WO2019030756A1

WO2019030756A1 - Stabilization of flame-retarded polymers

Info

Publication number: WO2019030756A1
Application number: PCT/IL2018/050875
Authority: WO
Inventors: Yaniv Hirschsohn; Eyal EDEN
Original assignee: Bromine Compounds Ltd.
Priority date: 2017-08-09
Filing date: 2018-08-07
Publication date: 2019-02-14

Abstract

A composition comprising: polystyrene; brominated poly[styrene-co-butadiene]; a heat stabilizer; a metal salt of stearic acid; characterized in that the composition further comprises a color stabilizer selected from nitrogen-containing compounds with carbonyl groups bonded to nitrogen atoms, e.g. hydrazide or oxaimide.

Description

Stabilization of flame-retarded polymers

Many polymers in commercial use contain flame retardants to reduce their flammability . The present invention relates to stabilization of polymers, such as polystyrene, against discoloration resulting from decomposition of bromine- containing flame retardants that are added to these polymers and that are prone to undergo degradation triggered by exposure to certain conditions, as described below.

The flame retardants under consideration are bromine-containing polymers produced by addition of elemental bromine to poly [ styrene-co-butadiene ] . Hereinafter, the name poly [ styrene- co-butadiene ] is used to denote a copolymer prepared from styrene and 1 , 3-butadiene monomers. The name poly [ styrene-co- butadiene] is not limited to any particular distribution of the monomeric repeating units. Preferred, however, are block copolymers with polystyrene terminal blocks, which are named herein polystyrene-jbl ock-polybutadiene-jbl ocic-polystyrene .

Because 1 , 3-butadiene ( CH2=CH-CH=CH2 ) can undergo both 1,2 addition and 1,4 addition, there are three types of repeat units in the resulting copolymer. One repeat unit corresponds to the styrene monomer, and the other two repeat units correspond to the different (1,2 and 1,4) addition forms of butadiene. It should be noted that the names poly [ styrene-co- butadiene] and polystyrene-jbl ock-polybutadiene-jbl ocic- polystyrene as used herein are not limited to any specific proportion of the 1,2 and 1,4 units.

On addition of elemental bromine to the aliphatic carbon-carbon double bonds of poly [ styrene-co-butadiene ] , for example, to polystyrene-jbl oc-k-polybutadiene-jbl ocic-polystyrene, a brominated polymer is obtained, as shown below:

where x, y and z indicate the numbers of the three types of repeat units, respectively. The bromination reaction takes place in a solvent, as described for example in WO 2008/021418, to afford a brominated poly [ styrene-co-butadiene ] . Such brominated products, in particular, polystyrene-jblock- brominated polybutadiene-jblock-polystyrene, are commercially available (e.g., FR-122P from ICL-IP) . Their weight average molecular weight is from 8,000 to 200,000, e.g., from 115,000 to 150, 000, with bromine content of not less than 60%, e.g., from 60% to 66%. The 1,2 addition product generally constitutes from 75-85% of the total butadiene content.

Brominated poly [ styrene-co-butadiene ] are very efficient flame retardants but unfortunately, they are prone to degradation due their inadequate stability at high temperatures. For this reason, brominated polymeric flame retardants are incorporated into a bulk polymer (for example, polystyrene) in conjunction with suitable stabilizers; the goal of the added stabilizer is to suppress degradation of the brominated polymeric flame retardant during processing. Commonly used stabilizers include hydrotalcite and epoxy-containing compounds, such as epoxy cresol novolac resin (Epon® 164) and F-2200 from ICL-IP, which has the followin formula:

Other stabilizers are the reaction products of epoxy novolac resin with tribromophenol and tetrabromobisphenol A, as described in PCT/IL2017/050854 (WO 2018/025266) . For the purpose of this invention, said additives that are used to prevent degradation of brominated poly [ styrene-co-butadiene ] during processing are named "heat stabilizers".

