Phosphorus-containing rigid polyurethane foams—II. Modifiers based on trimethyl phosphate

Eur. Polym.J. Vol. 28, No. 6, pp. 689-693, 1992 Printed in Great Britain. All rights reserved

0014-3057/92 $5.00 + 0.00 Copyright © 1992 Pergamon Press Ltd

PHOSPHORUS-CONTAINING RIGID POLYURETHANE FOAMS--II. MODIFIERS BASED ON TRIMETHYL PHOSPHATE E. TASHEV,l L. ZABSKI,2 S. SHENKOVl and G. BORISSOVl t Institute of Polymers, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria 2Institute of Polymer Chemistry, Polish Academy of Sciences, Zabrze, Poland

(Received 2 September 1991) Abstract--Phosphorus-containing modifiers, alkanolammonium salts, obtained by interaction of trimethyl phosphate and aminoalcohols, are used to obtain rigid polyurethane foams (PUFs) with various phosphorus contents. Some of their physical mechanical properties and resistance to combustion are examined. It is shown that these phosphorus-containing modifiers are effective additives for improving fire resistance of rigid PUFs. For a low concentration of phosphorus-containing modifier, the rigid PUFs have high fire resistance and good physical and thermomechanical properties.

INTRODUCTION Rigid polyurethane foams ( P U F ) a r e of considerable interest because of their excellent heat insulating properties and good physical mechanical indices. However, their high intrinsic surface contributes significantly to their combustibility, which is a considerable drawback and renders efforts to improve the resistance to combustion of special importance, One way to improve the properties of polymers and polymeric materials is modification by using phosphorus-containing monomers and oligomers. There are many phosphorus-containing modifiers used as fire retardants [1-8] but few of them have been applied industrially [9], mainly because the raw materials necessary for synthesis are not readily available and are too difficult to produce industrially, An important advantage of the modification of rigid P U F s with phosphorus-containing reaction additives is the inclusion of these substances in the polyurethane chemical structure; they do not diffuse towards the surface of the polymeric materials and remain more effective for longer periods [3, 10]. Their use is also favoured by the fact that the structure of the rigid P U F s , as opposed to the elastic materials, easily undergoes modification without detrimental effects. The present work refers to an investigation of the influence on some properties of the rigid PUFs, mainly on the resistance to combustion, of phosphorus-containing modifiers synthesized on the basis of trimethyl aminoalcohols,

phosphate

by

alkylation

of

EXPERIMENTAL PROCEDURES

Reagents Alkanolammonium salts of trimethyl phosphate with triethanolamine (P-3), N-methyl diethanolamine (P-4) and diethanolamine (P-5) were prepared by alkylation of the aminoalcohols with trimethyl phosphate, by the same procedure as the alkylammonium salts of dialkyl hydrogen phosphonates [1 I].

The starting materials, viz. diethanolamine, N-methyl diethanolamine, triethanolamine and trimethyl phosphate (Fluka), were purified by vacuum distillation. The polyols, Propylan RF66 (polyoxypropylenated sorbitol) and Propylan RF55 (a mixture of polyoxypropylenated aniline formaldehyde resin and triethanolamine) were obtained from Lankro Chemicals Ltd (U.K.); Fyrol 6 [diethyI-N,Nbis(2-hydroxyethyl)aminoethyl phosphonate, was a cornmercial product of Stauffer Chemical Co. (U.S.A.). The properties of the polyols and phosphorus-containing modifiers and their chemical structures are listed in Table I. For preparing rigid PUFs polymeric MDI-Desmodur 44V20 (Bayer AG, Germany), the catalyst Dabco R8020 (20% solution of triethylenediamine in N,Ndimethylethanolamine) (AIR Products and Chemicals Inc., U.S.A.), Silicon oil L5340 (Union Carbide Corp., U.S.A.) and foaming agent Arcton 11 (CFC13) (ICI Ltd, England) were also used.

