Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2002-01-15
2003-06-17
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S203000, C525S207000, C525S212000, C525S232000, C525S238000, C525S240000, C525S241000
Reexamination Certificate
active
06579944
ABSTRACT:
FIELD OF THE INVENTION
Polymer blends having a combination of elastic and thermoplastic properties, referred to as “thermoplastic vulcanizates” or “TPVs” (also referred to in the past as “thermoplastic elastomers” or “TPEs”) are made by dynamic vulcanization to provide desired hardness/softness, oil and temperature resistance, oxidation resistance, and processability, inter alia. In thermoplastic elastomers which are elastomeric alloys and not physical blends, the properties depend on the relative amounts of “hard” and “soft” phases provided by each component, and the properties of each component. To be of commercial value, the hard phase is typically provided by a readily available engineering thermoplastic resin, familiarly referred to as a “plastic” for brevity. Most commonly the plastic is chosen from polyesters, polyamides and polyolefins which provide a continuous phase of the hard phase in which dispersed domains of the “soft” phase of an elastomer are present. Optimizing the elastic recovery of a TPV and confirming the physical nature of its defined morphology, is the subject of this, invention. Confirmation is obtained with both photomicrographs and computer modelling. The photomicrographs are from an electron microscope, preferably a transmission electron microscope (TEM) Of particular interest are relatively “soft” blends of a vulcanizable (hereafter “curable” for brevity) rubber having controlled hardness less than about 90 Shore A. Such blends are exceptionally resistant to oil swelling, and to compression set. The term “elastomer” is used herein to refer to a vulcanized blend of polyolefin and rubber which may be formulated to exhibit varying degrees of elasticity such that a test strip 2.5 cm wide and 25 mm thick may be stretched in the range from about 5% to 100% of its initial length and still return to it; further, such vulcanized elastomer is necessarily thermoplastic and re-processable.
The Problem
There is a market need for blends of polar engineering thermoplastics containing a dispersed “polar rubber” phase and a continuous “plastic” phase, which blends have high elastic recovery. The term “elastic recovery” refers to the proportion of recovery after deformation and is quantified as percent recovery after compression. A TPV having a volume fraction of rubber particles greater than about 0.7 may have an elastic recovery in the range from about 50% to 60% at 50% compression; to get a higher elastic recovery one may modify the composition of the particular rubber dispersed in the continuous plastic phase, the ratio of the dispersed and continuous phases, the amounts and composition of the curing agent(s) used, the amount of processing oil, and other ingredients, and other factors, with the expectation that, with enough trial and error, one can make a TPV with an elastic recovery in the range from about 60% to 65%. How these factors influence the morphology of a TPV has been the subject of much study. Very little of this study has been devoted to identifying the key morphological requirement in a TPV which is most likely to provide much higher elastic recovery than one would normally expect of the same TPV produced according to prior art procedures not specifically directed to the formation of the critical morphology.
BACKGROUND OF THE INVENTION
Elastic recovery is that fraction of a given deformation that behaves elastically; a perfectly elastic material has a recovery of 100% while a perfectly plastic recovery has no elastic recovery. (see
Whittington's Dictionary of Plastics
3rd Ed. 1993 Technomic Publishing). Elastic recovery is an important property of a TPV which is expected to behave like a natural rubber for examples in application where a TPV is used in dynamic applications such as in hoses, and in sealing applications.
To date, a TPV is formulated with specified components including in addition to the rubber and plastic, plasticizers, processing aids and fillers, by melt-blending the ingredients within generally defined processing parameters, until by trial and error, a usable TPV is made. A “usable TPV” is one which can be used in a marketable product. In particular, how the components are confined in a mixing and melt-blending means, the rate at which mixing energy is inculcated, the time over which the components are melt-blended, and the conditions under which the TPV is cooled are derived from experience and by trial and error. Though it is likely, with all the work directed to the production of TPVs over the past decade, that TPVs having optimum morphology may have been produced; but if they have been, the morphology produced was accidentally produced. An improvement in elastic recovery was generally sought by varying the curing agent for the rubber, and also the processing oil, processing aid, and filler. No one has recognized, much less identified, the critical morphological feature directly responsible for producing elastic recovery substantially greater than that which is obtained if the critical feature is lacking in a usable TPV.
A usable TPV, contains particles of rubber the majority of which, that is greater than 50% by volume, are in the size range less than about 5 &mgr;m, some being as large as 10 &mgr;m and others being as small as 0.1 &mgr;m or smaller. Particles smaller than 0.1 &mgr;m are believed to be portions fractured from larger particles while the TPV is being melt-blended, and this very small size serves to define them as “very small” particles. A TPV preferred for its superior physical properties and acceptable elastic recovery has relatively large domains of rubber the majority of which are in the size range from about 1-5 &mgr;m, preferably 1-3 &mgr;m, and this size range serves to define them as “large particles”. The shape of all particles resembles that of a distorted ellipsoid or elongated ovoid, and this shape is particularly evident in large particles. The remaining rubber particles, in the size range larger than a “very small” particle and smaller than the mean diameter of the “large particles”, are defined as “small particles” or “mid-range particles” which also are generally ellipsoidal in shape. Because of the shape, the “diameter” referred to is the effective diameter, that is, the diameter the particle would have had if it was spherical. The elongated ovoid shape of the particles allows a high packing fraction of rubber particles in a unit volume of TPV, this being a characteristic of a usable TPV. The number of very small particles is of minor consequence in a TPV; the number of small and large particles is not. To date, there has been no clear teaching as to what effect the size of the particles and their distribution has in a TPV particularly with respect to its elastic recovery.
The morphology of various TPVs has been characterized in an article titled
Morphology of Elastomeric Alloys
by Sabet Abdou-Sabet and Raman P. Patel (Rubber Chem. & Tech., Vol 64, No. 5,pg 769-779, Nov.-Dec. 1991). Several variables affecting the morphology are identified, including the molecular weight of EPDM and PP; the ratio of EPDM to PP; degree of crosslinking; and types of crosslinks; but the effect of the thickness of a ligament, or the volume of continuous plastic phase between adjacent particles was not appreciated. The term “ligament” as used herein refers to the material of the continuous plastic phase connecting two adjacently disposed particles, and the “thickness of a ligament” refers to the minimum narrowed distance between two adjacent particles.
The origin of the overall elastomeric-like stress-strain behavior of a TPV including a large percentage of recoverable strain upon unloading is addressed in publications by Kikuchi et al (1992), Kawabata et al (1992) and Soliman et al (1999). In an article titled Origin of Rubber Elasticity in Thermoplastic Elastomers Consisting of Crosslnked Rubber Particles and Ductile Matrix, by Y. Kikuchi, T. Fukui, T. Okada and T. Inoue (Jour. of Appl. Polym. Sci. 50, 261-271 (1992), the strain recovery of a TPE is analyzed using a two-dimensional model for a two-phase sy
Abdou-Sabet Sabet
Boyce Mary C.
Kear Kenneth Emery
Shaw Karla Drew
Advanced Elastomer Systems LP
Lobo Alfred D.
Nutter Nathan M.
Skinner William A.
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