The aforementioned heat stabilizers are added in the production of extruded polystyrene, e.g.,:

manufacture of concentrate formulations with high content of brominated poly [ styrene-co-butadiene ] {known in the art as masterbatch formulations; such concentrates enable the industry to employ the flame retardant in a pelletized form instead of a powder } ; and

manufacture of finished formulations, such as extruded (XPS) polystyrene foam.

But it has been observed that polystyrene formulations that are flame retarded with brominated poly [ styrene-co-butadiene ] exhibit discoloration problems in the presence of metal salts of stearic acid, e.g., zinc stearate (a common lubricant additive incorporated into various polymers including polystyrene) . The color change is a noticeable indication to decomposition of brominated poly [ styrene-co-butadiene ] affected by zinc stearate. Because the industry is encouraged to promote the use of recycled polystyrene from various manufacturers, zinc stearate may ultimately find its way to the finished formulations even if not intentionally intended to be employed in such formulations, for example, in extruded (XPS) polystyrene foam manufactured from scarp polystyrene "contaminated" with zinc stearate. A similar problem was reported in WO 98/16579 for polystyrene samples that were flame retarded with a different type of bromine-containing compound - hexabromocyclododecane . Zinc stearate added - samples illustrated in WO 98/16579 exhibited a tendency to change color, that is, to form black spots.

It has now been found that the incompatibility of brominated poly [ styrene-co-butadiene ] with metal salts of stearic acid could be solved with the aid of certain stabilizers, named herein color stabilizers, to distinguish them from the aforementioned chief (heat) stabilizers (which mainly act as thermal stabilizers and/or acid scavengers) .

The present invention is primarily directed to a composition comprising :

polystyrene ;

brominated poly [ styrene-co-butadiene ] ;

a heat stabilizer;

a metal salt of stearic acid;

characterized in that the composition further comprises a color stabilizer selected from nitrogen-containing compounds with carbonyl groups bonded to nitrogen atoms, in particular compounds having the functional group -C (0) -NX-NX-C (0) - (X is hydrogen or alkyl; that is, hydrazide compounds) and compounds having the functional group -NX-C (0) -C (0) -NX- (X is hydrogen or alkyl; that is, oxaimide compounds) .

Preferred are compounds having a configuration consisting of two symmetrical units of the formulas [R-C (0) -NX- ] 2 or [R-NX- C(0)-]2, with antioxidant functionality incorporated into R, such as optionally substituted phenol, for example, phenol bearing bulky groups, such as one or more -C(CH3)3 groups, known as hindered phenols. For example, the hindered phenol is di- tert-butyl-hydroxyphenol , e.g., the terminal group of R is 3 ' , 5 ' -di-tert-butyl-4 ' -hydroxyphenyl .

A preferred symmetrical hydrazide compound of the formula [R-C (0) -NX- ] 2 has the structure depicted below {chemically named 1 , 2-bis ( 3 , 5-di-tert-butyl-4-hydroxyhydrocinnamoyl ) hydrazide } :

The compound is also named antioxidant 1024 and is commercially available as Irganox® MD 1024 and Songnox® 1024.

A preferred symmetrical oxaimide compound of the formula [R-NX-C (0) - ] 2 has the structure depicted below {chemically named 2,2' -Oxamido bis [ ethyl-3- ( 3 , 5-di-tert-butyl-4-hydroxyphenyl ) propionate] } :

The compound is also named antioxidant 697 and is commercially available as Naugard ® XL 1.

The experimental results reported below indicate that color changes of metal stearate-containing polystyrene formulations that are flame retarded with brominated poly [ styrene-co- butadiene] are effectively inhibited by small amounts of the compounds [R-C (0) -NX-] 2 or [R-NX-C (0) -] 2.

In some preferred compositions of the invention, the concentration of the polystyrene is from 85 to 98%, e.g., from 90 to 98% by weight based on the total weight of the composition. The concentration of the flame retardant, namely, the brominated poly [ styrene-co-butadiene ] is from 0.5 to 5%, e.g., from 1.5 to 3% by weight based on the total weight of the composition. The concentration of the heat stabilizer is from 0.1 to 0.5%, e.g., from 0.2 to 0.4% by weight based on the total weight of the composition.