Preparationof rigid polyurethane foams The rigid PUFs were prepared by the so-called "oneshot" procedure. All starting materials were mixed vigorously (the quantities are listed in Table 2) and the mixture was poured into a container with dimensions 18 x 18 x 22 cm. The PUFs were kept at 60 ° for 24 hr and, after bringing to room temperature, were cut into samples for the various studies.

Investigationof the rigid polyurethane foams Thermomechanicalproperties. The softening points of the rigid PUFs were determined from thermomechanical measurements according to DIN 53424-1964 Standard. The softening point (Ts) so determined was the temperature corresponding to a 10% deformation of a foam sample under a load of 0.0245 MPa (0.25 kg/cm~) and a rate of heating 50°/hr on a Vicat apparatus (Germany). The specimens had dimensions 4 x 4 x 2 cm. Compressive strength. The determination of the compressive strength of the rigid PUFs was performed on a FU 1000¢instrument (Germany) using samples with dimensions of 5 x 5 x 5 crn. The compression both parallel and perpendicular to the growth of the PUF causing a 10% deformation was determined. Resistance to combustion. The resistance to combustion of the rigid PUFs was studied by both the limiting oxygen 689

690

E. TASHEVet al. Table I. Characteristics of polyols and modifiers Compound Propylan RE55

Chemical formula H (X)~OCH2-~CH------~H20(X)n n

Propylan RF66

OH No. (rag OH/g)

P (%)

Funct.

495

--

6

495

-

3,65

+ N[CHECH20(X)nH]3 Fyrol 6

(C2 H30)2--P(O) CH2 N (CH2CH2 OH)2

440

12.16

2

P-3

(CH30)2P(O) O- N+ (CH2 CH20H)3

581

10.73

3

P-4

CH3 (CH3 O)2P(O) O- N+ (CH2CH2 OH)2

432

11.97

2

P-5

(CH30)2 P(O) O- N + (CH2 CH2 OH)2

685

12.65

3

I

CH~3 ~'H 3

/

\

H

CH 3

*X = --CH2---CH--O--

Table 2. Generalprescription for preparing rigid polyurethane foams PUFs Composition of starting mixture Phosphorus-containing modifiers

% by weight

x 30 70-x 1.5-2.0 0.5-2.0 29-35 121-135

polyol propylan RF 66 polyol propylan RF 55 Silicone L 5340 Catalyst Dabco R 8020 Foaming agent Arcton 11 Desmodur 44V20(NCO/OH= 1.05)

index (LOI) method (ASTM D-2863) and flame propagation by ASTM D-1692. The LOIs were determined on a module FTA instrument (Stanton Redcroft, U.K.) using samples with dimensions of 15 x 1.3 x 1.3 cm. Samples with dimensions of 15 x 5 x 1.3 cm were used for flame propagation evaluation [1]. RESULTS AND DISCUSSION

It was interesting to study the properties of rigid PUFs modified with alkanolammonium salts of trialkyl phosphates and comparing the results with those obtained using the industrial antipyrene Fyrol 6. The phosphorus-containing modifiers used by us were obtained by alkylation [11-13] of triethanolamine, N-methyl diethanolamine and diethanolamine with trimethyl phosphate,

In our paper [12] we established that the main products of the reaction of aminoalcohols with dialkyl hydrogen phosphonates at temperatures below 90 ° were alkanolammonium salts. The modifiers P-4 and Fyrol 6 are phosphorus-containing compounds with two functional groups (bydroxyl), P-3 and P-5 with three (either three hydroxyl or two hydroxyl and one NH respectively). By introduction of these modifiers into the polymer, the phosphorus atom is on the side of the main chain. P-3, P-4 and P-5 are bonded to the polymer with a salt connection, while Fyrol 6 is covalently bound. The modifiers were introduced into the rigid PUF compositions up to 20% by wt instead of Propylan RF55, according to the standard procedure (Table 2). For each individual phosphorus-containing modifier, various PUFs with different phosphorus contents were obtained. Also rigid PUFs with Fyrol 6 as modifier were obtained for comparing the properties of the newly synthesized polyurethanes and mainly for comparing the fire resistance. Depending on the amount of the flame retardant present in the polyol mixture, various amounts of the Dabeo R8020 catalyst were used. The studied modifiers contain a quaternary nitrogen atom loaded positively, which possesses certain catalytic activity [14-18]. Actually, the times for foaming, gel formation, surface drying O II

