The present invention relates to a polymer composition comprising (A) a first heterophasic propylene copolymer, (B) a second heterophasic propylene copolymer, (C) a flame retardant composition and (D) an aromatic phosphate ester. The present invention also relates to a process for the preparation of the said polymer composition and use of the said polymer composition.

Polymer compositions with flame retardant properties is well known in the art, e.g. EP3255095 discloses a flame-retardant polyolefin resin composition comprising polypropylene, melamine pyrophosphate, piperazine pyrophosphate, glass fiber and zinc oxide.

EP3202841 discloses a flame-retardant polypropylene resin composition comprising a polypropylene resin, glass fiber, phosphorous-containing flame retardant (FP2200 from Adeka), antioxidant, neutralizing agent and a surfactant.

WO2018/019762A1 discloses a flame retardant polypropylene composition comprising

(A) a polypropylene-based polymer,

(B) a first flame retardant in an amount of 15 to 40 wt% of the total composition, wherein the first flame retardant is in the form of particles comprising ammonium polyphosphate and at least one phosphate selected from the group consisting of melamine phosphate, melamine polyphosphate, melamine pyrophosphate, piperazine phosphate, piperazine polyphosphate, piperazine pyrophosphate,

2-methylpiperazine monophosphate, tricresyl phosphate, alkyl phosphates, haloalkyl phosphates, tetraphenyl pyrophosphate, poly(2-hydroxy propylene spirocyclic pentaerythritol bisphosphate) and poly(2,2-dimethylpropylene spirocyclic pentaerythritol bisphosphonate) and

(C) a second flame retardant in an amount of 0.1 to 15 wt% of the total composition, wherein the second flame retardant comprises an aromatic phosphate ester.

The combination of stiffness and flame retardant property of such polymer composition makes it a promising candidate as material for enclosure of batteries in electrical vehicle (EV). To improve the production efficiency of an EV battery, there is a desire to perform the assembly the enclosure of an EV battery by a snap-fit mechanism. A snap-fit mechanism typically comprises at least one snap-fit joint, wherein the snap-fit joint consists of a protruding edge on one part to be assembled and a snap-in area on another part to be assembled. During the assembly of two parts with a snap-fit mechanism and upon closing the distance between the two parts, the protruding edge is first deformed, e.g. bended, then the protruding edge fits in the snap-in area and recovers to its initial shape where the protruding edge and the snap-in area form a snap-fit joint. The material used in the snap-fit mechanism should have sufficient flexibility so that the protruding edge does not break during the deformation.

The presence of the flame retardant agent in the polymer composition often leads to brittleness and makes the polymer composition unsuitable to be used in a battery enclosure assembled by a snap-fit mechanism. Therefore there is a need of a polymer composition having excellent flame retardant performance and sufficient flexibility. Preferably the polymer composition has a density of lower than 0.93 g/cm3 for the purpose of weight reduction of an electrical vehicle. The object of the present invention is achieved by a polymer composition comprising

(A) a first heterophasic propylene copolymer, wherein the MFI of the first heterophasic propylene copolymer is in the range from 3.6 to 14 g/10min as measured according to ISO1133:1-2011 at 230°C, 2.16kg, wherein the first heterophasic propylene copolymer comprises a first propylene-based matrix and a first ethylene-propylene copolymer, wherein the amount of the first ethylene-propylene copolymer is in the range from 18 to 26 wt% based on the total amount of the first heterophasic propylene copolymer;

(B) a second heterophasic propylene copolymer, wherein the MFI of the second heterophasic propylene copolymer is in the range from 18 to 51 g/10min as measured according to

ISO1133:1-2011 at 230°C, 2.16kg, wherein the second heterophasic propylene copolymer comprises a second propylene-based matrix and a second ethylene-propylene copolymer, wherein the amount of the second ethylene-propylene copolymer is in the range from 15 to 23 wt% based on the total amount of the second heterophasic propylene copolymer;

(C) a flame retardant composition comprising at least one phosphate;

(D) an aromatic phosphate ester, wherein the amount of the first heterophasic propylene copolymer is in the range from 10 to 38 wt%, the amount of the second heterophasic propylene copolymer is in the range from 35 to 60 wt%, the amount of the flame retardant composition is in the range from 13 to 33 wt%, the amount of the aromatic phosphate ester is in the range from 0.3 to 13 wt% based on the total amount of the polymer composition.

The inventor of the present invention found the polymer composition according to the invention has superior flame retardant properties and sufficient flexibility to be used in the material of an EV battery enclosure suitable for snap-fit assembly. In the context of the present invention “flexibility” is quantified as elongation at break in a tensile measurement according to ISO527- 1 :2019 with 1 A specimen.