Metal stearate may be purposely included in the composition to serve a useful function, in which case its concentration may be from 0.1 to 0.5% by weight. The color stabilizer should preferably be added such that the weight ratio metal stearate : [R-C (0) -NX-] 2, or likewise, metal stearate : [R-C (0) -NX- ] ₂, is from 5:1 to 1:5, preferably from 1:1 to 1:3, more preferably from 1:1 to 1:2, for example, from 1:1 to 1:1.5.

But as pointed out above, remnant metal stearate from recycled sources can also cause discoloration problems. The amount of remnant metal stearate may typically be up to 0.5% by weight of the polymer composition of the invention. A suitable amount of a color stabilizer to counter the effect of such remnant metal stearate and guarantee color stability when recycled polystyrene suspected to contain metal stearate is used a raw material would be from 0.01 to 1.0%, for example, from 0.05 to 0.5%, preferably from 0.1 to 0.4%, based on the total weight of the composition.

Therefore, in one embodiment of the invention, the polystyrene composition is preferably a polystyrene foam, either extruded (XPS foam) or expanded (EPS foam) . Melt processing methods to produce such foams and conventional additives employed in such processes are known in the art and are described, for example, in US 2008/0096992 and WO 2010/080285.

Accordingly, another aspect of the invention is a process for preparing flame retarded, discoloration resistant polystyrene foam from metal stearate-containing recycled polystyrene, comprising feeding said recycled polystyrene, brominated poly [ styrene-co-butadiene ] , a heat stabilizer and a color stabilizer selected from [R-C (0) -NX-] ₂ and [R-NX-C (0) - ] ₂ as defined above to the extruder, injecting a foaming agent (e.g., CO2 and/or halogenated hydrocarbons, e.g., fluorine/chlorine substituted hydrocarbons) into the extruder gas entrance, extruding the mass, cooling and processing into shaped foam articles, e.g., panels. Melt-blending in the extruder, e.g., in a twin-screw extruder, may be achieved at temperature in the range from 70 to 240°C, e.g., 160 to 230 °C.

The ingredients (bulk polystyrene and the various additives) may be fed separately to the extruder, or may be premixed to form a dry blend which is then charged into the extruder. Alternatively, some of the additives may be added via the masterbatch route, namely, a masterbatch formulation consisting essentially of a polystyrene carrier (which may be the same or different from the bulk polystyrene of the finished formulation), high content of brominated poly [ styrene-co- butadiene] and the heat stabilizer, for example, 40-60 : 35-45 : 15-5, e.g. 50 : 40 : 10 masterbatch formulation, may be used. Masterbatch formulations are obtained by melt-blending the polymeric carrier and the additives to obtain pellets with high concentration of the flame retardant and the heat stabilizer associated therewith. Masterbatch pellets are in many cases favored by the industry as these solid forms are more convenient for handling, shipping and feeding into the extruder than powdery materials.

As to the color additive, e.g., a compound of the formulas [R-C (0) -NX-] 2 or [R-NX-C (0) -] 2 which is available in the market as a powder can be added either to the masterbatch formulation alongside the brominated poly [ styrene-co-butadiene ] and its chief heat stabilizer; or mixed separately with the bulk polymer (e.g., by being separately added to the extruder) . Examples

Preparation 1

To prepare masterbatch pellets, polystyrene (PS 124n pellets from BASF), FR-122P and F-2200HM were fed in 50:40:10 weight ratio into a twin-screw co-rotating extruder ZE25 with L/D=32 with open vent at 7^th zone. The operating parameters of the extruder were as follows:

Barrel temperatures were set at (from feed end to discharge end) : Ti=no heating, T₂=160°C, T₃=160°C, T₄=160°C, T₅=160°C, T₆=160°C, T₇ vent = 160°C, T₈=160°C; T₉ die= 180°C.

Screw rotation speed: 350 rpm

Feeding rate: 10-11 kg/hour.

The strands produced were cooled in a water bath and pelletized in a pelletizer 750/3 from Accrapak Systems Ltd. The resultant pellets were dried in a circulating air oven at 80°C for 3 hours.

