Impact Modified Poly(alkylene Terephthalates)

1. Field of the Invention

This invention relates to impact modification of poly(alkylene terephthalates).

2. Description of the Prior Art

Although impact modifiers for poly(alkylene terephthalates) based on rubbers of polybutadiene, butadiene-styrene copolymers, butadiene-acrylonitrile copolymers, ethylene-propylene rubbers, polyisobutene and polyisoprene have been suggested, no one has suggested impact modifiers based on acrylic rubbers which are multi-phase composite interpolymers. See, for example, U.S. Pat. No. 3,919,353. Although such acrylic rubber-based composite interpolymers have been suggested as impact modifiers for thermoplastic polymers in general, they have especially been suggested for vinyl halide polymers and acrylic polymers.* It was unexpected that such impact modifiers should also function well to modify poly(alkylene terephthalates) because acrylic polymers and vinyl halide polymers are both considered amorphous plastics whereas poly(alkylene terephthalates), especially poly(butylene terephthalate), are generally considered to be a crystalline plastics. It would be considered unexpected to those skilled in the art that the modifiers and techniques used to obtain enhancement of specific properties in one plastic can be extended to the other plastic.

Combination impact modifier melt strength improvers based on multiple stage polymers having an epoxy functional final stage have been disclosed by Lane, Ser. No. 631,761 filed Nov. 13, 1975 now U.S. Pat. No. 4034013. Lane's materials operate by significantly increasing melt viscosity. However it is frequently desired by processors of poly(alkylene terephthalates), especially in injection molding of PBT, to achieve impact modification without significantly increasing melt viscosity.

It is therefore an object of the invention to provide an impact modified poly(alkylene terephthalate). Another object of the invention is to provide a process for improving impact strength of poly(alkylene terephthalate) without significantly increasing melt viscosity. These objects and others as will become apparent from the following disclosure are achieved by the present invention which comprises a blend of about 99 to 60 percent by weight of a poly(alkylene terephthalate) and about 1 to 40 percent by weight of a multi-phase composite interpolymer comprising about 25 to 95 weight percent of a first elastomeric phase polymerized from a monomer system comprising about 75 to 99.8 percent by weight of a (C.sub.1 to C.sub.6) alkyl acrylate, 0.1 to 5 percent by weight cross-linking monomer, and 0.1 to 5 percent by weight graftlinking monomer, and about 75 to 5 weight percent of a final, rigid thermoplastic phase free of epoxy groups polymerized in the presence of said elastomeric phase.

The poly(C.sub.1 to C.sub.6 alkylene terephthalates) which are modified in accordance with this invention can be poly (methylene terephthalate), poly(ethylene terephthalate), poly (propylene terephthalate), poly(butylene terephthalate), poly(pentylene terephthalate), poly(cyclohexene terephthalate), and the like. Poly(butylene terephthalate) is especially suitable to impact modification in accordance with this invention. Suitable amounts of impact modifier are from 1 to 40 percent by weight of the blend. Below 1 percent, no significant effect is seen, and it is preferred to have at least 10 percent by weight of the impact modifier for significant improvement. Above 40 percent by weight, no further improvement is economically achieved. The impact modifier is a multi-phase composite interpolymer comprising about 25 to 95 weight percent of a first elastomeric phase and about 75 to 5 weight percent of a final rigid thermoplastic phase. One or more intermediate phases are optional, for example a middle stage polymerized from about 75 to 100 percent by weight styrene. The first stage is polymerized from about 75 to 99.8 weight percent C.sub.1 to C.sub.6 acrylate resulting in an acrylic rubber core having a T.sub.g below about 10.degree. C. and crosslinked with 0.1 to 5 weight percent crosslinking monomer and further containing 0.1 to 5 percent by weight graftlinking monomer. The preferred alkyl acrylate is butyl acrylate. The crosslinking monomer is a polyethylenically unsaturated monomer having a plurality of addition polymerizable reactive groups all of which polymerize at substantially the same rate of reaction. Suitable crosslinking monomers include poly acrylic and poly methacrylic esters of polyols such as butylene diacrylate and dimethacrylate, trimethylol propane trimethacrylate, and the like; di- and trivinyl benzene, vinyl acrylate and methacrylate, and the like. The preferred crosslinking monomer is butylene diacrylate. The graftlinking monomer is a polyethylenically unsaturated monomer having a plurality of addition polymerizable reactive groups, at least one of which polymerizing at substantially different rate of polymerization from at least one other of said reactive groups. The function of the graftlinking monomer is to provide a residual level of unsaturation in the elastomeric phase, particularly in the latter stages of polymerization and, consequently, at or near the surface of the elastomer particles. When the rigid thermoplastic phase is subsequently polymerized at the surface of the elastomer, the residual unsaturated addition polymerizable reactive group contributed by the graftlinking monomer participates in the subsequent reaction so that at least a portion of the rigid phase is chemically attached to surface of the elastomer. Among the effective graftlinking monomers are allyl group-containing monomers of allyl esters of ethylenically unsaturated acids such as allyl acrylate, allyl methacrylate, diallyl maleate, diallyl fumarate, diallyl itaconate, allyl acid maleate, allyl acid fumarate, and allyl acid itaconate. Somewhat less preferred are the diallyl esters of polycarboxylic acids which do not contain polymerizable unsaturation. The preferred graftlinking monomers are allyl methacrylate and diallyl maleate. A most preferred interpolymer has only two stages, the first stage comprising about 60 to 95 percent by weight of the interpolymer and being polymerized from a monomer system comprising 95 to 99.8 percent by weight butyl acrylate, 0.1 to 2.5 percent by weight butylene diacrylate as crosslinking agent, 0.1 to 2.5 percent by weight allyl methacrylate or diallyl maleate as a graftlinking agent, with a final stage polymerized from about 60 to 100 percent by weight methyl methacrylate. The impact modifiers of the present invention do not function as melt viscosity increasers as do the modifiers described in U.S. patent application Ser. No. 631,761 filed Nov. 13, 1975 now U.S. Pat. No. 4034013, since they do not contain a reactive outer stage; more particularly they do not contain reactive epoxide units in the outer stage.

