Flame Retardant Hot Melt Adhesive Compositions Using Brominated Styrene/Atactic Polypropylene Graft Copolymers
Flame retardant hot melt adhesives include atactic polypropylene having grafted thereon a ring-halogenated styrene. The compositions may also include homopolymers of the styrene monomer or equivalents, as well as non-halogenated atactic polypropylene. Tackifying agents, waxes, anti-oxidants and other additives may also be included.
Flame Retardant Hot Melt Adhesive Compositions Using Brominated Styrene/Atactic Polypropylene Graft Copolymers, E. J. Termine, R. W. Atwell and N. A. Favstritsky, United States Patent No. 5,041,484.
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of compositions useful in the manufacture of hot melt adhesives and to the adhesive compositions themselves, and particularly to the preparation of flame retardant hot melt adhesives containing atactic polypropylene.
2. Description of the Prior Art
Atactic–also known as amorphous–polypropylene (APP) was for years an undesirable by-product of isotactic polypropylene production. Due to its low price, APP soon found its way into many applications, including use in adhesive formulations. Eventually, opportunities arose for APP adhesives requiring that they possessed some degree of flame retardancy.
A hot melt adhesive is generally manufactured from a mixture of three components: a thermoplastic resin capable of providing cohesion of the mixture, a petroleum resin having a tackifying effect, and a paraffin or a microcrystalline polymeric wax capable of adapting the viscosity of the mixture to the processing and use conditions of the adhesive. In choosing the thermoplastic resin part of the mixture, good properties are sought in respect to heat stability and adhesiveness. The thermoplastic resins commonly used in compositions for the manufacture of hot melt adhesives have included copolymers of ethylene and vinyl esters, particularly vinyl acetate, or copolymers of ethylene and alkyl acrylates, particularly ethyl acrylate and butyl acrylate.
A typical hot melt adhesive formulation is described in U.S. Pat. No. 4,325,853, issued to Acharya et al. on Apr. 20, 1982. This patent indicates that a typical adhesive comprises, for example, ethylene copolymer, a tackifier and a wax. The Acharya patent is directed to the particular use of alkenyl succinic anhydride in combination with ethylene copolymer and a tackifying resin. It is indicated that wax-like materials and anti-oxidants may also be used. Among the ethylene copolymers mentioned are ones with vinyl acetate or with acrylic and methacrylic acid.
A general form of hot melt adhesive is also described in U.S. Pat. No. 4,404,299, issued to Decroix on Sept. 13, 1983. The Decroix patent indicates the adhesive to conventionally include a thermoplastic resin, a tackifying resin, and a paraffin or other wax-like substance for modifying the melt viscosity. Particularly described in the Decroix patent is the use of a terpolymer of ethylene and minor parts of alkyl acrylate or methacrylate and of maleic anhydride.
While atactic polypropylene (APP) has been found to be suited for use in hot melt adhesives, there has remained a desire to improve the flame retardancy of APP without adversely affecting the other desirable properties of APP. Approaches to this problem have included the modification of APP directly or the addition of flame retardants into the overall adhesive formulation.
Improvement of flame retardancy has relied on modifications to atactic polypropylene, or on additives for the APP, but disadvantages have been associated with both approaches. Although a vast number of modified polypropylene compositions have been described or theorized in the prior art, few if any suitable flame retardant APP derivatives have been identified. Similarly, numerous additives for increasing the flame retardancy of atactic polypropylene have been studied and some are commercially available. Nonetheless, there is at present no commercially available flame retardant additive for atactic polypropylene which provides adequate retention of physical properties, and demonstrates high thermal stability and absence of solids at processing temperatures. The present invention contemplates a modification of atactic polypropylene which yields a composition that retains the desirable physical properties of atactic polypropylene, and avoids the disadvantages of alternate approaches.
Hot melt adhesive formulations can be made flame retardant by the addition of halogenated liquid and/or solid additives. However, liquid additives weaken the adhesive bond strengths by dilution and plasticization of the adhesive formulation. On the other hand, solid additives act as fillers, and can result in settling and lower tensile strengths. Present adhesive dispensing systems are not generally equipped with stirrers, and solid insoluble flame retardant additives which settle out of the adhesive can clog feed lines.
