Surface Modification of Polymers To Achieve ... - ACS Publications

Chapter 15

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 22, 2018 | https://pubs.acs.org Publication Date: July 21, 1995 | doi: 10.1021/bk-1995-0599.ch015

Surface Modification of Polymers To Achieve Flame Retardancy Charles A. Wilkie, Xiaoxing Dong, and Masanori Suzuki

1

Department of Chemistry, Marquette University, Milwaukee, WI 53233

Methacrylic acid has been grafted onto both acrylonitrile-butadiene -styrene terpolymer or styrene-butadiene block copolymer by the anthracene sensitized photoproduction of hydroperoxides. The grafted methacrylic acid has been converted to its sodium salt by treatment with aqueous sodium hydroxide. The TGA residue that is obtained at 800°C is greater than that expected based on the starting materials. Preliminary cone calorimetry results indicate that a graft layer of sodium methacrylate increases the time to ignition and also decreases the peak heat release rate. This procedure is presented as a general approach to flame retardation for a variety of polymers.

There are numerous additives that have been used asflameretardants for a wide variety of polymers. These include materials such as alumina trihydrate, which endothermically decomposes to lose water, thus removing heat and diluting the flame; halogens, which function principally by quenching the radicals that make up the flame; and phosphorus, which may function in either the vapor phase in a fashion similar to that of the halogens or in the condensed phase by either changing the mode of degradation of the polymer or by the formation of char. These three systems describe the processes that are currently known about the functioning of flame retardants. Each of these presents its unique problems. Alumina trihydrate must be used at very high loadings and this has an adverse effect on the physical

1

Permanent address: Japan Synthetic Rubber Co., 100, Kawajiri-Cho, Yokkaichi, Mie, 510, Japan

0097-6156/95/0599-0236S12.00/0 © 1995 American Chemical Society Nelson; Fire and Polymers II ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


15.

WILKIE E T AL.

Flame Retardancy in Modified Polymers

237

properties of the polymer. The halogens are widely applicable to a variety of polymers but they may present environmental problems. Condensed phase additives change the mode of thermal degradation so that flammable gases are not produced and, instead char is obtained. These are very useful but each polymer is different and generally requires a unique additive system to achieve flame retardancy. One approach is to investigate the effect of a wide variety of additives on the thermal degradation of a particular polymer and to use that information to design a suitable flame retardant. In this laboratory, we have followed that approach for many years and have examined the effects of many different additives on the thermal degradation of poly (methyl methacrylate). The additives that have been investigated include: red phosphorus (7-2), Wilkinson's catalyst, ClRh(PPh ) (3-4), Nafions (5), copolymers of 2-sulfoethyl methacrylate and methyl methacrylate (6), a variety of transition metal halides, MnCl , CrCl , FeCl , FeCl , NiCl , CuCl , and CuCl (7-9), various phenyltin chlorides (10), tetraphenyl- and tetrachloro-tin (77), and diphenyl disulfide (Chandrasiri, J. A. and Wilkie, C. Α.; Polym. Degrad. Stab., in press). A breakthrough in flame retardancy studies could be achieved if a general approach could be devised that 1) could be used at low loadings; 2) was compatible with the environment; and 3) had the possibility of applicability to a wide variety of polymers. In pursuing these ideas, we have concluded that a surface treatment that will form an adherent, thermally insulating char layer under thermal conditions would be advantageous. To accomplish this, one needs to 1) identify a suitable char forming material and 2) develop some process for its attachment to the surface of the polymer. In several publication, McNeill (12-15) has described the thermal degradation of a variety of methacrylate polymers and has shown that substantial char is produced when these are thermally degraded. This data is shown in Table I. 3

2

2

3

2

3

3

2

TABLE I Thermal Degradation of Salts of Poly (Methacrylic Acid) Cation +

H Li Na K Cs Mg Ca Sr Ba +

+

+

+

2 +

2+

2+ 2+

Onset temperature of degradation, °C 200 400 400 400 400 200 280 320 400

% Residue at 500°C 3 54 64 66 82 31 57 61 70

Identity of Residue "C" Li C0 + C NajCC^ + C K C0 + C Cs C0 + C MgO + C CaC0 + C SrC0 + C BaC0 + C 2

2

3

3

2

Nelson; Fire and Polymers II ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

3

3

3

3


238

FIRE AND POLYMERS II

We have decided that grafting of these polymers onto the substrate polymers offers an excellent opportunity to achieve flame retardation; methacrylic acid and its sodium salt have been selected for the initial study as the char forming monomers. Grafting offers great versatility because of the variety of ways in which it may be initiated. These include various chemical initiators, photochemical initiation, and initiation by high energy radiation. Geuskens has shown that grafting may be accomplished by the anthracene-sensitized photoproduction of hydroperoxides on the substrate and the thermal degradation of these hydroperoxides in the presence of suitable monomers (16-17); this grafting approach has been adopted for this study. This paper describes our recent work in this area on flame retardation of acrylonitrile-butadiene-styrene terpolymer, ABS (Suzuki, M. and Wilkie, C. Α.; Polym. Degrad. Stab., in press), and styrenebutadiene block copolymer, SBS (Dong, X.; Geuskens, G.; and Wilkie, C. Α.; Eur. Polym. J., in press). EXPERIMENTAL The grafting reaction was carried out as previously described (77, Suzuki, M. and Wilkie, C. Α.; / . Polym. Sci.: Part A: Polym. Chem., in press). The ABS or SBS was compression molded in a heated press to obtain films of about 200 micron thickness and anthracene was permitted to migrate into these films from a methanolic solution. Irradiation of the films with a lamp that emits between 350 and 400 nm produces hydroperoxides on the butadiene portion of the polymer. The hydroperoxidized films were heated in aqueous solutions of methacrylic acid to produce a graft layer of methacrylic acid on the film. The films were then soaked in a dilute solution of aqueous sodium hydroxide to convert the acid to the sodium salt. The percent of grafting is defined as follows: M -M g

u g

Weight % Grafting =

χ 100

where M = mass of grafted sample and M g

ug

= mass of sample before grafting.