(CH30)3~

+ N(CH2CH2OH)2

) ( C H 3 0 ) 2 - - ~ - - O - N + (CH2CH2OH)2

I~ where R = H (P-5); CH3 (P-4); CH2CH2OH (P-3). The reaction proceeded at 85-90 ° without by-products, and so is very suitable for industrial production,

g/

~CH 3

and foam growth are decreased by increasing the amount of the modifier even with reduction of the amount of catalyst. Only for modifier P-5 up to

Phosphorus-containing rigid PUFs--II

No. I

Table 3. Characteristicsof foaming of the rigid polyurethanefoams PUFs Time for: Gel Surface Foam % by wet Foaming formation d r y i n g growth Density modifier P(%) (sec) (see) (see) (sec) (kg/m3) -0 14 70 90 90 29.2

P-3 2

5

0.23

14

48

60

65

29.9

3 4 5

I0 15 20

0.47 0.70 0.93

15 15 14

49 45 45

60 55 55

67 65 65

30.0 29.3 30.5

P-4 6 7 8 9

5 10 15 20

0.26 0.53 0.80 1.08

14 13 13 13

52 42 39 37

62 49 45 45

70 57 52 50

30.0 30.5 30.9 32.0

P-5 10 11 12 13

5 10 15 20

0.28 0.55 0.80 1.08

16 16 15 I1

103 80 73 60

132 105 94 78

145 110 110 85

29.0 28.8 29.3 29.0

15% by wt was there some increase of the times (Table 3). The density of the rigid PUFs is practically constant, i.e. 30 + 2 kg/m 3 (Table 3). The heat stability of PUFs expressed by the softening point (Ts) slightly increases by using a modifier (Fig. 1). The effect can be explained by the presence of an ionogenic group in the molecule of the modifier, causing better electrostatic interactions between macromolecules and, as a result, an increase of the softening point, The presence of modifier (phosphorus) into the polyurethane composition influences also the mechanical properties, i.e. compressive strength. Figure 2 shows the relationship between the compressive strength and phosphorus content. The modifiers P-3, P-4and P-5 increase the compressive strength parallel to the growth of the foam. For example, the compressive strength of an unmodified sample is 218 kPa and a sample modified with 20% by wt P-5 is 248.1 kPa. For P-3 and P-4, there is a maximum at about 10% by wt of modifier. The compressive strength perpendicular to the growth is also increased by P-4 and P-5, while it is slightly decreased by P-3. It can be summarized that introduction of phosphrous-containing modifiers based on alkanolammonium salts of trialkyl phosphates does not worsen the mechanical c:~ •=

180 -

t-®

691

o

and thermomechanical properties of the rigid PUFs; on the contrary, they slightly improve them. By using modifiers P-1 and P-2 on the basis of dimethyl hydrogen phosphonate [18], there was a slight decrease of compressive strength both parallel and perpendicular to the growth of the foam, while the modifiers P-3, P-4 and P-5 increase the compressive strength. Probably, the presence of a P-H group in the modifier P-i and P-2 has a negative effect. The rigid PUFs modified with alkanolammonium salts of trimethyl phosphate exhibit increased resistance to combustion. The relationship between flame propagation (determined as the length of the burntup part of the sample) and the phosphorus content is depicted in Fig. 3. According to the test (ASTM D-1692), the modified polyurethanes are classified as polymers with self-extinguishing properties (SE). From the character of the curves, it can be seen that the modifiers P-3, P-4 and P-5 are more efficient as burning retardation agents than the industrially used Fyrol 6 (curve 4). The difference in efficiency becomes even clearer on examining the data from the studies on the flammability of the PUFs carried out by the LOI method (ASTM D-2863). As can be seen from the