The final stage monomer system can be comprised of C.sub.1 to C.sub.16 methacrylate, styrene, acrylonitrile, alkyl acrylates, allyl methacrylate, diallyl methacrylate, and the like, as long as the over T.sub.g is at least 20.degree. C. Preferably the final stage monomer system is at least 50 weight percent C.sub.1 to C.sub.4 alkyl methacrylate. It is further preferred that the final stage polymer be free of units which tend to degrade poly(alkylene terephthalates), for example acid, hydroxyl amino, and amide groups.

In the following Examples, the abbreviations in accordance with the following Table are used:

Ba = n-butyl acrylate

Ea = ethyl acrylate

Eha = 2-ethylhexyl acrylate

Bda = 1,3-butylene diacrylate

AlMA = allyl methacrylate

St = styrene

Mma = methyl methacrylate

Lma = lauryl methacrylate

An = acrylonitrile

Dalm = diallyl maleate

Pbt = poly(butylene terephthalate)

Bd = butadiene

Aa = acrylic acid

Dmaema = dimethylaminoethyl methacrylate

Aam = acrylamide

Hema = 2-hydroxyethyl methacrylate

Pet = poly(ethylene terephthalate)

A single slash is used between monomers of a single stage and a double slash is used as a shorthand method of indicating a separation between stages or phases. The first stage to be polymerized is written first before the double slash, and subsequent stages are written subsequently.

Using conventional emulsion polymerization, multiple stage impact modifiers according to the invention of the following formulations are prepared:

Blends of modifiers and resins were extrusion compounded using a 1 inch diameter, 24:1 L/D machine with a 2 stage 3.0/3.5:1 screw. Extrusion temperatures were 450.degree.-525.degree. F. Dried extruded pellets were molded using a 1.5 oz. reciprocating screw injection molder with an ASTM family mold. Processing temperatures were again 450.degree.-525.degree. F. with a mold temperature of ca. 150.degree. F. and a 30-45 sec. cycle time. Impact strengths were determined by ASTM D-256 using 1/8 inch thick milled notched injection molded bars.

In all of the following examples, poly(butylene terephthalate) (Eastman Chemical Products Tenite 6PRO brand) was first used alone and then under the same conditions with specified percentages of the modifiers prepared in accordance with prior examples.

The impact modifier of Example 1 also functions very well to improve poly(propylene terephthalate) and poly(cyclohexane terephthalate), improving the notched Izod impact strength thereof.

To show the beneficial effect of the invention when the poly(alkylene terephthalate) is PET, the procedure used in Example 26-42 was used except AKZO Arnite A-160 PET, I.V. = 1.0, was used instead of PBT, and mold temperature was 130.degree.-150.degree. F. with 40 sec. cycle time using a ram injection molder instead of a reciprocating screw type.

In amorphous form, i.e., when a cold mold is used, 10% of the modifier of Example 26 increased the notched Izod impact strength from 0.24 .+-. 0.07 to 1.02 .+-. 0.13. In crystalline form, i.e., when the molded part is post-headed for 30 mins. at 140.degree. C., the notched Izod of the unmodified was 0.33 .+-. 0.12 while the 10% Example 26 molding had a notched Izod of 0.79 .+-. 0.09.

To show the improved thermal stability of modifiers used in the invention versus butadiene based analogues, thermal gravometric analysis (TGA) and differential scanning colorimetry (DSC) were done on both the Example 13 and the Example 16 modifiers. The TGA results are reported in the following Table:

The DSC results (run at 20.degree. C./min.) were that the Example 13 modifier shows a strong exotherm at 200.degree. C., whereas the Example 16 modifier did not begin to exotherm until 235.degree. C. Further, the Example 13 material charred at 235.degree. C. whereas Example 16 did not char even at 325.degree. C.

These data show that modifiers used in the invention are far more thermally stable at typical PBT processing temperatures than butadiene-based analogues.