Background of the invention

The demand for polyamide (PA) is expected to grow in view of the importance of applications such as power distribution, industrial control, insulation systems and connectivity systems. Thus, a need exists for polyamides, either unfilled or reinforced, e.g., with glass fibers (GF) , which show reduced flammability alongside good electrical and mechanical properties. We use the notation PA/GF _X (for example, PA 6, 6/GFso) to indicate a polyamide that is filled with glass fibers and the concentration of the reinforcing filler, e.g., 30% by weight.

Flammability characteristics of plastics are often quantified by a method specified by Underwriter Laboratories standard UL 94, where an open flame is applied to the lowermost edge of a vertically mounted test specimen made of the tested polymer formulation. The specimens used in the UL 94 test method vary in thickness (typical thicknesses are ~3.2 mm, ~1.6 mm, ~0.8 mm and ~0.4mm) . During the test, various features of the flammability of the test specimens are recorded. Then, according to the classification requirements, the polymer formulation is assigned with either V-0, V-l or V-2 rating at the measured thickness of the test specimen. Polymer formulation assigned with the V-0 rating is the least flammable. In the UL-94 burning test, the thinner the specimens are, the longer the burning time.

One approach towards reducing the flammability of polyamides, to achieve UL 94 V-0 ratings at 0.4-0.8 mm wall thickness, is by the addition of a brominated flame retardant. Generally, the performance of brominated flame retardants depends heavily on the inclusion of antimony trioxide (Sb2Os) in the flammable polymer. Antimony trioxide acts as a synergist to enhance the action of the brominated flame retardant. Hence, Sb20s often accompanies the brominated flame retardant in many polymeric formulations, and polyamide is no exception. Such polyamide formulations, that are described for example by Weil and Levchik in "Current Practice and Recent Commercial Developments in Flame Retardancy of Polyamides" (Journal of Fire Science, Vol. 22, 2004, p.251-263) show that flame retardation of polyamides with the brominated flame retardant /Sb2O3 system is quite efficient, with filled polyamides (e.g., PA 6, 6/GFso) achieving UL-94 V- 0/0.8 mm rating at a reasonable loading of the flame retardant system.

However, the electrical properties of bromine-added polyamides are often not satisfactory, e.g., to serve as insulating materials with high tracking resistance that are required for many applications. The electrical breakdown property is quantified using the Comparative Tracking Index (CTI) , which is a measure of the susceptibility of solid insulating materials to surface tracking when exposed to an electrolyte under the application of voltage. CTI indicates the voltage which causes the sample to undergo carbonization. CTI is measured according to IEC 60112 with standard 0.1% NH4CI electrolyte solution; see experimental section below. The higher the CTI the better the resistance of the sample to electrical discharge etc. The maximum voltage applied in the IEC 60112 method is 600 V. But many commercial polyamide grades, e.g., PA 6, 6/GF2o-3o, that are formulated with brominated flame retardants to get UL 94 V-0 rating at 0.4 or 0.8 mm, possess relatively low CTI, i.e., of less than 400 V (https://docplayer.net/163527753-For-the- electrical-electronic-markets.html; see Chart 2) .

It was recently shown in US 2020/0115551 that unfilled polyamide can be formulated with a brominated flame retardant to achieve UL 94 V-0 rating at a thickness as low as 0.4 mm in the absence of Sb ₂ O ₃ , reaching CTI of 550 V. However, to offset the omission of the powerful Sb ₂ O ₃ synergist, a non-halogenated flame retardant was added to the polyamide (Examples 3-5 of US 2020/0115551) .

Thus, there exists a need for a bromine-based flame-retardant system that can be added to polyamide, to effectively reduce the flammability of polyamide formulations, especially filled polyamide grades, such as PA 6, 6/GFIS-35 formulations, to achieve UL 94 V-0 at 0.8 mm thickness or less, and at the same time enhance the CTI of the polyamide.

The invention

Experimental work reported below shows that the use of a brominated flame retardant (BFR) in combination with boron phosphate (BPO4) confers the desired flammability and electrical properties on polyamides. Notably, polyamides such as PA 6, 6/GF3o can be flame retarded effectively with the aid of the abovementioned combination in the absence of antimony trioxide. The BFR/BPO4 combination performs well at reasonable concentrations in polyamides to achieve UL 94 V-0 rating, comparable to the concentrations of the BFR/Sb ₂ O3 combination that are recommended for the same purpose. The difference is seen in the electrical properties; the experimental results shown below indicate that polyamide formulations of the invention reach CTI as high as 600 V.