State of the art methods for rendering APP flame retardant consist of blending with inert additives, an approach which has not been entirely successful. Sedimentation of the additive in the melt vessel or transfer lines, thermal instability, volatility, migration out of the adhesive, high viscosity and poor adhesive strength are problems associated with various additive-type flame retardants. We have found that ring brominated styrene, when grafted to APP, provides a novel composition that is useful as a flame retardant hot melt adhesive. Furthermore, adhesive strength and melt viscosities are actually improved over the base polymer.
One method of conferring flame retardancy to APP is to chlorinate it by grafting chlorine atoms directly onto it. G. M. Ronkin, et al., “Structure and Properties of Chlorinated Polypropylene”, Plast. Massy, 1987, 3, 20-23, report that the direct chlorination of isotactic and amorphous atactic polypropylene provides flame resistance and self-extinguishing properties, but with decreased strength. In U.S. Pat. No. 3,291,862, issued to W. B. Armour, et al. on Dec. 13, 1966, it is similarly disclosed that chlorinated atactic polypropylene yields fire retardation accompanied by increased elongation and decreased tensile and peel strength. While this approach would solve the problem of flame retardant sedimentation, the relatively low strength of the non-aromatic carbon-chlorine bond is not sufficient to prevent thermal degradation over the long periods and high temperatures to which hot melt adhesives are subjected.
Certain molecular flame retardants have been grafted onto APP by other researchers. M. Hartman, et al., “Graft Copolymerizations of Vinylphosphonic Acid Derivatives on Atactic Polypropylene”, Z. Chem., 1980, 20(4), 146-7, describe the use of vinyl phosphonates to flame retard APP. There is no indication that the grafts are useful as flame retardant adhesives.
The most widely used method to produce a flame retardant hot melt adhesive is to add a non-reactive halogenated material. In U.S. Pat. No. 4,169,082, issued to Kusterer on Sept. 25, 1979, there is disclosed the use in a hot melt adhesive of commercially available chlorinated (preferred) or brominated waxes in combination with paraffin wax, a halogen scavenger, and flame and smoke suppressants. The use of chlorinated paraffin as an additive to atactic polypropylene in a hot melt adhesive is described in U.S. Pat. No. 3,803,067, issued to Kehr, et al. on Apr. 9, 1974. M. Kellner, et al., in Czechoslovakia Patent No. 176,612, issued on Jan. 15, 1979, disclosed the use of polychlorobiphenyl with ethylene vinyl acetate copolymer and/or atactic polypropylene for use in a hot melt adhesive. While these approaches have succeeded in minimizing unmelted solids in the composition, the additives introduce problems of poor thermostability in the case of the paraffin and unacceptable toxicity for the PCB.
Various non-halogenated monomers have been grafted to polypropylene in accordance with the prior art. For example, maleic anhydride is commonly grafted to atactic polypropylene to increase adhesive bond strength. In U.S. Pat. No. 4,049,602, issued to Albers on Sept. 20, 1977, there is disclosed the grafting of unbrominated styrene to atactic polypropylene for the purpose of producing an inherently white adhesive without the use of opacifying fillers.
U.S. Pat. No. 3,034,939, issued to Newkirk, et al. on May 15, 1962, discusses fire extinguishing properties for a hot melt adhesive composition containing chlorinated biphenyls and/or chlorinated paraffins as flame retardants. The use of the former material is now EPA regulated due to PCB contamination problems. The latter material is subject to dehydrochlorination if allowed to stand at the 270.degree.-350.degree. F. temperature required for use. Additionally, antimony trioxide and calcium carbonate (which tend to settle out) are required. There is no adhesive data in the Newkirk, et al. patent.
In U.S. Pat. No. 4,279,808, issued to Hornbaker et al. on July 21, 1981, there is described a method for the preparation of polybromostyrene resin by the addition polymerization of nuclear brominated styrene. The Hornbaker et al. patent is limited to the addition polymerization of bromostyrene in the presence of specified rubbery polymers, namely SBR rubber (butadiene-styrene copolymers), EPR rubber (ethylenepropylene copolymers), EPDM rubber (i.e. terpolymers of ethylene, propylene and a diene monomer), polyisoprene rubber (e.g. cis-1,4-polyisoprene and trans-1,4-polyisoprene), Neoprene (i.e. polymers and copolymers of 2-chloro-1,3-butadiene), cis-1,4-polybutadiene, and polybutadienes having mixed structures (e.g. cis-1,4; trans-1,4 and 1,2 structures), with the polybutadienes being particularly preferred.
