Compositions – Compositions containing a single chemical reactant or plural... – Organic reactant
Reexamination Certificate
1995-06-07
2001-03-06
Henderson, Christopher (Department: 1713)
Compositions
Compositions containing a single chemical reactant or plural...
Organic reactant
C252S182170
Reexamination Certificate
active
06197213
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the prevention of scorching prior to crosslinking of a peroxide or azo compound crosslinkable thermoplastic and/or elastomeric composition.
A major difficulty in using organic peroxides or azo compounds in crosslinking (curing) elastomeric and thermoplastic materials applications is that they may initiate premature crosslinking (i.e. scorch) during compounding and/or processing prior to the actual phase in the overall process when curing is desired. With conventional methods of compounding, such as milling, Banbury, or extrusion, scorch occurs when the time-temperature relationship results in a condition where the peroxide or azo initiator undergoes thermal decomposition, initiating the crosslinking reaction whereby gel particles in the mass of the compounded polymer may be formed. The presence of these gel particles leads to inhomogeneity of the final product. Excessive scorch reduces the plastic properties of the material so that it can no longer be processed, resulting in the loss of the entire batch.
Therefore, it has been widely accepted that the peroxide of choice must have a high enough activation temperature so that compounding and/or other processing steps can be successfully completed prior to the final curing step. Thus one method of avoiding scorch is to use an initiator that is characterized by having a high 10 hour half-life temperature. The disadvantage to this approach is that one subsequently obtains a longer cure time, which results in lower throughput. High cure temperatures can be used but this runs into the disadvantage of higher energy costs.
A further way of avoiding scorch is to lower the compounding and/or processing temperature to improve the scorch safety margin of the crosslinking agent. This option however may be somewhat limited in scope depending upon the polymer and/or process involved. In addition, curing at the lower temperature requires longer cure times and results in lower throughput. Prior to the present invention, certain additives were incorporated into compositions which reduced the tendency for scorching. For example, British patent 1,535,039 discloses the use of organic hydroperoxides as scorch inhibitors for peroxide-cured ethylene polymer compositions. U.S. Pat. No. 3,751,378 discloses the use of N-nitroso diphenylamine or N,N′-dinitroso-paraphenylamine as retarders incorporated in a polyfunctional acrylate crosslinking monomer for providing long Mooney scorch times in various elastomer formulations. U.S. Pat. No. 3,202,648 discloses the use of nitrites such as isoamylnitrite, tert-decyl nitrite and others as scorch inhibitors for polyethylene. U.S. Pat. No. 3,954,907 discloses the use of monomeric vinyl compounds as protection against scorch. U.S. Pat. No. 3,335,124 describes the use of aromatic amines, phenolic compounds, mercaptothiazole compounds, bis(N,N-disubstituted thiocarbonyl)sulfides, hydroquinones and dialkyldithiocarbamate compounds. The use of mixtures of the active compounds in preventing scorch is neither taught nor suggested. U.S. Pat. No. 4,632,950 discloses the use of mixtures of two metal salts of disubstituted dithiocarbamic acid, wherein one metal salt is based on copper. This reference does not teach the use of such mixtures with neat peroxides. For some applications, it is desirable or mandatory to use liquid or neat peroxides, as described in this current invention. One such application is in extruded compounding. A common commercial process technique employs a liquid peroxide which is sprayed onto polymer pellets or granules to coat them prior to extrusion compounding. This can provide increased production efficiency and eliminates physical handling of hazardous compounds. This reference patent teaches that at least one filler must be present. The scorch resistant systems described in this reference are not effective in polyolefins specifically LDPE, LLDPE, or HDPE. The present invention is effective in polyolefin systems. Moreover, this reference does not teach the use of mixtures of hydroquinones and metal salts of disubstituted dithiocarbamic acid.
When employing these prior art methods for extending scorch time, the cure time and/or final crosslink density of the cured composition can be adversely affected, leading to a decrease in productivity and/or product performance. The present invention overcomes the disadvantages of the prior art in that an improvement in scorch at compounding temperatures is achieved without significant impact on the final cure time or crosslink density. This is achieved by incorporation of the cure retarding composition at low additive levels, thereby limiting the effect on properties. In addition, significant scorch protection is achieved, since the use of the combination of the hydroquinones and a sulfur accelerator of the dithiocarbamate or thiuram class results in a synergistic effect on scorch time at the low additive levels employed.
SUMMARY OF THE INVENTION
The present invention provides in a first composition aspect a scorch retarding composition comprising a hydroquinone and at least one sulfur accelerator.
The tangible embodiments of this composition aspect of the invention possess the inherent applied use characteristics of being scorch retarders showing greater effect than equivalent amounts of either component used separately when incorporated into polymeric compositions which are crosslinkable by free radical initiation while not substantially adversely affecting final cure time or properties.
Special mention is made of compositions of the first composition aspect of the invention which additionally comprise a coagent.
The invention also provides in a second composition aspect a scorch retarding, curing/crosslinking composition comprising a free radical initiator selected from the group consisting of organic peroxides, azo compounds and mixtures thereof, and the scorch retarding composition of the first composition aspect of the invention.
The tangible embodiments of this second composition aspect of the invention possess the inherent applied use characteristics, when blended into conventional thermoplastic and/or elastomeric polymers as a crosslinking agent, of providing improved scorch protection for the blended system while not substantially adversely affecting final cure times or characteristics.
This invention also provides in a third composition aspect a crosslinkable composition comprising a peroxide or azo compound crosslinkable thermoplastic and/or elastomeric polymer, and a scorch retarding curing/crosslinking composition as defined in the second composition aspect of the inventions.
The invention also provides in an improved process for the preparation of a crosslinkable composition comprising a peroxide or azo compound crosslinkable thermoplastic and/or elastomeric polymer and a free radical initiator selected from the group of organic peroxides, azo compounds and mixtures thereof wherein said polymer is compounded with said free radical initiator, the improvement comprising performing said compounding in the presence of a scorch retarding composition of the first composition aspect of the invention.
Special mention is made of processes of this process aspect of the invention wherein the scorch retarding composition additionally comprises a coagent.
In the practice of this invention, the preferred blends of hydroquinones and sulfur accelerators exhibit acceptable solubility in the free radical initiators when the selected free radical initiator is a liquid or low melting solid. Thus, this new technology will allow for a pumpable or a meterable homogeneous crosslinking system that provides ease of handling and greater worker safety as well as longer compounding times for better mixing due to the improved scorch protection provided.
Where homogenous liquid or low melting solid crosslinking compositions are not normally used such as in rubber compounding, and the selected scorch retarding crosslinking composition is liquid, the hydroquinone, peroxide, sulfur accelerator and optional coagent(s)
Hibbard Edward Phillip
Kmiec Chester Joseph
Novits Michael Fred
ATOFINA Chemicals, Inc.
Bright Royal E.
Henderson Christopher
Marcus Stanley A.
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