Accordingly, the invention is primarily directed to a polyamide composition comprising a polyamide, a brominated flame retardant and boron phosphate (or boron pyrophosphate, or boron polyphosphate) . The invention also relates to a method of reducing the flammability and enhancing the comparative tracking index of a polyamide to achieve UL 94 V-0 rating and reach 500 < CTI < 600 V, e.g., 550 < CTI < 600 V, which method comprises adding a brominated flame retardant and boron phosphate to the polyamide .

The polyamides for use in the invention are of course commercially available and can be prepared by well-known methods, e.g., polycondensation reactions of dibasic carboxylic acids and diamines (or amino-carboxylic acid) and ring-opening polymerization of lactams, that are described, for example, in Nylon Plastics Handbook M., ed. I. Kohan, 1995, Munich, Hanser.

The monomers used in the preparation of polyamides by polycondensation reactions are aliphatic and/or aromatic dicarboxylic acids and aliphatic and/or aromatic diamines, hence the polyamides may be wholly aliphatic/cycloaliphatic, wholly aromatic or mixed aliphatic-aromatic. Major dicarboxylic acid monomers include adipic acid, 2,2,4 and 2 , 4 , 4-trimethyl adipic acid, azelaic acid, sebacic acid, 1 , 1 O-decanedicarboxylic acid, 1 , 12-dodecanedioic acid, isophthalic acid and terephthalic acid. Major diamine monomers include hexamethylenediamine, tetramethylenediamine, 2,2,4- trimethyl hexamethylenediamine, 2 , 4 , 4-trimethyl hexamethylenediamine, 1 , 4-diaminobutane, 1,12- dodecanediamine and xylylenediamine . As an example of aminocarboxylic acids, 11-amino undecanoic acid could be mentioned.

For example, aliphatic polyamides obtained by polycondensation reactions of dibasic acid HO2C (CH2) xCChH and diamine NH2(CH2) _y NH2 can be used in the invention. PA 6,6 is produced this way (x=4, y=6, i.e., from adipic acid and hexamethylenediamine; namely, poly (hexamethylene-adipamide ) .

As mentioned above, another commercially important process to obtain aliphatic polyamides is the ring-opening polymerization of lactams, such as the polymerization of caprolactam to give PA 6 (polycaprolactam) , which takes place in water with cationic or anionic initiator. Other useful lactam monomers besides caprolactam include capryllactam and lauryllactam.

Thus, specific polyamides that can be used in the invention, obtainable by the synthetic methods set out above, are aliphatic polyamides such as PA 6, 6, PA 6, PA 6,9, PA 6,10, PA 6,12, PA 11, PA 12, PA 12,12, PA 4, 6 and partially aromatic polyamides such as polyxylylene adipamide and polyphthalamides formed by the reaction of ferephthalic acid/ Isophthalic acid with aliphatic diamines, such as hexamethylene diamine.

The concentration of the polyamide in the composition of the invention is from 30 to 80 % by weight, e.g., in filled grades it may vary in the range from 30 to 60% and in unfilled grades it is usually from 60 to 80% (based on the total weight of the composition) . The term polyamides, as used herein, includes copolyamides, such as those described in length in US 2020/0115551, e.g., the polymerization product of diamine (for example, hexamethylenediamine) , dibasic acid (for example, adipic acid) and a monomer capable of self-condensation (for example, caprolactam) to form a co-polyamide such as PA 6, 6/6. Polyamide blends, obtained for example by melt-mixing in an extruder two types of polyamides, such as PA 6, 6 and PA 6, to give PA 6, 6 + PA 6 blend (PA 6, 6 is usually the predominant component) can also benefit from the invention.

