FLAME RETARDANT POLYAMIDES BACKGROUND OF THE INVENTION

This invention relates to flame retardant polyamide compositions and more particularly to such compositions containing a combination of melamine and phenol-aldehyde resins.

The use of melamine and melamine derivatives such as melamine cyanurate as flame retardant agents for polyamides has been known for many years (U.S. Patent 3,660,344 and U.S. Patent Re 30,402). In the case of polyamide 6.6, which is specifically used for its better thermal aging properties versus polyamide 6, the use of melamine as the flame retardant of choice has not been widely used commercially due to the fact that its volatility can lead to large amounts of white mold deposit and as previously noted (U.S. Patent 4,525,505) melamine can bleed from molded parts under various simulated use conditions. The alternative to melamine has been the use of melamine derivatives or its condensation products such as melamine cyanurate; but, although these have presented solutions to the melamine blooming issues, they have proved difficult to scale-up to typical commercial processes without some loss of flammability control. The volatility rate of melamine from polyamides containing it is comparable to melamine itself because of the incompatibility of melamine with polyamides.

It has now been found that phenol-aldehyde resins, such as phenol-formaldehyde resins sold under the name novolac, reduce and in some cases eliminate mold deposit and humid blooming of the melamine during compounding and molding of the polyamide compositions. Polyamide compositions containing a combination of melamine and a phenol-aldehyde resin according to the invention have the advantages of having a flame resistant rating of VO under EL-94 and retaining their physical and electrical properties.

According to the present invention, provided is a flame retardant polyamide composition which comprises:

(a) 45-95% by weight of a polyamide, preferably 65-95% by weight, and most preferably 75-90% by weight; (b) 2-30% by weight of melamine, preferably 2-15% by weight, and most preferably 5-15% by weight; and

(c) 2-30% by weight of a phenol-aldehyde resin, preferably 2-15% by weight, and most preferably 5-15% by weight; and optionally

(d) 0-50% by weight, preferably 0-30% by weight, of at least one of a reinforcing agent or filler. When a reinforcing agent and/or filler are used, they are preferably used at a concentration of 10-30% by weight.

Polyamides useful in the compositions of this invention are well known in the art and embrace those semi-crystalline and amorphous polymers having a molecular weight of at least 5000 and commonly referred to as nylons. Suitable polyamides include those described in U.S. Patent Nos.2,071,250, 2,071,251, 2,130,523, 2,130,948, 2,241,322, 3,312,966, 2,512,606, and 3,393,210. The polyamide can be produced by condensation of equimolar amounts of a saturated dicarboxylic acid containing from 4 to 12 carbon atoms with a diamine, in which the diamine contains from 4 to 14 carbon atoms. Excess diamine can be employed to provide an excess of amine end groups over carboxy end groups in the polyamide. Mixtures of polyamides can also be used, as can block polymers and copolymers.

Examples of polyamides include copolymers of polyhexamethylene adipamide (66 nylon), polyhexamethylene azellamide (69 nylon), polyhexamethylene sebacamide (610 nylon), and polyhexamethylene dodecanoamide (612 nylon), the polyamide produced by ring opening of lactams, i.e. polycaprolactam, polylauric lactam, poly-11- amino-undecanoic acid, bis (paraaminocyclohexyl) methane dodecanoamide. It is also possible to use in this invention polyamides prepared by the copolymerization of two of the above polymers or terpolymerization of the above polymers or their components, e.g., for example, an adipic, isophthalic acid hexamethylene diamine copolymer. Specifically, modified PA6.6 (particularly a 6.6-6 copolymer), PA6, PA6.10, PA6.12, PAH, PA12, PA12.12, PA6/6.6 etc. may be used. Particularly preferred because of high physical properties is a 6.6 polymer.

There are no special restrictions as to molecular weight of the polyamides. Preferable are polyamides with a relative viscosity (RV) according to ASTM D789 of 20 to 70, preferably 30-60.

Phenol-aldehyde resins are linear condensation resins which are obtained by condensing phenols such as phenol, cresol, bis-phenol A, resorcinol, or phenol-cresol and higher alkyl substitutions of them, and

aldehydes such as formaldehyde or acetaldehyde. Such resins are sold commercially under the name novolac. A phenol-formaldehyde resin is preferred.