(A) First heterophasic propylene copolymer and (B) Second heterophasic propylene copolymer Heterophasic propylene copolymers, also known as impact propylene copolymers or propylene block copolymers, are an important class of polymers due to their attractive combination of mechanical properties, such as impact strength over a wide temperature range and their low cost. These copolymers find a wide range of applications ranging from the consumer industry (for example packaging and housewares), the automotive industry to electrical applications.

Heterophasic propylene copolymers are generally prepared in one or more reactors, by polymerization of propylene in the presence of a catalyst and subsequent polymerization of an ethylene-a-olefin mixture. The resulting polymeric materials are heterophasic, but the specific morphology usually depends on the preparation method and monomer ratios used.

The first heterophasic propylene copolymer and the second heterophasic propylene copolymer can be produced using any conventional technique known to the skilled person, for example multistage process polymerization, such as bulk polymerization, gas phase polymerization, slurry polymerization, solution polymerization or any combinations thereof. Any conventional catalyst systems, for example, Ziegler-Natta or metallocene may be used. Such techniques and catalysts are described, for example, in W006/010414; Polypropylene and other Polyolefins, by Ser van der Ven, Studies in Polymer Science 7, Elsevier 1990; W006/010414, US4399054 and US4472524. Preferably, the first heterophasic propylene copolymer and second heterophasic propylene copolymer are produced in the presence of Ziegler-Natta catalyst.

The first heterophasic propylene copolymer and second heterophasic propylene copolymer may be prepared by a process comprising the steps of

- polymerizing propylene and optionally ethylene and/or a-olefin in the presence of a catalyst system to obtain the propylene-based matrix and

- subsequently polymerizing ethylene and a-olefin in the propylene-based matrix in the presence of a catalyst system to obtain the dispersed ethylene-a olefin copolymer. These steps are preferably performed in different reactors. The catalyst systems for the first step and for the second step may be different or same. The first heterophasic propylene copolymer comprises a first propylene-based matrix and a first ethylene-propylene copolymer, wherein the amount of the first ethylene-propylene copolymer is in the range from 18 to 26 wt%, preferably in the range from 20 to 24 wt% based on the total amount of the first heterophasic propylene copolymer. The first propylene-based matrix typically forms the continuous phase in the first heterophasic propylene copolymer. The amounts of the first propylene-based matrix and the first ethylene-propylene copolymer may be determined by 13C-NMR, as well known in the art. Preferably the amount of moieties derived from ethylene in the first ethylene-propylene copolymer is in the range from 15 to 65 wt%, preferably from 30 to 57 wt%, more preferably in the range from 45 to 52 wt% based on the total amount of the first ethylene-propylene copolymer.

The first propylene-based matrix preferably consists of a first propylene homopolymer and/or a first propylene copolymer consisting of at least 96 wt% of propylene monomer units and at most 4 wt% of comonomer units selected from ethylene monomer units and a-olefin monomer units having 4 to 10 carbon atoms.

Preferably, the comonomer in the first propylene- a-olefin copolymer is selected from the group of ethylene, 1-butene, 1-pentene, 4-methyl-1 -pentene, 1-hexen, 1-heptene and 1-octene, and is preferably ethylene.

It is common general knowledge that the first propylene-based matrix and the first ethylene- propylene copolymer are not the same.

Preferably, the first propylene-based matrix consists of a first propylene homopolymer. The fact that the first propylene-based matrix consists of a first propylene homopolymer is advantageous in that a higher stiffness is obtained compared to the case where the first propylene-based matrix is a first propylene copolymer. The melt flow index (MFI) of the first heterophasic propylene copolymer is in the range from 3.6 to 14 g/10min, preferably in the range from 5.1 to 12 g/10min, preferably from 6.7 to 9.8 g/10min g/10min as measured according to ISO1133:1-2011 at 230°C, 2.16kg.

The second heterophasic propylene copolymer comprises a second propylene-based matrix and a second ethylene-propylene copolymer, wherein the amount of the second ethylenepropylene copolymer is in the range from 15 to 23 wt%, preferably in the range from 19 to 22 wt% based on the total amount of the second heterophasic propylene copolymer. The second propylene-based matrix typically forms the continuous phase in the second heterophasic propylene copolymer. The amounts of the second propylene-based matrix and the second ethylene-propylene copolymer may be determined by 13C-NMR, as well known in the art. Preferably the amount of moieties derived from ethylene in the second ethylene-propylene copolymer is in the range from 16 to 63 wt%, preferably from 31 to 51 wt%, more preferably in the range from 40 to 45 wt% based on the total amount of the second ethylene-propylene copolymer.

The second propylene-based matrix preferably consists of a second propylene homopolymer and/or a second propylene copolymer consisting of at least 96 wt% of propylene monomer units and at most 4 wt% of comonomer units selected from ethylene monomer units and a-olefin monomer units having 4 to 10 carbon atoms.