The brominated flame retardant added to the polyamide is preferably a bromine-containing polymer. Different types of bromine-containing polymers can be used, for example: i) Brominated polystyrene, having the following formula: (n=degree of polymerization; m= 1,2, 3, 4 or 5)

Brominated polystyrene is prepared by methods known in the art (see US 4,879,353 and US 5,532,322) . Suitable grades have weight average molecular weight in the range of about 500, 000-600,000, with bromine content preferably exceeding 60 or even 65 % by weight e.g., 66% (that is, average of 2-3 bromine atoms per aromatic ring in the polymer backbone chain) . Such brominated polymers, in the form of a free-flowing powder or in granular form, are available on the market, e.g., from ICL-IP (FR 803P) . ii) Poly (pentabromobenzyl acrylate) , represented by the following formula:

(n=degree of polymerization)

The polymer (abbreviated PBBPA) is produced by polymerizing the corresponding monomer pentabromobenzyl acrylate, either in bulk (in an extruder at a temperature in the range from 120°C to 290°C as described in US 4,996,276) , or in solution, see US 4,128,709 or 6,028,156. The polymer is also available on the marketplace, being sold by ICL-IP (FR-1025) . iii) brominated epoxy resins and end-capped derivatives thereof, represented by the Formula (I) :

(m=degree of polymerization) ; wherein Ri and R2 are independently selected from the group consisting of the following:

Preferred are epoxy-terminated resins (that is, resin having glycidyl end groups) represented by Formula (la) : with weight-average molar mass between 2500 and 30000, preferably from 15000 to 25000. The epoxy resins are obtainable by reacting tetrabromobisphenol A with epichlorohydrin and then further reacting with bisphenol A. Commercially available examples include F-2100 from ICL-IP, with molecular weight of about 20,000 or F-3100 (end-capped) with molecular weight of about 15, 000. Boron phosphate (BPO4) for use in the invention is available on the marketplace from various suppliers in a powder form and can be synthesized by the reaction of equimolar amounts of phosphoric acid (H3PO4) and boric acid (H3BO3) as described, for example, in US 2,200,734 and Acta Chim. Slov. 1999 46(2) pp . 161-171. Boron phosphate is used commercially as a catalyst and has been considered as a slow-release boron fertilizer (US 3, 655,357) . Boron phosphate has been also mentioned as an additive for thermoplastics and was incorporated into a few polyamide- containing composition (see EP 714951) , but it appears that hitherto its combined action as an adjunct to brominated flame retardants along with CTI enhancement in polyamides has not been reported. CTI-enhancing amount of boron phosphate is added to the polyamide composition, for example, from 4 to 10% by weight based on the total weight of the composition.

Both unfilled and filled polyamides could benefit from the invention. The addition of fillers improves the mechanical properties of the polymer. Filler loading in polyamide formulation may be from 1 to 50%, e.g., from 5 to 40%, based on the total weight of the polyamide formulation. One important type of filler consists of reinforcing glass fibers, which are often pre-coated by manufacturers to improve their compatibility with the polymer in question (e.g., polyamide in the present case; for example, a silane coating) . The major constituents of glass fibers applied for reinforcing polyamides intended for use in electrical devices are alumino-borosilicates ; such type of glass in known as E-glass. The glass fibers are in the form of filaments with diameter in the range from Ip to 30p, and are applied in the form of chopped strands with length in the range from 2 to 10 mm, e.g., from 3 to 4.5 mm. The concentration of the glass fibers is usually from 5% to 40%, e.g., from 10% to 40%, preferably from 20% to 40%, e.g., from 25% to 35% by weight, based on the total weight of the polyamide composition. Preferred compositions of the invention are therefore designated PA/GF20-40 and PA/GF25-35, such as PA/GF25 and PA/GF30. Other common secondary fillers include glass microspheres and minerals such as talc, mica, and clay.

Bromine content of commercially available flame retardants vary from ~50 to 80% by weight. The bromine content of the brominated flame retardants mentioned above (FR-803P, FR-1025 and F-2100) are Br FR-803P= 6%, Br FR-IO25=71% and Br F-21OO=52% . It is convenient to express the loading of a brominated flame retardant in a flammable polymer by reporting the amount of bromine supplied by the flame retardant to the polymer. The amount of bromine added to a flammable polymer upon incorporation of a brominated flame retardant is calculated by multiplying the bromine content of the flame retardant by the weight concentration of the flame retardant added to the polymer composition.

The experimental results reported below show that to achieve satisfactory reduction in flammability and appreciable CTI enhancement in filled polyamide such as glass reinforced polyamide, the total concentration of the [bromine/BPCh ] in the polyamide formulation is from 18 to 30% (20 to 25%) by weight and the combination is proportioned 3:1 to 2:1, e.g., about 2.5:1 (bromine: BPO4) . That is, the concentration of bromine is adjusted in the range from 10 to 20%, e.g., 13 to 20%, e.g., 14 to 18% by weight, with not less than 5% by weight of boron phosphate, e.g., from 6 to 10% by weight, based on the total weight of the composition.