The masterbatch pellets were used in the next examples to add the flame retardant FR-122P into polystyrene via a masterbatch route .

Example 1 (reference) and 2 (of the invention)

Flame retarded polystyrene formulations were prepared by feeding polystyrene (PS 145D pellets from BASF) and the additives set out in Table 1 into a twin-screw co-rotating extruder ZE25 with L/D=32 with open vent at 7^th zone. The operating parameters of the extruder were as follows:

Barrel temperatures were set at (from feed end to discharge end) : Ti=no heating, T₂=160°C, T₃=160°C, T₄=180°C, T₅=180°C, T₆=180°C, T₇ vent = 180°C, T₈=190°C; T₉ die= 200°C.

Screw rotation speed: 350 rpm.

Feeding rate: 10-11 kg/hour. Table 1

The strands produced were cooled in a water bath and pelletized in a pelletizer 750/3 from Accrapak Systems Ltd. The resultant polystyrene pellets were dried in a circulating air oven at 80°C for 3 hours.

To evaluate the effect of the added stabilizer to protect zinc stearate- containing formulation against change in color under severe environment, the polystyrene pellets were press molded in a press (from Labtec) at 200°C for 3 minutes, into cylindrical bodies with diameter of 7cm and height of 3mm, and the so-formed test specimens were subjected to visual evaluation to determine change in color. The photos shown in Figure 1 clearly indicate that in the absence of the stabilizer of the invention, the bromine-containing flame retardant degrades in a zinc stearate-containing polystyrene, seeing that the polystyrene sample turned black, whereas the stabilizer-added sample of Example 2 retained good color.

The Limiting Oxygen Index (LOI) measured for Examples 1 and 2 was 24.63% and 24.25%, respectively.

Claims

1) A composition comprising:

polystyrene ;

brominated poly [ styrene-co-butadiene ] ;

a heat stabilizer;

a metal salt of stearic acid;

characterized in that the composition further comprises a color stabilizer selected from nitrogen-containing compounds with carbonyl groups bonded to nitrogen atoms.

2) A composition according to claim 1, wherein the heat stabilizer is an epoxy-containing compound.

3) A composition according to claim 1 or 2, wherein said nitrogen-containing compound has a functional group chosen from -C (0) -NX-NX-C (0) - and NX-C (0) -C (0) -NX-, wherein X is hydrogen or alkyl.

4) A composition according to claim 3, wherein the color stabilizer is hydrazide of the formula [R-C (0) -NX- ] 2 or oxaimide of the formula [R-NX-C (0) - ] 2 with R being an organic moiety comprising (an optionally) substituted phenol.

5) A composition according to claim 4, wherein R comprises hindered phenol bearing one or more -C(CH3)3 groups.

6) A composition according to any one of the preceding claims, wherein the color additive is hydrazide of the structure

7) A composition according to any one of the preceding claims, comprising :

from 90 to 98 wt% polystyrene;

from 0.5 to 5 wt% brominated poly [ styrene-co-butadiene ] ;

from 0.1 to 0.5 wt% heat stabilizer;

from 0.1 to 0.5% wt% metal stearate;

wherein the metal stearate : heat stabilizer is from 1:1 to 1:3.

8) A composition according to any one of the preceding claims, wherein the amount of the color stabilizer is from 0.05 to 0.5% based on the total weight of the composition.

9) A composition according to any one of the preceding claims, comprising a polystyrene foam, which is extruded (XPS foam) or expanded (EPS foam) .

10) A process for preparing flame retarded, discoloration resistant polystyrene foam from metal stearate-containing recycled polystyrene, comprising melt-blending in an extruder said recycled polystyrene, brominated poly [ styrene-co- butadiene] , a heat stabilizer and a color stabilizer selected from [R-C (0) -NX-] 2 and [R-NX-C (0) - ] ₂ as defined in previous claims above, injecting a foaming agent into the extruder gas entrance, extruding the mass, cooling and processing into shaped foam articles.