Preferably, the comonomer in the second propylene- a-olefin copolymer is selected from the group of ethylene, 1-butene, 1-pentene, 4-methyl-1 -pentene, 1-hexen, 1-heptene and 1-octene, and is preferably ethylene.

It is common general knowledge that the second propylene-based matrix and the second ethylene-propylene copolymer are not the same. Preferably, the second propylene-based matrix consists of a second propylene homopolymer. The fact that the second propylene-based matrix consists of a second propylene homopolymer is advantageous in that a higher stiffness is obtained compared to the case where the second propylene-based matrix is a second propylene copolymer.

The melt flow index (MFI) of the second heterophasic propylene copolymer is in the range from 18 to 51 g/10min, preferably in the range from 22 to 42 g/10min, more preferably from 26 to 36 g/10min as measured according to ISO1133:1-2011 at 230°C, 2.16kg.

In a preferred embodiment that the MFI of the first heterophasic propylene copolymer and the MFI of the second heterophasic propylene copolymer are in the preferred range, the polymer composition shows even better balance between flame retardant performance and flexibility.

In a preferred embodiment, the first heterophasic propylene copolymer consists of the first propylene-based matrix, the first ethylene-propylene copolymer and optional additives, wherein the first propylene-based matrix consists of a first propylene homopolymer, wherein the total amount of the moieties derived from ethylene is in the range from 7.0 to 14 wt%, preferably in the range from 8.8 to 12 wt%, even more preferably from 9.5 to 11 wt% based on the total mount of the first heterophasic propylene copolymer.

In a preferred embodiment, the second heterophasic propylene copolymer consists of the second propylene-based matrix, the second ethylene-propylene copolymer and optional additives, wherein the second propylene-based matrix consists of a second propylene homopolymer, wherein the total amount of the moieties derived from ethylene is in the range from 5.0 to 12 wt%, preferably in the range from 6.7 to 11 wt%, even more preferably from 8.2 to 10 wt% based on the total mount of the second heterophasic propylene copolymer.

(C) Flame retardant composition The flame retardant composition comprises at least one phosphate, wherein the phosphate is preferably selected from the group consisting of melamine phosphate, melamine polyphosphate, melamine pyrophosphate, piperazine phosphate, piperazine polyphosphate, piperazine pyrophosphate,

2-methylpiperazine monophosphate, tricresyl phosphate, alkyl phosphates, haloalkyl phosphates, tetraphenyl pyrophosphate, poly(2-hydroxy propylene spirocyclic pentaerythritol bisphosphate), poly(2,2-dimethylpropylene spirocyclic pentaerythritol bisphosphonate).

The flame retardant composition is preferably in the form of particles. Preferably the flame retardant composition has a normal particle size distribution (D50) of at least 8 microns as determined by Mastersizer 2000 available from Malvern.

Preferably, the amount of phosphate in the flame retardant composition is in the range from 40 to 75 wt% as measured after treating with nitric acid using ICP-OES spectrometer (iCAP 6300 Duo available from Thermo Fisher) Preferably, the flame retardant composition comprises piperazine pyrophosphate, melamine phosphate and zinc oxide.

Preferably, the amount of piperazine pyrophosphate is in the range from 40 to 69 wt%, more preferably from 50 to 67 wt%; the amount of melamine phosphate is in the range from 29 to 49 wt% and the amount of zinc oxide is in the range from 1 to 10 wt% based on the total amount of the flame retardant composition.

(D) Aromatic phosphate ester

Preferably, the aromatic phosphate ester is selected from the group consisting of resorcinol bis(diphenyl phosphate); tetraphenyl resorcinol bis(diphenylphosphate); bisphenol A bis(diphenyl phosphate); bisphenol A diphosphate; resorcinol bis(di-2,6-xylyl phosphate), phosphoric acid, mixed esters with [1 , 1 '-biphenyl]-4-4'-diol and phenol; phosphorictrichloride, polymer withl ,3-benzenediol, phenylester;

1 ,3-phenylene-tetrakis(2,6-dimethylphenyl)diphosphate; isopropenylphenyl diphenyl phosphate;

4-phenylphenolformaldehyde phenylphosphonate; tris(2,6-xylyl) phosphate; resorcinol bis(di-2,6-xylyl phosphate); bisphenol S bis(diphenyl phosphate); resorcinol-bisphenol A phenyl phosphates. Preferably, the aromatic phosphate ester is added as a liquid.

Preferably, the aromatic phosphate ester is bisphenol A bis(diphenyl phosphate).

