Poly(tetramethyleneterephthalate) Crosslinked by Irradiation

The crosslinking of many polymers by irradiation or by chemical means to improve their properties is a relatively new technology which has been found useful with many polymers, e.g., polyethylene, polyvinylchloride, polyoxymethylenes, polyvinylidene difluoride, etc. It is also known that the addition to such polymers of monomers which function as crosslinking promoters will have beneficial effects, e.g., as disclosed in U.S. Pat. Nos. 3,215,671 and 3,494,883. However, for some polymers only one or a few monomers function effectively in this regard and even for those polymers for which a substantial number of monomers are beneficial, there is a significant amount of selectivity and, in many cases, a drastic difference in effectiveness of monomers. Other patents disclosing such technology are U.S. Pat. Nos. 2,965,553; 3,137,674; and 3,580,829. Furthermore, it is known that the property of heat recoverability may be imparted to crosslinked polymers, e.g., according to the process of Cook et al U.S. Pat. No. 3,086,242, the disclosure of which is incorporated by reference herein.

However, as indicated in U.S. Pat. No. 3,142,629, not all polymers are susceptible to improvement by crosslinking, even with the addition of monomers which promote crosslinking in other polymers. Among polymers in wide use, polyethyleneterephthalate is perhaps the most significant polymer which has not been successfully crosslinked by irradiation. A closely related polymer, poly(tetramethyleneterephthalate) has likewise been found by applicant to realize no crosslinking when irradiated.

It has been found that the addition of N,N'-m-phenylenedimaleimide is uniquely effective in imparting crosslinkability to poly(tetramethyleneterephthalate), e.g., when the resulting mixture is irradiated. Accordingly, it is an object of the present invention to provide a useful crosslinked poly(tetramethyleneterephthalate)-containing polymer. It is a further object of the present invention to devise methods and compositional blends which impart desired modifications in properties of the crosslinked poly(tetramethyleneterephthalate)-containing polymer.

There is no departure from the scope of the present invention when other moieties are blended or copolymerized through the agency of a monomer with poly(tetramethyleneterephthalate), provided that such blending or copolymerization does not destroy the capacity of the resulting product to be crosslinked after the addition of N,N'-m-phenylenedimaleimide.

However, a particularly preferred blending material is the block copolymer consisting of two or more alternating segments of polytetramethylene ether with poly(tetramethyleneterephthalate). Such copolymers are sold under the name Hytrel, e.g., Hytrel 4055, Hytrel 5555 and Hytrel 6355 and have the general formula: ##SPC1##

In which x and y may be varied over a wide range. Two or more alternating segments of [CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 O] and ##SPC2## are possible.

The amount of N,N'-m-phenylenedimaleimide which may be used in the present invention will, as a minimum, be that effective to permit crosslinking of a given poly(tetramethyleneterephthalate)-containing polymer. In general, from about one to about three parts per hundred parts of N,N'-m-phenylenedimaleimide per hundred parts of polymer will give beneficial results. However, given the concept of the present invention, only routine experimentation will be required to establish an effective crosslinking amount of N,N'-m-phenylenedimaleimide.

Radiation dose levels to achieve crosslinking according to the present invention may range from about 2 to about 80 megarads or more, but a dose of about 10 to 40 megarads is preferred. For most purposes, a dose of about 20 megarads will be effective.

The present invention is further illustrated by the following examples of specific embodiments thereof.

One hundred parts of Tenite 6PRO, a poly(tetramethyleneterephthalate) manufactured by Eastman Chemical Products, Inc., was combined with 3 parts of N,N'-m-phenylenedimaleimide (HVA-2 manufactured by du Pont which has a melting point of about 200.degree.C and is stable at temperatures up to 250.degree.C). Samples of this material were irradiated using a 1 mev electron beam to the dose levels shown in Table I. As indicated in Table I, one set of samples was irradiated at room temperature, a second set of samples was irradiated at room temperature and then immediately placed in an oven at 145.degree.C, and a third set of samples was irradiated while sitting on a hot plate at 95.degree.C. After irradiation, each sample was expanded at 240.degree.C (the melting point of Tenite 6PRO is about 224.degree.C) and held at the expanded dimension until cooled to room temperature. The samples were then caused to undergo heat recovery by heating to above the melting point in the absence of any physical restraint. The degree of expansion and degree of recovery are shown in Table I.