For example, when brominated polystyrene is used as a flame retardant it is added at a concentration of not less than 21%, e.g., from 22 to 26% by weight (based on the total weight of the polyamide composition) , to supply the required bromine loading (>13%) ; BPO4 is added at a concentration of 5 to 8%. As mentioned above, the BFR/BPO4 combination does not need the help of antimony-containing synergists. The composition of the invention is substantially antimony (e.g., Sb2O3)-free. By "substantially Sb2O3-free" is meant that the concentration of antimony trioxide in the composition is well below the acceptable amount used in plastic composites in conjunction with halogenated additives, e.g., not more 1.0% by weight, more preferably, up 0.5% by weight, e.g., 0.0-0.3% by weight (based on the total weight of the composition) . Most preferably, the compositions of the invention are totally devoid of antimony trioxide .

The compositions of the invention also include one or more antidripping agents such as polytetrafluoroethylene (abbreviated PTFE) in a preferred amount between 0.1 and 1.0 wt.%, more preferably between 0.3 and 0.7 wt.%, based on the total weight of the composition. PTFE is described, for example, in US 6, 503, 988.

For example, compositions comprising: from 30 to 50% by weight of a polyamide (e.g., 30-45%) , for example, PA 6, 6; from 10 to 35% by weight of a reinforcing filler (e.g., 25-35%) , for example glass fibers; from 10 to 20% by weight of bromine (e.g., 13 to 20%, 14 to 18%) , supplied by a corresponding amount of bromine-containing polymer selected from the group consisting of brominated polystyrene, poly (pentabromobenzyl acrylate) and brominated epoxy resin of Formula (Ta) ; from 4 to 10% of boron phosphate (e.g., 5 to 8%) ; from 0.1 to 1.0 % (e.g., 0.3 to 0.7%) by weight of PTFE; and from 1 to 5% (e.g., 2 to 4%) by weight talc; were shown to possess especially good flammability and electrical properties. That is, when compositions of the invention are tested in accordance with the UL-94 flammability test, at a thickness equal to 0.8 mm (or even 0.4 mm) , V-0 rating is measured; and when tested in accordance with IEC 60112 standard using 0.1% by weight NH4CI solution and platinum electrodes, CTI of 500 V or more (e.g., 500 < CTI < 600, 550 < CTI < 600 V) is measured .

Apart from the polyamide matrix, the reinforcing amount of glass fibers, the combination BFR/BPO4, an anti-dripping agent and a secondary mineral filler such as talc, the composition of this invention may further contain conventional additives, such as UV stabilizers, processing aids, antioxidants, lubricants, colorants, impact modifiers and the like. The total concentration of these auxiliary additives is typically not more than 3 % by weight, based on the total weight of the composition.

The preparation of the polyamide compositions of the invention (i.e., the blending of the polyamide and the other constituents) , and the shaping of the compositions into articles may be carried out by conventional methods such as melt-mixing (to form the blend) and injection molding or extrusion molding (to transform the blend into the final article) .

For example, the polyamide compositions are compounded by meltmixing the components, e.g., in a co-kneader or twin-screw extruder, wherein the mixing temperature, for most polyamides, is in the range from 250 to 350°C. It is possible to feed all the ingredients to the extrusion throat together, but it is generally preferred to first dry-mix some of the components, and then to introduce the dry blend into the main feed port of the extruder, with one or more of the ingredients being optionally added downstream. For example, the polyamide, the bromine containing flame retardant, boron phosphate, PTFE and talc and one or more of the conventional additives are dry blended and the blend is fed to the extruder throat, followed by the addition of glass fibers downstream. Process parameters such as barrel temperature, melt temperature and screw speed are described in more detail in the examples that follow.

The resultant extrudates are comminuted into pellets. The dried pellets are suitable for feed to an article shaping process, e.g., injection molding and extrusion molding. Polyamides are readily processable by injection molding. For example, after they have been dried to minimize moisture, PA 6, 6 and PA 6 are injection molded at melt temperature in the range from 250 to 290 °C, respectively, and mold temperature is from 70 to 90 °C. Process parameters are described in more detail in the examples that follow.

