EoW July 2010

EuroWire – July 2010

technical article

Improving themechanical

propertiesof non-halogenated

flame retardant compounds

By Jeremy R Austin, Herbert S.-I Chao, Sartomer Company

Abstract

Traditionally, plastic articles have been

rendered flame retardant by the intro-

duction of halogenated compounds, such

as tetrabromobisphenol A, or TBBPA.

Recently, a movement to non-halogenated

flame-retardants has been the focus of

academic and industrial research, but

these safer alternative technologies have

a deleterious impact on mechanical

properties.

Mineral fillers used as flame retardants

require in excess of 60% by weight loading

to fulfil flame requirements. In the current

study, functionalised liquid polybutadienes

(LPBD) are used to improve the elongation

and tensile strength of aluminium trihy-

drate (ATH) filled ethylene vinyl acetate

(EVA) compounds. Pre-dispersion of the

coupling agents onto ATH led to gains in

elongation of greater than 200%.

Low loadings of functionalities including

maleic anhydride, epoxy and amine were

proven to be most effective. Incorporation

of a di-acrylic ionic monomer provided

gains in tensile modulus unattainable by

the LPBD materials.

1 Introduction

Scientific studies have indicated that

halogenated flame retardants (HFR)

are widespread contaminants for the

environment. Hazardous emissions from

manufacturing, disposal or recycling of

plastic articles containing HFRs pose such

a serious threat that some HFRs have

already been removed from electronics

and household goods, and the European

Union has ratified regulations governing

the plastics industry to eliminate them.

With similar legislature impending on all

continents, several markets across the

plastics industry are seeking alternative

technologies.

Several non-halogenated flame retardants

(NHFR), such as ammonium phosphates,

melamine compounds, nanoclays or

hydrated minerals, have been identified.

Aluminium trihydrate (ATH) is a recognised

flame retardant filler for polymers, and

is free from halogens. Typically, flame

retardants act to delay ignition by

depriving the fire of fuel, or suppressing

the ignition temperature.

ATH however, releases water vapour

during decomposition, which is believed

to withdraw heat from the substrate and

dilute the fuel supply. Once charred, the

residue of Al

inhibits migration of

oxygen and volatile compounds released

by the polymer that can further proliferate

the exothermic reaction.

In most applications, a simple replacement

strategy can be employed, where one

NHFR can replace an HFR. In some

instances, such as the case of hydrated

minerals such as aluminium trihydrate

or magnesium hydroxide, the transition

is more difficult. In order to achieve the

required flame retardance high loadings of

ATH are necessary, often in excess of 60%

by weight.

Once the volume fraction of inorganic

filler exceeds 50% there is a marked

deterioration of physical properties in the

compound. Plentz

et al

demonstrated

that in PP compounds containing ATH

there existed a relationship between the

filler loading and aggregate size.

This finding indicated that not only are

physical properties compromised by the

elevated filler loading, but that the ATH

also would aggregate as the loading

increased. Studies have shown that the

addition of a functionalised polymer is

an effective method to modify the inter-

facial adhesion at the organic/inorganic

boundary in polymer composites

2, 3,4

Mai

et al

demonstrated that incorporation

of graft modified acrylic acid into PP-ATH

compounds causes a chemical interaction

between the carboxyl and hydroxyl groups

in the polymer and filler respectively.

Improving the interfacial adhesion was

shown to improve both the thermal and

mechanical properties.

Similarly, Wang

et al

introduced maleic

anhydride grafted EPR into a PP-Mg(OH)

compound, and found that the EPR-g-MA

resided exclusively at the interface.

Encapsulating the Mg(OH)

improved

the dispersion of the filler, which was

manifested as improved impact strength.

Plentz

et al

introduced an acrylic acid

functional PP to their PP-ATH system, and

demonstrated that improved interaction

at the interface caused an increase in melt

flow index and improved tensile and flex

strength.

In all three cases, the functionalised

additives interacted with the filler to

overcome the deleterious effects of high

hydrated mineral filler loadings.

Traditional

functionalised

materials

have been investigated to overcome the

deficiencies in flame retardant compounds

containing ATH.

The current evaluation examines the effect

of low molecular weight functionalised

liquid polybutadienes (LPBD) as interfacial

modification agents in a 60% filled ethy-

lene vinyl acetate (EVA) wire and cable

(W&C) system. Feedback from industry has

indicated that migrating to an ATH solution

reduces the tensile strength, ductility and

flow to such a degree that the material

cannot function in W&C.

Having a low molecular weight is thought

to be advantageous to better seek and

adhere to the filler surface, thereby

enhancing the interfacial modification.

The type and loading level of the

functionality was varied to assess the

appropriate chemistry to best improve

EVA-ATH compounds.