Polycarbonates, copolyestercarbonates, and polysiloxane copolycarbonates are high polymers produced by the condensation or intercondensation of a dihydroxy compound and a diacid or reactive derivative thereof such as an acid halide. When the dihydroxy compound is bisphenol-A and the acid derivative is phosgene, a simple polycarbonate (PC) polymer results. Similarly terephthalic acid and ethylene glycol intercondense to form polyethylene terephthalate (PET). Since these polymers are polyesters of bifunctional precursor monomers, it is theoretically possible for the reaction mixture to go entirely to completion and create one entire reaction vessel filling molecule. In practice, of course, this does not occur because as the polymerization increases the average chain length of the polymer increases, the viscosity of the reaction medium increases and the reaction probability decreases because there are progressively fewer complementary reactive species in a unit volume of the reaction vessel. Thus the reaction slows and eventually terminates on the basis of the statistics of reaction probability and the statistics of the polymer chain conformation because a reactive acid-derived terminus is statistically unlikely to find and react with a reactive hydroxyl terminus.
In producing these types of polyester polymers, endcapping or chain terminating agents are employed. In order to effectively terminate the growing end of a polymer molecule, these chain terminating or endcapping species must be monofunctional such that when reaction occurs with the growing end of the polymer molecule, further growth in the chain length of the particular polymer molecule is terminated. Thus depending on the statistical mechanics of polymer growth, there should be at least a rough correlation between the quantity of chain terminating agent, on a molar basis, and the average molecular weight of the polymer. Indeed, one function of endcapping agents, aside from the elimination of reactive ends, is to regulate the average molecular weight of the polymer being synthesized.
Typical endcapping agents have been monofunctional compounds of low molecular weight, high reactivity, readily available and cheap. Additionally such compounds have been monofunctional analogs of one or the other bifunctional monomers being polymerized. Thus in the case of polycarbonates, typical endcapping agents are various phenols such as phenol, tertiary-butyl-phenol, and para-cumyl-phenol. Other endcapping agents have been disclosed such as chromanyl in U.S. Pat. No. 3,697,481 to Bialous et al. herewith incorporated by reference. In general, aromatic polycarbonates and polycarbonate copolymers may be produced by various methods such as shown in U.S. Pat. Nos. 3,635,895 and 4,001,184, herewith incorporated by reference.
Variations in the mole ratio between the chain terminating compounds, such as phenol, and the chain growing compounds, such as bisphenol-A and phosgene, lead to the ability to control the molecular weight of the resulting polymer. Higher levels of chain terminating agents in the reaction mixture tend to lead to lower average molecular weights or shorter average polymer chain length. Conversely, lower levels of chain terminating agents in the reaction mixture tend to lead to higher average molecular weights or longer average chain length.
Frequently there are additional considerations or advantages associated with the choice of a particular chain terminating agent. Being esters, polymers such as polyesters, copolyestercarbonates, polycarbonates, polysiloxane copolycarbonates and the like are susceptible to hydrolysis and trans-esterification. A chain terminating agent that reduces the susceptibility of these polymers to hydrolysis or trans-esterification can impart improved properties to the polymer as well as functioning as a polymer chain length regulator during synthesis.
When put to use, these polymers may be alloyed with other polymers and/or compounded with various stabilizing and functionalizing additives. The additive compounds or mixtures of additive compounds are typically incorporated to prohibit undesired reactions of the polymer to the physical or chemical challenges experienced either during the process of converting the polymer to a useful article of manufacture or during the useful life of the manufactured article containing the stabilized polymer. These physical and chemical challenges include among others, slow oxidation, rapid oxidation (combustion), photolytic degradation, thermal degradation, and hydrolytic degradation. Consequently, depending on a particular polymer, there are to be found various stabilizer compounds available commercially either singly or in combination that improve or render more stable one or more of the physical or chemical properties of the polymer.
A particular problem associated with the polycarbonate family of polymers is stability to photolytic degradation, especially that caused by ultraviolet radiation. There are accordingly a large variety of stabilizer compounds useful to impart an improved resistance to the effects of ultraviolet radiation upon polycarbonate polymers. Among these stabilizer compounds are the phenolically substituted benzotriazole compounds. At low levels of addition to the polymer formulation, below about 0.5 to about 1.0 weight percent, the benzotriazole ultraviolet stabilizers generally disperse or dissolve in the polymer matrix in a satisfactory fashion and generally impart the desired ultraviolet resistance to the polymer. At higher levels, above about 2 to about 3 weight percent, the benzotriazole stabilizers have a tendency to undergo migration, phase separation, and plate out. This is a significant problem for certain extruded, laminated or layered sheet formulations where the function of the sheet is to provide a protective function for structural or glazing sheet thereunder, because when the stabilizer compound undergoes a phase separation the effective quantity of stabilizer compound present in the polymer matrix is reduced. Additionally, the stabilizer that migrates form the polymer matrix coats and/or plugs the manufacturing process equipment, causing surface defects and other quality problems in the articles being manufactured. This results in increased downtime of the manufacturing equipment for cleaning.
A previous approach exemplified by the teachings of U.S. Pat. No. 4,153,780 (the '780 patent) where phenolically substituted benzotriazoles, active for imparting ultraviolet resistance to polymers, are chemically bound as an endcapping agent to the polycarbonate polymer through the phenolic hydroxyl moiety. This approach incorporates the phenolically substituted benzotriazole as a chain stopping agent into the polymeric molecule. However, by the formation of a covalent chemical bond between the phenolic oxygen of the substituted benzotriazole and the terminal chloroformate group of the growing polycarbonate polymer, the ability of the phenolically substituted benzotriazole to function as an ultraviolet stabilizer is greatly reduced or altogether destroyed. Apparently, the phenol hydroxyl group of the phenolically substituted benzotriazole must be capable of forming a hydrogen bond in order for the molecule to function as an inhibitor for the degradative effects of ultraviolet radiation. While Applicant subscribes to this view as a matter of information and belief as however, the operability of Applicant's invention does not depend on this particular theoretical mechanism. While the incorporation of the benzotriazoles as taught in the '780 patent may render polycarbonates somewhat more stable to ultraviolet radiation, on a comparative basis the addition of an equivalent amount of free, as opposed to polymer bound, benzotriazole stabilizer compound to polycarbonates generally produces a better stabilizing effect in the polymers being treated therewith. Consequently, the benefit that might be achievable by chemical incorporation of the stabilizer molecule into the polymer is more than offset by a loss in efficacy caused by the changes in chemical bonding forced upon the stabilizer molecule when the stabilizer molecule is incorporated into the polymer.
Typically the stabilizer compounds are of a significantly lower molecular weight by comparison to the polymer being stabilized. This large difference in molecular weight leads to problems that are generally categorized as compatibility problems, i.e. the stabilizer may not be soluble in the polymer or because of its low molecular weight, the stabilizer has a tendency to volatilize or migrate out of the polymer matrix. A stabilizer that will not dissolve or disperse in the polymer to be stabilized does not impart any useful benefit to the polymer. Likewise a stabilizer that volatilizes or migrates out of the polymer matrix also does not impart any useful benefits to the polymer, and causes problems during manufacturing. The famous so-called "new car" smell is due to the migration and/or volatilization of various polymer stabilizing additives and plasticizers from the polymeric formulations widely employed in the manufacture of automobiles.