Thermogravimetric analysis was carried out on a Omnitherm TGA 1000M at a scan rate of 20°C/min to a maximum temperature of 800°C. The oxygen index was measured on a home built apparatus using bottom ignition. Cone calorimetry per ASTM Ε 1354-92 was performed using a Stanton Redcroft/PL Thermal Sciences instrument at 25 kW/m in the horizontal orientation. The 0.25 inch thick samples were mounted using the edge retainerframeand wire grid; the mass was approximately 75 grams. Exhaust flow was set at 24 L/sec. and the spark was continuous until the sample ignited. TGA-FTIR analysis was performed using a Cahn thermogravimetric analyzer coupled to a Mattson Instrument FTIR spectrometer. The evolved gases were sampled with a sniffer tube that extends into the sample cup and admits only some of the gases to the infrared spectrometer. Sample size used was about 40 mg with a heating rate of 20°C per minute and an 2


15.

WILKIE E T A L .


239

3

inert gas purge of 30-50 cm /min. Evolved gases were transferred to a heated 10 cm gas IR cell by a heated quartz tube. Spectra were identified by visual identification as well as from searching spectral data bases. RESULTS AND DISCUSSION


Three criteria have been selected to judge the efficacy of this flame retardant technique; these are: char yield in a thermogravimetric analysis experiment, cone calorimetry, and oxygen index. Thermogravimetric Results. The TGA curve for polymethacrylic acid gives a 5 % residue while a 13% residue is obtained for the polymer obtained by treatment of polymethacrylic acid with sodium hydroxide. Poly(sodium methacrylate) obtained by the homopolymerization of sodium methacrylate gives a 55% residue. This indicates that treatment of the acid with sodium hydroxide is an inefficient way to obtain the grafted sodium salt. Unfortunately it has not been possible to directly graft sodium methacrylate onto polymers at this time and the methacrylic acid followed by sodium hydroxide treatment must be followed. For a polymer which completely volatilizes in the TGA experiment and has 10 weight % grafted methacrylic acid, one expects a residue of 5% of 10% or 0.5%. If the observed residue is greater than this, this indicates that the substrate which would normally volatilize under these conditions, has been retained within the sample. TGA curves for unmodified SBS and a sample which has been grafted with 60 weight % methacrylic acid and converted to the sodium salt are shown in Figure 1 and the TGA data for grafted SBS is shown in Table II. Theoretical residues have been calculated using the residue obtained for the homopolymers. For the sodium salt, since the graft was formed by treatment of the acid with sodium hydroxide, the 13% residue obtained from the polymer prepared in this way was used. TABLE II TGA residue for MAA and NaMAA grafted SBS weight % grafted

MAA residue

theoretical MAA residue

NaMAA residue

theoretical NaMAA residue

0 10 20 30 40

0 3.1 4.6 4.3 3.9

0 .5 1.0 1.5 2.0

0 8.7 13.6 16.0 25.1

0 1.3 2.6 3.9 5.2



240

FIRE AND POLYMERS II

One immediately notices that the residues for the grafted samples are larger than the predicted values based upon the amount of grafted methacrylic acid. The residues from sodium methacrylate grafted samples show a general increase with weight % grafted methacrylic acid, indicating that as the amount of graft layer increases, the amount of protection offered to the substrate is increased and more SBS is retained by the coating. The residues for methacrylic acid grafted samples show much more variation, it is likely that this coating is less insulating than that from sodium methacrylate and thus is less retentive of substrate. Similar results have been obtained for grafted ABS samples. These results are presented in Table III.

TABLE III TGA residue for MAA and NaMAA grafted ABS

weight % grafted

MAA residue

theoretical MAA residue

NaMAA residue

theoretical NaMAA residue

0 1 10 20 30 40 50

4.0 4.3 7.3 4.4 5.1 4.0 7.6

4.0 4.0 4.4 4.8 5.2 5.6 6.0

4.0 5.3 5.8 15.0 15.2 19.8 24.4

4.0 4.1 5.3 6.6 7.9 9.2 10.5

Again the TGA residues are larger than expected from the graft layer only and must indicate that the substrate participates in the char formation. For the sodium salt case, there is an increase in the char yield with an increase in the amount of grafting. Again the acid shows much more variation and the same reason is offered, the char from the sodium salt, consisting of primarily sodium carbonate with a small amount of elemental carbon, is a better insulator and more retentive of substrate. Figure 2 presents the TGA curve for an ABS sample which contains 50 weight % sodium methacrylate as well as for unmodified ABS. The initial degradation is due to the elimination of water from the methacrylic acid. The onset of degradation occurs at the same temperature in ABS, sodium methacrylate obtained from the acid by treatment with sodium hydroxide, and in the grafted sample. Grafting does not appear to effect the onset of degradation but it does change the extent of degradation. On the whole, the TGA results indicate that there is participation of the substrate, whether SBS or ABS, in char formation. It is likely that there is some



15.

WILKIE ET A L .


241

Figure 1. TGA curves for unmodified SBS and SBS to which has been grafted 60 weight % methacrylic acid which is then converted to the sodium salt. Rate is 20°C per minute under dinitrogen. 120110100-

c α

Μ

ι

Surface Modification of Polymers To Achieve ... - ACS Publications

Recommend Documents