Other commercial flame retardants, such as decabromodiphenyl oxide (DDPO) are also used in flame retardant hot melt adhesives, as described in U.S. Pat. No. 4,727,107, issued to McConnell, et al. on Feb. 23, 1988. DDPO is an insoluble solid melting at 300.degree.-315.degree. C., which acts as a filler and can settle out of the adhesive formulation. The McConnell patent claims the use of decabromodiphenyl ether in flame retardant hot melt adhesives for polyurethane foams and fabrics. The use of antimony oxide is also claimed. These materials will settle out of the adhesive mixture. Also, formulating the adhesive is cumbersome due to the pre-melting and mixing of the flame retardant and antimony oxide with polyester to form a concentrate which is then back added to the base polyester for application. No adhesive data is given in the McConnell patent.
Outside of the field of flame retardancy, various modifications to polyolefins have been proposed. For example, U.S. Pat. No. 3,177,270, issued to Jones et al. on Apr. 6, 1965, describes a method for modifying polyolefins for the purpose of improving tensile strength, elongation and/or flexural modulus. The Jones et al. patent specifically describes the preparation of ethylene polymer which is modified with styrene, a styrene/acrylonitrile mixture, dichlorostyrene or a mixture of isomeric vinyltoluenes. The Jones et al. patent additionally lists other possible polymeric substrates for use in the described method as including polypropylene, polyisobutylene, polybutene, and copolymers of ethylene and propylene, ethylene and isobutylene, and ethylene and butene. The patent indicates that other copolymers that can be used include those containing a predominant amount of one or more aliphatic olefins chemically combined or interpolymerized with a minor proportion of another monoethylenically unsaturated organic compound, such as copolymers of a predominant amount of ethylene and a minor proportion of styrene, vinyl acetate or methyl methacrylate. Possible graft monomers are listed as including styrene, vinyltoluene, vinylxylene, ethylvinylbenzene, isopropyl styrene, para-tert-butyl styrene, dichlorostyrene, bromostyrene, fluorostyrene, or mixtures thereof with acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, methyl methacrylate or maleic anhydride.
U.S. Pat. No. 4,179,401, issued to Garnett et al. in 1979, relates to a process for producing a heterogenous catalyst for the hydrogenation, hydroformylation, isomerization, cracking or dehydrogenation of organic molecules. The Garnett process comprises the steps of radiation grafting a monomer having an alpha-unsaturated bond to a metal or an organic polymer and complexing a nitrogen, halogen, or phosphorous containing group to the monomer. The Garnett et al. patent lists many possible polymer/monomer combinations. Identified polymer substrates included polyvinyl compounds, polyolefins, polyvinylidenes, polysiloxanes, polydienes, polyethers, polyimides, polysulphones, polyesters, polyamides, polyurethanes, polycarbonates and polyureas. Listed as possible monomers for use in the described process were p-nitrostyrene, p-amino styrene, p-chlorostyrene, vinyldiphenylphosphine, cis bis (1,2-diphenylphosphino) ethylene, triallylphosphine, divinylphenylphosphine and many more.
The graft atactic polypropylene polymers used in the present invention do not suffer the drawbacks of having insoluble flame retardants which act as fillers, increase melt viscosity, and require the use of solid antimony synergists or other flame retardant enhancing agents. Since the grafts are polymeric and compatible in the adhesive formulation, they do not perform like liquid flame retardants which can weaken and migrate from adhesive bonds.
SUMMARY OF THE INVENTION
In one aspect of the present invention there is provided a flame retardant hot melt adhesive composition including atactic polypropylene having grafted thereon a ring-halogenated styrene. The flame retardant graft copolymer is represented by the formula: ##STR1## in which n is an integer>1, APP is atactic polypropylene, and S is a side chain grafted to the APP and having monomeric units of the formula: ##STR2## wherein x=1 to 4, R.sub.1 is H or CH.sub.3, and R.sub.2 is H or a C.sub.1-4 lower alkyl group. The compositions may also include homopolymers of the styrene monomer or an equivalent, as well as non-grafted atactic polypropylene. Tackifying agents, waxes, anti-oxidants and other additives may also be included.
It is an object of the present invention to provide hot melt adhesives which have desirable physical properties and improved flame retardancy.
A further object of the present invention is to provide hot melt adhesives which do not include flame retardant additives which adversely affect physical properties, such as melt viscosity, or which settle out of the composition.
Further objects and advantages of the present invention will be apparent from the description which follows.