Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-07-19
2003-03-04
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S405000, C524S413000, C524S415000, C524S419000, C524S420000, C524S416000, C524S436000, C524S437000, C524S494000
Reexamination Certificate
active
06528558
ABSTRACT:
FIELD OF INVENTION
The present invention relates to the flame retardation of polymeric compositions and especially compositions including fillers such as glass fibers, and, contrary to prior art teachings,without the use of red phosphorus and of halogen-based additives. This application correspond to Disclosure Document 457665 of Jun. 14, 1999.
BACKGROUND INFORMATION AND PRIOR ART
Flame retardation of polymers is at present mandatory for many of their application in many countries due to strict laws and regulations. Of particular importance are the requirements of polymeric devices, used in electrical, electronic and communication systems, which in many cases contain sizable amounts of glass fibers and other fillers. Accordingly, a number of systems for flame retarding various polymers were developed. These flame-retarding systems usually involve mixing or blending of the plastics with one or more flame retarding chemical additives. Most of these chemicals are either based on halogens or on red phosphorus and constitute low molecular weight compounds. They are in many cases applied in conjunction with co-additives or synergists such as antimony trioxide in the case of halogen compounds. The halogen-based additives provide a reasonable protection from fire hazards, however, they suffer from a number of serious inadequacies, which are responsible for difficulties in their application and use; they generate upon combustion highly-corrosive hydrogen halides and toxic substances. Red phosphorus which is used for glass-fiber containing polyamides can cause the emission of the undesirable toxic phosphine gas.
An important development in flame retardancy employed the principle of intumescence. According to this principle, the flame retardant additives form during the first stages of pyrolysis and combustion a foamed porous barrier which is impermeable to the combustible gases evolved during pyrolysis and to the molten polymer and prevents their flow to the flaming surface. In addition, it is believed that the intumescent barrier hinders the convection of the heat generated in the combustion from entering into the plastic. The additives used in the intumescent systems comprise a “catalyst”, usually ammonium polyphosphate (APP), a char-forming agent, in most cases a polyhydric alcohol such as pentaerythritol and a blowing agent usually a nitrogeneous material, such as melamine, guanidine or urea, which produce non-combustible gases. The APP is assumed to serve as a dehydration catalyst of the polyhydric alcohol and the dehydration is believed to occur via phosphorylation of the hydroxyl groups of the alcohol as well as of the hydroxyl groups formed by oxidation of the methyl groups of the polymer during combustion. Subsequent thermal dephosphorylation produced double bonds and crosslinks, which led to char structure foamed by the evolving gases from the blowing agent.
Whereas the intumescent systems provided a reasonable degree of flame retardance for a number of polymers such as polyethylene and polypropylene, it could not advantageously be applied to compositions containing glass fibers or other fillers. The presence of the glass fibers prevented the formation of the impermeable barrier essential for effective flame retardance. The char-forming additives appear not to be compatible with filler materials.
OBJECT
It is a primary object of the invention to improve on existing flame retardation techniques of polymeric composition.
SUMMARY OF THE INVENTION
It has surprisingly been found, that a high degree of flame retardancy can be imparted to glass fiber-containing polymers by using APP without char-forming agents, but with relatively small amounts of metal-based catalysts and of sulfur compounds. These formulations are very effective. It has unexpectedly and surprisingly been found, and this is the most striking feature of this invention, that the addition to an ammonium polyphosphate-based formulation of a relatively small amount of certain sulfur derivatives brings about a dramatic enhancement of the flame retardant effectivity of the system. The additives, according to the present invention, are less toxic and less corrosive than the formulations used in the art of flame retarding of polymers today. They are also readily available and relatively inexpensive.
The invention is applicable to a considerable number of thermoplastic polymers, including, as examples, polyamide (PA) 6 and 66, PA 11, PA 12, PA 4.4, PA 6.3, PA 6.4, PA 6.10, PA 6.12; polybutylene terephtalate (PBT); Polyethylene terephtalate (PET) and other polyesters saturated and unsaturated; polystyrenics; polyacrylics; polyurethanes; polycarbonates polyethylene (PE); Polypropylene (PP) and blends and copolymers of the above, as well as epoxy resins.
The function of the sulfur derivatives in their surprising effect is not exactly understood. It is believed that the effect is based on a specific new quasi-intumescent effect, which produces upon pyrolysis and combustion a non-dripping and non-ignitable surface with a relatively small amount of char. The sulfur derivative appears to be a more effective catalyst for the dehydration, cross-linking and char formation than APP alone. The sulfation and desulfation occur more rapidly than the phosphorylation and dephosphorylation. The char is formed both by the sulfation and the phosphorylation routes, but the char obtained appears to be a more effective, more compact and less penetrable surface barrier. The sulfur compounds may act as synergists of the APP, similar to the effect of antimony trioxide in the case of halogen-based additives.
The sulfur compounds applicable in the present invention are numerous and diversified. The common feature of all sulfur-based materials of this invention is their ability to interact at the pyrolysis and ignition temperatures, i.e., 350-500° C., with the other ingredients of the system i.e. the polymer, APP, glass fibers, and others. They should be stable enough at the processing temperatures, so as not to degrade markedly the polymer, but should react and produce the desirable effect of flame retardancy during combustion. Since the reactivity of various polymers at the high combustion temperatures differs, it is necessary to adapt to given polymeric substrates suitable sulfur derivatives.
The sulfur derivatives used in the present invention include inorganic and organic compounds of several valencies, such as: −2; 0; +4; +6. The minus two valency compounds are predominantly metallic sulfides, particularly of low water solubility and difficult to hydrolyze and in particular of heavy metals such as: ZnS
1
, GeS
2
, MoS
2
, MnS, Sb
2
S
3
, Sb
2
S
5
, and other heavy metal sulfides as described, for example, in Fritz Ephraim Inorganic Chemistry, fourth edition, page 229. Although many heavy metal sulfides may be objectionable for use as additives, in several cases due to toxicity (e.g. lead sulfide), and in other cases due to their dark color (Cu, Mo, Ni, Co, Ag), they could advantageously be used in the present invention. Of particular importance is ZnS due to its white color, high temperature stability, low cost and the fact that it is being used as a pigment additive to plastics to impart a white color. ZnS is thus known to be compatible with polymers. It has surprisingly been found that already small amounts of ZnS, in the range of 1-3 weight % of a polymer composition, yield a pronounced flame retardancy effect. At the combustion temperature, in the presence of air, the zinc sulfide is oxidized to higher valency products such as sulfur, zinc sulfoxylate, ZnSO
2
, thiosulfates, sulfites and finally sulfates. These oxidation reactions, which usually do not produce hydrogen sulfide, are rapid at the ignition temperature, but the oxidation products, which are more reactive than the sulfide, interact with the polymer and the other ingredients of the plastic composition to render the flame-retarding surface barrier.
It has surprisingly been ascertained that when applying sulfur compounds of the valencies between 6 and −2, high de
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