material. Matrix Assisted Laser Desorption/Ionization Mass Spectrometry was used to investigate changes of deca-BDE during processing and aging. No evidence of degradation was found. In this investigation nothing has appeared indicating that HIPS containing deca- B D Eis not well suited for recycling.

Introduction The need for fire resistant polymeric materials is continuously growing. This is partly due to the increasing use of for example, electronic equipment and polymeric building materials and partly caused by more stringent flammability requirements.

© 2001 American Chemical Society

In Fire and Polymers; Nelson, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

281


282 Decabromodiphenylether (deca-BDE) is a widely used, high performance fire retardant (FR) for thermoplastics. It also has a low toxicity and bioaccumulation (1). In Europe in recent years, there has been a general desire to restrict the use of halogen containing compounds partly based on perceived recycling difficulties. One way of reducing the displacement of halogen containing materials in the environment or decreasing the need for costly destruction of such materials is recycling. In order to be accepted as construction materials, however, mechanical, durability, aesthetical, and other properties must be retained in the recycled materials. One can, however, suspect that deca-BDE is not fully stable during aging and processing. Instead, from a durability point of view harmful degradation products like H B r might be formed. It is well known that an autocatalytic formation of HCI causes serious degradation of polyvinylchloride (PVC) (2). The suspected instability in combination with the high concentration of decaB D E normally used in HEPS might seriously restrict the possibility for recycling. The purpose of this study was, therefore, to examine i f HIPS with F R (FRHIPS) in comparison with HIPS without F R (NFR-HIPS) can be recycled without serious loss of important properties, including fire performance. HIPS with and without deca-BDE has, for that reason, been subjected to accelerated thermo-oxidative aging corresponding to a normal in-door service time of ten years and analyzed with respect to mechanical properties as well as F R retainment. Other additives like antimony oxide were not considered to be unstable in the same way as deca-BDE and should not affect the recycling potential of FRHIPS in a negative way.

Experimental Description Problems and Solution The conclusion, based on initial Differential Scanning Calorimetry (DSC), was that we could try thermo-oxidative aging at 95 °C, although the position of the glass transition region was difficult to evaluate. After a short period of aging, it became obvious that the aging temperature was too high, resulting in shrinkage and distortion of samples. A more thorough examination with D S C indicated the glass transition region to be between 87 and 95 °C. A second aging series was started at 80 °C, in order to age well below the glass transition region. If aging was performed at temperatures in the transition region or above, we would most probably introduce irrelevant degradation processes.



283 Since the aging temperature was reduced, the aging time should be increased in order to obtain the desired aging effect. The acceleration factor assumed for the thermo-oxidative aging corresponds roughly to a doubling of the aging rate for each increase of 10°C. For a simulated service time of ten years, this would lead to an aging time of about 1400 hours at 80°C for our samples. The estimation above is based on the assumption that the rubber phase in impact modified polystyrene is the most vulnerable one (3,4). According to Dixon, it is recommended to use a relatively low activation energy (60-70 kJ/mol) when calculating the acceleration factor in order not to overestimate the expected service life(5). Most likely the expected service life will be closer to twenty years then the ten years at which this study is aimed.

Materials and Sample Preparation The HIPS materials were delivered by Nova Chemicals. The stabilizer packages for both grades were normal for HIPS, according to the supplier. Both materials were extruded to about 0.2 mm thin strips by using a single screw Brabender P L E 651 extruder. The temperature profile in the extruder was 160, 180, 200, 200, and 210°C and the speed of rotation of the screw was 60 rpm. The temperature profile used is recommended by the HIPS producer and has been verified by B A S F and Dow. Thicker test specimens were also manufactured by injection molding according to ISO 294 and test specimen type I B . The following parameters were used: melt temperature 230°C, injection pressure 1000 bar, and a follow-up pressure of 700 bar. These parameters are recommended for virgin HIPS. The reason for using two different sample thicknesses is that the thick samples correspond to dimensions of normal F R containing HIPS products, while in thin samples thermo-oxidative degradation is not as restricted by oxygen diffusion as for thick samples. Consequently, degradation will be more severe in the thin samples. The degradation of a thin sample can also be regarded as corresponding to the degradation of a thin surface region of a thick sample. Moreover, loss of additives depends on the relation between exposed surface area and bulk volume. The thick samples were used for performing U L 94 testing to establish the ignition behavior of the F R and NFR-HIPS material, both before and after aging and recycling.

Aging 3

Aging of HIPS and HIPS-FR took place in 0.3 m Salvis heating cabinets. The temperature was 80 °C and the duration of aging was 1400 hours corresponding to about ten years of normal room temperature service. About 50 test specimens of each type were aged.