The amount of the first heterophasic propylene copolymer is in the range from 10 to 38 wt%, preferably from 15 to 30 wt%, more preferably from 20 to 28 wt%, even more preferably from 22 to 26 wt%, the amount of the second heterophasic propylene copolymer is in the range from 35 to 60 wt%, preferably from 40 to 55 wt%, more preferably from 42 to 50 wt%, the amount of the flame retardant composition is in the range from 13 to 33 wt%, preferably from 18 to 27 wt%, more preferably from 19 to 24 wt%, the amount of the aromatic phosphate ester is in the range from 0.3 to 13 wt%, preferably from 1.8 to 8.0 wt%, more preferably from 2.5 to 4.5 wt% based on the total amount of the polymer composition.

Preferably the polymer composition according to any one of the preceding claims, wherein the polymer composition has a UL94 rating of V0 at 0.8mm, wherein the specimens are conditioned in an environment of 23°C, 50% RH for 48 hours after forming and prior to UL94 measurement.

Preferably the polymer composition has a tensile elongation at break of at least 42 % as measured according to ISO527-1:2019 using 1A specimen.

Preferably the density of the polymer composition is in the range from 0.956 to 1 .200 g/cm3, preferably from 0.989 to 1 .158 g/cm3, more preferably from 1.002 to 1.098 g/cm3 as measured according to ISO 1183-1:2004. The present invention further relates to a process for the preparation of the polymer composition according to the invention comprising the sequential steps:

- Providing the first heterophasic propylene copolymer, the second heterophasic propylene copolymer, the flame retardant composition and the aromatic phosphate ester;

- Mixing the first heterophasic propylene copolymer, the second heterophasic propylene copolymer, the flame retardant composition and the aromatic phosphate ester in an extruder.

The present invention further relates to the use of the polymer composition according to the invention for improving flame retardant performance and flexibility of an article, wherein the article is preferably a battery enclosure.

The present invention further relates to an article comprising the polymer composition according to the invention, wherein the amount of the polymer composition is at least 95 wt%, preferably at least 98 wt% based on the total amount of the article, wherein the article is preferably a battery enclosure.

Experiment

Material

595A is a propylene homopolymer commercially available from SABIC, having an MFI of 47 g/10min as measured according to ISO1133:1-2011 at 230°C, 2.16kg.

591A is a propylene homopolymer commercially available from SABIC, having an MFI of 6 g/10min as measured according to ISO1133:1-2011 at 230°C, 2.16kg.

87MK40T is a heterophasic propylene copolymer commercially available from SABIC, having an MFI of 8 g/10min as measured according to ISO1133:1-2011 at 230°C, 2.16kg. 87MK40T comprises 22 wt% ethylene-propylene copolymer and the total amount of moieties derived from ethylene in 87MK40T is 10.5 wt% as determined by NMR. 511MK40T is a heterophasic propylene copolymer commercially available from SABIC, having an MFI of 30 g/10min as measured according to ISO1133: 1-2011 at 230°C, 2.16kg.511 K40T comprises 21 wt% ethylene-propylene copolymer and the total amount of moieties derived from ethylene in 511 K40T is 9 wt% as determined by NMR.

FP2500S is a flame retardant composition commercially available from Adeka ADK STAB FP- 2500S. FP-2500S is a flame retardant composition according to the most preferred embodiment of the invention.

CR741 is Bisphenol-A Bis(Diphenyl Phosphate) commercially available from Daihachi.

Additive package: The additive package comprises 30 wt% stabilizer, 15 wt% anti-dipping agent (TSAN F449 from SABIC), 8 wt% nucleating agent, 7 wt% slipping agent, 40 wt% color masterbatch. The weight percentage is based on the total amount of the additive package.

Sample preparation

All IBs and CEs were prepared in a twin screw extruder. The formulations of lEs and CEs are in

Table 1

Table 1 Formulations of lEs and CEs lEs and CEs obtained from the extruder were in pellet form. The pellets were further injection molded into specimens for measurements. Measurement

Melt flow index (MFI) was measured according to ISO1133:1-2011 at 230°C, 2.16kg.

Tensile properties were measured according to ISO527-1 :2019 using 1A specimen.

Density was measured according to ISO 1183-1:2004. Flame retardant measurement was taken according to UL94. Specimens with different thicknesses were used, the specimens were conditioned prior to the measurement under the following two conditions:

Condition 1 : After injection molding and prior to flame retardant measurement, the specimens were conditioned in an environment of 23°C, 50% RH for 48 hours. Condition 2: After injection molding and prior to flame retardant measurement, the specimens were conditioned in a first environment of 70°C, 50% RH for 168 hours, then conditioned in a second environment of 23°C, 20% RH for 4 hours.

Result

The properties of lEs and CEs are shown in Table 2 Table 2 Properties of I Es and CEs According to Table 2, only lEs demonstrate superior FR performance, MFI and tensile elongation at break.