(1) + P-3

~"

(2) o P-4

~

(3) • P - 5

e-

Perpendicular (4) [] P-3 (5) • P-4

(2) o P-4 300 -

~, 2s0

g

Parallel

(1) + P-3 ( 3 ) , P-5

.~---'-a~--=+

(6) zx P-5 0

';

~-+-~-

_; 200

"°~ 170 ~'~.'_ = F.~~

z

• 150 -._> ~

1oo ~ ' ~ n

'S

u

-~-

JJ

_~.

,,.-,

E t-

16o

I

I

I

0.4

0.8

1.2

C o n t e n t of p h o s p h o r u s (%)

Fig. 1. Relationship between the content of phosphorus in the rigid PUFs and their thermomechanical properties expressed by the temperature of softening (DIN 53424).

~~

o

50 0

I

I

I

0,4

0.8

1.2

C o n t e n t of p h o s p h o r u s (%)

Fig. 2. Relationship between the content of phosphorus in the rigid PUFs and their compressive strength (parallel and perpendicular to the growth).

692

E. TASHEVet al. ,~

Table 4. ALe1 A + B .In(l + Cp) =

15

(1) + P-3

o. 10 -'1

.c: ~,

" ®

5 0

~

(2) 0 P-4

~x'~

-(3)A 5 (•3 )P-5

")-t~

0

x"x.~ I

0.4

~ I

0.8

Modifier P-3 P-4 P-5

0.0919 0.I 137 0.1485

B

5.1165 4.9636 4.8516

Cc

0.994l 0.9936 0.9925

SEE 0.1952 0.1758 0.1998

A and B are coefficients;Cc is a correlationcoefficient;SEE is the standard error of the determination.