284 Tensile and Impact Strength Testing


A n Instron 5566 tensile tester equipped with a non-contacting Instron 2663 video extensiometer was used to acquire data for tensile stress and elongation at break according to ISO 527. Crosshead speed was 20 mm/min. The impact strength test was performed according to ISO 179 using a Zwick Impact Strength Tester. For each test above at least ten specimens were used. Mean values and standard deviations were calculated.

Determination of Total Bromine Content A Micromass Platform Inductive Coupled Plasma-Mass Spectrometer (IC MS) was used to determine the total bromine content in extruded FR-HIPS before and after aging. Ground FR-HIPS was mixed with potassium hydroxide, hydrazine sulfate, and benzoic acid. The mixture was burned in a bomb calorimeter in oxygen in order to convert all bromine compounds into bromide, which was subsequently dissolved in water. The solution was injected into the ICP instrument for total bromine content detennination. Both unaged and aged materials were investigated.

Degradation of deca-BD£ In order to investigate i f deca-BDE degrades due to aging and/or processing, pure deca-BDE and deca-BDE removed from unaged and aged FRHIPS were examined with Matrix Assisted Laser Desorption and Ionization Mass Spectrometry ( M A L D I - M S ) . The M A L D I instrument used was a Bruker BIFLEX™III. The 337 urn nitrogen U V laser has an intensity of 107 W c m and a pulse length of 5 ns at ~ 1 Hz. The mass resolution in the time-of-flight mass spectrometer is >10,000. F R was removed from HIPS by a procedure involving dissolution of the plastic in tetrahydrofuran (THF) and filtering of the solution in order to remove carbon black. The polymer and the fire retardant were separated by gel permeation chromatography (GPC). The fraction containing the F R was collected and the solvent was evaporated. For the M A L D I analysis T H F was used as solvent and picolinic acid (PA) as matrix. Irganox 1010 and Irganox 1076 were added and used for mass calibration. 2


285 Fire Classification


In order to determine whether the material being tested passed the required ignitability behavior both before and after aging and recycling, injection molded samples were subjected to classification according to U L 94. The NFR-HIPS samples were classified according to the "horizontal burn" characterization in the U L 94 standard while the FR-HIPS samples were subjected to classification according to the "vertical burn" classification described in UL94. The results are summarized in the next section.

Results Mechanical Testing The elongation at break for both the aged and unaged extruded F R - and NFR-HIPS is shown in Figure 1.

NFR-HIPS

FR-HIPS

NFR-HIPS

FR-HIPS

Figure I: Elongation at break for extruded FR-HIPS and NFR-HIPS before and after aging. The values for both NFR-HIPS and FR-HIPS have decreased after thermooxidative aging. The decrease after aging is, however, more pronounced for the NFR-HIPS.



286 The corresponding results for injection molded HIPS are shown in Figure 2. There is only a minor change due to aging for the FR-HIPS, while the N F R HIPS shows a very distinct reduction in elongation at break, i.e., greater than 50%. The explanation for the difference in behavior between extruded and injection molded test specimens (Figures 1 and 2) is probably the restriction in oxygen diffusion for thicker injection molded samples, which reduces thermooxidative degradation inside the samples. Most probably the FR-HIPS samples contain a higher % of stabilizer than the NFR-HIPS samples. The material used in this study is commercial grade and thus exact information concerning the composition is not available.

Figure 2: Elongation at break for injection molded FR-HIPS and NFRHIPS before and after aging. Although elongation at break is regarded to be more sensitive to aging of polymeric materials than tensile stress, the latter is also of interest. In many cases stress at break increases by aging. This is generally due to an increase in cross-linking of the polymer as a part of the aging process (6). The value of stress at break for extruded test specimens is shown in Figure 3 Comparing the results for FR-HIPS one finds a very small change between aged and unaged samples. For NFR-HIPS, on the other hand, there is a significant increase in stress at break indicating that some kind of degradation has taken place as a result of aging. Again, this is probably due to a higher portion of stabilizers in the FR-HIPS than in the NFR-HIPS.



287

NFR-HIPS

FR-HIPS

NFR-HIPS

FR-HIPS

Figure 3: Stress at break for injection molded FR-HIPS and NFR-HIPS before and after aging. Results for the impact strength measurements regarding injection molded HIPS are shown in Figure 4. The result is quite apparent. N o significant change is found for FR-HIPS, while a 50% reduction has occurred for NFR-HIPS.

NFR-HIPS

FR-HIPS

NFR-HIPS

FR-HIPS

Figure 4: Impact strength for injection molded FR-HIPS and NFR-HIPS before and after aging.