Other suitable phenol-containing compounds from which useful resins can be produced are n- and isoalkyl phenols which contain up to 8 carbon atoms in the alkyl groups, naphthols, hydroxy diphenyls, hydroxy diphenyl ethers, hydroquinone, pyrocatechol, bis-(hydroxy-phenyl) alkanes or cycloalkanes having up to 20 carbon atoms, bis (hydroxyphenyl) sulphides (or sulfones) and hydroxy benzoic acids. The phenol-aldehyde resins can be prepared by known methods such as described in Houben-Weyl, Methoden der organischen Chemie, Vol. 14/2, 4th edition pages 274 et seq.

Cross-linked phenol-aldehyde resins can also be used if a resin of higher melt viscosity is desired. Cross-linking can be conducted in a known manner using an acid such as phosphoric acid or boric acid, PC1 ₃ , or polyfunctional epoxides or isocyanates or a formaldehyde donor.

Preferred phenol-aldehyde resins have softening temperatures in the range of 40 to 200°C, preferably above 100°C; and molecular weights (Mw) in the range of 5,000-30,000, preferably 10,000-20,000.

The flame retardant polyamide compositions can also contain conventional additives, fillers, and reinforcing agents such as lubricants, mold-release agents, stabilizers, dyes, pigments, color concentrates, flow agents, glass and organic fibers, chalk, clay, quartz, and hydrated mineral fillers such as magnesium hydroxide. Fillers and reinforcing agents, when used, are used at concentrations of 10-50% based on the weight of the composition, preferably 10-30% by weight.

The compositions of the invention are prepared by thoroughly mixing or dissolving melamine in the phenol-aldehyde resin, or the two can be added simultaneously or separately to the polyamide during a compounding step. Any of the other fillers and additives typically used in polyamide compounding, i.e., in an extruder /screw combination specifically intended for the compounding of powders and glass into thermoplastic resin, can also be added at the same time. The resulting extrudate is cooled, pelletized and dried before molding the resulting composition into shaped articles. In the examples which follow, test bars at each condition described in the UL94 test were tested in order to give a percentage of the

total which fail by igniting the cotton. This percent failure can then through experience of the manufacturing process and the variability in the test method itself be correlated with the ability to reliably make commercial flame retardant resins which consistently pass the UL94 test at VO. Relative viscosity (RV) is measured according to ASTM D-789 in

90% formic acid. The Comparative Tracking Index (CTI) is conducted in accordance with IEC 112. Weight loss (%) is carried out by Isothermal Thermogravimetric Analysis (TGA) at 280°C.

The invention can be further understood by the following examples in which parts and percentages are by weight unless indicated otherwise.

EXAMPLES Example 1

Eighty (80) parts by weight of a polyamide 66/6 copolymer (90% by weight 66/10% by weight 6 with RV 46-52) was tumble-blended with 10 parts by weight of melamine powder and 10 parts by weight of phenol- formaldehyde resin (novolac HRJ 10424 having a softening point of 120°C) together with minor amounts of process aids and lubricants typical for the compounding of polyamides. The resulting physical blend was then fed to a 28 mm twin screw extruder fitted with vacuum venting. The extruder barrel temperature settings were in the 260-270°C range with a die temperature of 270°C and screw speed of 300 rpm.

The resulting strand was cooled in water and pelletized. The resulting granules were vacuum dried at 80°C to a moisture content of less than 0.2%. The dried granules were then molded into the required test pieces using a standard thermoplastic molding machine using a melt temperature of 270°C and mold temperature of 60-70°C.

The above procedure was repeated using (B) 15 parts of melamine and 5 parts of phenol-formaldehyde resin, and (C) 15 parts of melamine, 0 parts of phenol-formaldehyde resin, and 85 parts of copolymer. The results are shown in Table 1.

Table 1

A B C

Polyamide 66/6 80 80 85

Copolymer (90/10) Melamine 10 15 15

Novolac HRJ 10424 10 5 0

UL94 Flammability at 1.6 mm V-O V-0 V-0

% Wt Loss in 10 min at 280°C 2.5 3.5 7

Blooming (Visual) After 5 Days at 70°C/100% RH None None Yes

CTI (Volts) >600 >600 >600