As can be seen from Table I, the static modulus at 240.degree.C (M.sub.100) for each sample increased with increasing dose levels, thereby estabilishing that crosslinking occurred in each case. In marked contrast, a sample of Tenite 6PRO containing no HVA-2 showed no measurable static modulus even after a dose of 80 Mrads, thereby establishing that no measurable crosslinking occurs when the polymer alone is irradiated.

The elastic memory (or heat recovery) data shown in Table I establish that the crosslinked polymer has excellent performance in this regard, with expansions of over 300% possible and recovery close to or at 100% in each case. When noncrosslinked polymer is expanded, it is found that high expansions, e.g., 100% or more, are not usefully obtained because the degree of recovery decreases with increasing expansion, e.g., an expansion of 80% at 145.degree.C will recover less than 50% of the expanded dimension when heated at 195.degree.C, and even at low expansions, e.g., 40% or less, complete recovery is not possible, e.g., an expansion of 40% at 145.degree.C will recover less than 70% of the expanded dimension when heated at 195.degree.C.

It may also be noted from Table I that heating either during or immediately after irradiation (beaming) resulted in a greater degree of crosslinking at equivalent dose levels as shown by the higher static modulus obtained when such heating occurred.

Test specimens in the form of injection molded ASTM D-638, type IV, tensile dumbbells and bars were prepared from Tenite 6PRO and a 75:25 Tenite 6PRO/Hytrel 4055 blend. Some specimens of each were irradiated to doses of 20 and 80 megarads. Ultimate tensile strength, ultimate elongation, and impact strength are increased by the presence of Hytrel, but tensile strength at yield and flexural elastic modulus are reduced by its presence. Broadly speaking, flexibility is increased and stiffness decreased by the presence of Hytrel. Some representative results are shown in Table II. Each irradiated specimen contains 3 parts per hundred of HVA-2.

ASTM D638 was used to measure ultimate tensile strength, tensile strength at yield and ultimate elongation while ASTM D256 was used to measure impact strength. ASTM D790 was used to measure flexural elastic modulus.

Samples of Tenite 6PRO containing 3 parts per hundred of HVA-2 were irradiated to doses of 10, 20, 40 and 80 megarads. Notched impact strength determinations were then made for samples given no further treatment and for samples heated to 240.degree.C and then quickly quenched in ice water. At each dose level, it was found that the impact strength of the quenched samples was at least about twice that of the samples given no further treatment.

From the foregoing, it will be apparent that the addition of N,N'-m-phenylenedimaleimide converts poly(tetramethyleneterephthalate) from a non-crosslinkable material to a material capable of being crosslinked. The specificity of such effects is indicated by the fact that N,N'-m-phenylenedimaleimide does not produce beneficial results when added to polyoxymethylenes which embrittle seriously when irradiated in spite of the fact that triallyl cyanurate does produce beneficial results in polyoxymethylenes and both triallyl cyanurate and N,N'-m-phenylenedimaleimide produce beneficial results in polyvinylidenefluoride.

The crosslinked poly(tetramethyleneterephthalate) products of the present invention have good electrical insulation properties, good low temperature properties and excellent solvent resistance. Thus, such products are useful for electrical insulation, heat recoverable parts such as those disclosed in Wetmore U.S. Pat. No. 3,243,211, the disclosure of which is incorporated by reference herein, and the like.

When poly(tetramethyleneterephthalate) is blended with a block copolymer consisting of two or more alternating segments of polytetramethylene ether and poly(tetramethyleneterephthalate), the amount of copolymer added may vary over wide limits depending on the properties desired. The blend may contain up to 50% or more of the copolymer.

Tenite 6PRO, a poly(tetramethyleneterephthalate) containing N,N'-m-phenylenedimaleimide was fabricated into sheet form. Following irradiation crosslinking, this sheet was vacuum formed easily into various shapes. Sheet of Tenite 6PRO not containing N,N'-m-phenylenedimaleimide and not crosslinkable is very difficult to vacuum form because at the forming temperature the plastic lacks sufficient melt strength to prevent excessive sagging or melting. Crosslinking increases melt strength (viscosity) permitting heating of sheet for vacuum forming without excessive sagging or melting.