288 Thus, all mechanical testing performed indicate that the FR-HIPS used in this study withstands thermo-oxidative aging much better than does NFR-HIPS. It should, however, be emphasized that the reason for this result is not necessarily due to the presence of deca-BDE. A s explained above this effect is probably due to a co-additive.


Total Bromine Content

The total bromine content was determined by ICP-MS before and after aging for extruded HIPS. Thin extruded films were investigated as they were subjected to more severe degradation compared to the thicker injection molded samples. Moreover, the thin extruded films have a larger exposed surface area relative to the bulk volume compared to the thicker injection molded samples. Large surface area relative to bulk volume can cause higher emission of additives. Thus, these samples are a more severe test of the retention of the fire retardant. Table I. Total bromine content in unaged and aged F R - H I P S . 79

Aged Unaged

8 ,

Br

11.1 11.1

Br

11.1 11.1

As is seen in Table I, the bromine content is 11 mass% of deca-BDE in unaged and aged FR-HIPS. Both isotopes, B r and B r , were used in the determination and they gave the same results. Thus, no significant loss of decaB D E was observed. It must be pointed out that this investigation was focused on the durability of FR-HIPS material, i.e., on possible significant losses of deca-BDE which might affect the fire retarding properties. Although very important from an environmental point of view, possible losses of small amounts of deca-BDE to the environment during ageing or processing were outside of the scope of this investigation. 79

81

Degradation of deca-BDE Ionization in M A L D I is regarded as very "soft", i.e., only molecular ions are formed of analyte molecules. In the case of deca-BDE, however, there is a significant debromination in the ionization process. This effect is also seen in negative ion spectra where Br", Br ", and Br " ions are detected. The 2

3


289 debromination occurs both for pure deca-BDE and for deca-BDE extracted from unaged and aged FR-HIPS.

a + +


Pure DBDE

M

(M-3Brf {M-4 Br)* 1

I b

*

Br)

(M- Br)*

IV Γ

I

DBDE from injection molded HiPS

1 c DBDE from injection molded and aged H I P S

1

1 400

S00

600

700

800

900

1000

m/z Figure 6a-c: Schematic MALDI MS-spectra for pure deca-BDE, deca-BDE from unaged FR-HIPS, and deca-BDE from aged FR-HIPS, respectively. Comparing the spectra in Figure 6 one finds no significant differences, indicating that degradation of deca-BDE, other than that caused by the ionization process, does not take place. Hamm et. al. have investigated HIPS containing deca-BDE after repeated processing of the material. They found no degradation of the fire retardant (7).

Fire Classification The results of the fire classification of both aged and unaged FR- and N F R HIPS indicated that the NFR-HIPS did not retain its H B rating after aging while the FR-HIPS retained its V 0 rating. The results are summarized in Table II.

Table II. Results of U L 94 classification of F R - and N F R - H I P S strips FR-HIPS Aged Unaged

V0 V0

NFR-HIPS Failed H B HB


290


Conclusions Mechanical tests clearly show that HIPS containing deca-BDE has improved aging properties compared to the HIPS used in this study, without deca-BDE. In fact, the investigation indicates that unaged FR-HIPS has slightly better mechanical properties than unaged NFR-HIPS. There is no indication of loss of deca-BDE due to aging. N o degradation of deca-BDE caused by aging was found. Additional work should, however, be done in order to confirm this result. Importantly, the fire retardant behavior of the FR-HIPS is retained after aging. Thus, the FR-HIPS is able to pass its fire retardant properties onto a new generation of products. One can conclude, that in this investigation nothing has appeared indicating that HIPS containing deca-BDE is not well suited for recycling.

References 1.

2. 3. 4. 5. 6. 7.

Norris, J . M . ; Ehrmantraut, J.W.; Gibbons, C.L.; Kociba, R.J.; Schwetz, B . A . ; Rose, J.Q., Humiston C.G.; Jewett, G.L.; Crummett, W . B . ; Gehring, P.J.; Tirsell, J.B., Brosier, J.S. Appl. Polym. Sym. 1973, 22, 195-219. Salman, R.S.; Al-Shama’a, N . D . Polym.-Plast. Technol. Eng. 1991, 30(4), 343-349. Shimada, J.; Kabuki, Κ. J. Appl. Polym. Sci. 1988, 12, 671-685. Kelen, T., Polymer Degradation, Van Nostrand Reinhold Company, New York, 1983. Dixon, R.K., IEEE. Transaction on Elec. Insul. 1980, E1-15,No. 4, 331340. Grassie, N.; Scott, G . Polymer Degradation and Stabilisation, Chapter 4, Cambridge University Press, 1985. Hamm, S., Schulte, J., Strikkeling, M., Maulshagen, Α., Poster at Dioxin 99, Venice, 1999.


Processing and Thermo-Oxidative Aging of High-Impact Polystyrene

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