Articles molded from the polyamide compositions, especially thin-walled parts, form another aspect of the invention. Specific examples of articles (e.g., made of PA 6, 6 or PA 6) include electric and electronic parts, in applications of power distribution (such as circuit breakers, domestic/industrial switches and sockets, contactor housing, refrigerator electronic power module housing) and insulation systems (such as coil bobbins, electrical engine insulation parts, stator housing and stator coils) ; connectivity systems and cable ties.

The combination consisting of BFR/BPO4 was shown to be effective in reducing the flammability of polyamides, including filled polyamides such as glass reinforced PA/GF20-35, providing compositions having good electrical properties. Another aspect of the invention is therefore an additive mixture, comprising a bromine-containing polymer, such as brominated polystyrene, and boron phosphate (with bromine : BPO4 weight ratio preferably in the range from 3:1 to 2:1) , that is able to reduce flammability and increase CTI in antimony-free glass-reinforced thermoplastics. That is, when used in forming a glass-reinforced thermoplastic composition, the additive mixture can impart a UL- 94 flammability rating of V-0 at a thickness equal to 0.8 or 0.4 mm; and when the antimony-free glass-reinforced thermoplastic composition is tested in accordance with IEC 60112 standard using 0.1% by weight NH4CI solution and platinum electrodes, the comparative CTI measured is from 500 to 600 V, e.g., 500 to 600 V.

Examples

Materials and methods

The materials used for preparing the PA 6 , 6/GFso formulations are tabulated in Table 1 ( FR is abbreviation of flame retardant ) :

Table 1

Flammability properties

A direct flame test was carried out according to the Underwriters-Laboratories standard UL 94 in a gas methane operated flammability hood, applying the vertical burn on specimens of 0 . 8 mm or 0 . 4 mm thickness .

Mechanical properties

The Notched I zod impact test was carried out according to ASTM D256- 81 using Instron Ceast 9050 pendulum impact system . Tensile properties were determined according to ASTM D638 using Zwick Z- 010 material testing machine ( type 2 dumbbells were used, with a speed test of 5 mm/min) .

Thermal properties

Heat distortion temperature ( abbreviated HDT ; this is the temperature at which a polymer sample deforms under a speci fic load) was measured according to ASTM D-648 with load of 1820 kPa and heating rate of 120°C/hour. The instrument was HDT/VICAT - Instron HV3S.

Electrical properties

Comparative Tracking Index (CTI) was measured according to the International Electrotechnical Commission (IEC) STANDARD publication 112 (DIN EN 60112) . The test was performed with a solution of NH4CI 0.1% (solution A) and Pt electrodes.

Examples 1 (comparative) and 2 to 4 (of the invention)

PA 6,6/GFso formulations

To prepare the formulations tabulated in Table 3, PA 6, 6 and the additives (with the exception of the glass fibers) were preblended and the blend was fed via Feeder no. 1 into the main port of a twin-screw co-rotating extruder ZSK-18-ML (Coperion) . The glass fibers were added downstream, via Feeder no. 3 to the seventh zone of the barrel through a lateral feeder. The twin screw extruder was operated with barrel temperature profile of 235°C, 260°C, 260°C, 265°C, 265°C, 270°C, 275°C, die - 280°C, applying a screw rotation speed of 500-900 rpm and feeding rate of 8-10 kg/hour.

The strands produced were pelletized in a pelletizer 750/3 from Accrapak Systems Ltd. The resultant pellets were dried in a circulating air oven (Heraeus Instruments) at 120°C for 3 hours. The dried pellets were injection molded into 0.4 mm and 0.8 mm thick test specimens using Allrounder 500-150 from Arburg under the conditions tabulated in Table 2. The test specimens were conditioned for one week at 23°C and were then subjected to several tests to determine their properties. Table 2

The compositions and test results are set out in Table 3 .

Table 3 The results tabulated in Table 3 indicate that addition of boron phosphate to glass filled polyamide 6, 6, that was flame retarded with brominated polystyrene (bromine w/w concentration of the polyamide composition is ~15%) has two positive effects: flammability rating was improved from V-2 to V-0 at 0.8 mm, and CTI increased from 475 V to 600 V (Example 1 Example 2) . Furthermore, PA 6, 6/GFso that is UL 94 V-0 rated at 0.4 mm can also be achieved at an acceptable level of added bromine (Example 3 Example 4) .