4 1

A

I

1.2

Wang computer. Cc is a correlation coefficient, SEE is the standard error of the determination. The "~ Content of phosphorus (%) coefficient B, which can also be called a "coefficient Fig. 3. Phosphorus content of rigid PUFs vs flammability of efficiency", is of special importance in the evalu(given as the length of the burnt-up part, ASTM D-1692) ation of a given flame retardant. Comparison of the relationship. B coefficients for the modified rigid PUFs indicates that the efficiency of the phosphorus in the alkylated aminoalcohols with trimethyl phosphate is greater (B Fig. 4, LOI of the PUFs increases with increase of the is about 5) than that of Fyrol 6 (B is 3.04). This phosphorus content or at higher modifier content difference probably depends on the structure of the respectively. Above a certain amount of the latter, the compounds used. The modifiers P-3, P-4 and P-5 are curves become slightly sloped tending towards a alkanolammonium salts of trimethyl phosphate; limiting value. Similar observations have been re- Fyrol 6 is diethyl-N,N-bis (hydroxyethyl) aminoported in other studies and ascribed to char for- methyl phosphonate. The phosphorus moiety in it is marion catalysed by the phosphorus, while the LOI bonded covalently while, in the other modifiers, there of pure carbon [19] is about 36. It may be seen from is a salt connection and the presence of that ionogenic the curves that the modifiers possess greater efficiency group affects the properties of the modified polythan Fyrol 6. At 0.4% phosphorus content, LOI of urethanes. the PUFs modified with P-3, P-4 and P-5 are about 0.5 higher than that of Fyrol 6. The increase of LOI with increase of phosphorus content in the foam is in CONCLUSION agreement with the recently established relationship Rigid PUFs are obtained using as modifiers phosbetween LOI and the amount of phosphorus in rigid phorus and nitrogen-containing compounds viz. PUFs [20], and the linear character of the dependency alkylammonium salts of aminoalcohols (trican be described by the equation: ethanolamine, N-methyldiethanolamine and diALOI = A + B . l n ( l + Cp) ethanolamine) with trimethyl phosphate, and also Fyrol 6. It is shown that the modifiers P-3, P-4 and where P-5 are suitable reaction additives for rigid PUFs ALOI = L O I - ( L O I ) 0 . improving the fire resistance and some physical mechanical properties. These phosphorus-containing LOI is the oxygen index of the phosphorus-con- modifiers are more effective than the industrially used raining PUF (in % O2), (LOI)0 refers to the standard antipyren Fyrol 6. Comparison with the modifiers P-I PUF not containing phosphorus, Cp is the phos- and P-2(alkylammoniumsalts of aminoaicoholswith phorus content of the PUF (wt %), A and B are dimethyl hydrogen phosphonate)shows that P-3, P-4 constants, and P-5 are more effective. This equation has been compared [20] with that suggested by Kresta and Frish [19]; it provides a better correlation with the experimental data in the REFERENCES regions of lower phosphorus content in PUFs. 1. C. I. Hilado. Flammability Handbook for Plastics, 3rd The values for A and B in the equation (Table 4) Edn. Technomic, U.S.A. (1982). are determined by the least squares method using a 2. J. W. Lyons. The Chemistry and Uses o f Fire Retardants. Wiley-Interscience, New York (1970). (1) + P-3 3. K. C. Frisch and S. L. Reegen. Relationship between chemical structure and flammability resistance of polyx 24 - (2) o P-4 urethanes. In Flame Retardant Polymeric Materials. 1 +"~ (3) • P-5 ~......_,~.A-39 Plenum Press, New York (1975). .--q 23 - (4) v ~ " 4. A. J. Papa. Ind. Engng Chem. Prod. Res. Dev. 9, 478 '~~__~__~ 4 (1970). ® 22 >, - 5. K. Troev, K. Todorov and G. Borissov. J. appl. Polym. X o 21 ~ Sci. 29, 1701 (1984). 6. R. H. Rosenberg and R. S. Cooper. U.S. Pat. 3 294 710 •.ft, 20 (1966). m -~ 19 I i I 7. T. M. Beck and E. N. Walsh. U.S. Pat. 3 076 010 (1963). 0 0.4 0.8 1.2 8. K. Ashida, F. Yamauchi, M. Katoh and Y. Harada. J. Cell. Plast. 10, 181 (1974). Content of phosphorus (%) 9. U.S. Pat. 3 235 517 (1966). Fig. 4. Phosphorus content of rigid PUFs vs LOI (ASTM I0. H. E. Stepniczka. J. Fire Flammability, Fire Retard. D-2863) relationship. Chem. 1, 61 (1974).

Phosphorus-containing rigid PUFs--II 1I. E. Tashev, S. Shenkov, K. Troev and G. Borissov. Authorship certificate Bulgaria, 39 876 (1986). 12. E. Tashev, S. Shenkov, M. Troev and G. Borissov. Phosphorus Sulfur 56, 225 (1991). 13. N. T. Thuong. Bull. Soc. Chim. France 3, 9281 (1971). 14. Jpn. Kokai 63 265 909 (1988); Chem. Abstr. 111, 78799x (1989). 15. S. Arai, Y. Tamano and D. W. Lowe. J. Cell. Plast. 24, 284 (1988). 16. E. Ceausescu, C. Sdrul, M. Pirlog, L. Flerascu, A.

17. 18. 19. 20.

693

Hintz, L. Cornea, N. Lucaciu and M. Patru. Rev. Roum. Chim. 34, 437 (1989). B. Davies. Pigm. Resin Techn. 17, 2 (1988). E. Tashev, S. Shenkov, K. Troev, G. Borissov, L. Zabski and Z. Edlinski. Eur. Polym. J. 7,4, I101 (1988). I. E. Kresta and K. C. Frish. J. Cell. Plast. 11, 68 (1975). L. Zabski, W. Walczyk, D. Weleda and Z. Edlinski. Nehorlavost Polymernych Materialov, Vol. 1, p. 27. Dom Techniky CSVTS, Bratislava (1980).