Metallocenes toughen HDPE for thin-wall packaging.

Metallocene-type catalysts offer two routes to injection molding thinner-walled HDPE packaging than ever before possible. One route is to make unique impact modifiers for conventional HDPE resins. The other method is to create new "enhanced" HDPEs directly. Both approaches are being explored by Dow Plastics, Midland, Mich., using its Insite constrained-geometry catalyst technology.

Already in commercial production are Insite-based Affinity poly-olefin plastomers (POPs) with densities below 0.915 g/cc. These have been shown to be effective as impact modifiers for thin-wall HDPE and PP packaging. Still in development at Dow are enhanced HDPE resins made with Insite technology. These materials are said to show an optimized balance of stiffness, toughness, and processability in thin-wall packaging applications.

TIES THAT BIND

According to Dow development chemist Brian Faulkner, molders have taken to using higher-flow resins than were used several years ago because today's HDPE container molds are thinner and more difficult to fill. Molders are also continually moving to higher-flow resins in order to achieve container light weighting and cycle-time reductions to remain competitive. Faulkner says thin-wall containers are being molded with HDPE shaving a melt index of 60 g/10 min or more, while containers that must withstand abuse at low temperatures are typically molded with resins near a 40 MI. To achieve sufficient modulus to allow for stacking of containers, resins with densities between 0.950 and 0.952 g/cc are typically required, he notes.

Conventional polyethylene technology offers limited options for producing faster-flowing HDPE resins with acceptable toughness and stiffness, says Faulkner. While significant improvements in processability can be achieved by lowering molecular weight or raising melt index, there is a corresponding loss of impact resistance. Choosing a conventional resin with a somewhat lower density, below 0.950 g/cc, can provide improved impact but with a corresponding loss in stiffness.

The options to improve impact performance have been limited because the lower-molecular-weight HDPE resins that are currently used in thin-wall packaging applications have very low tie-molecule concentrations, Faulkner says. The tie molecules provide connectivity between the crystalline regions in polyethylene. That connectivity is what provides the resin's toughness. Insite catalyst technology has been found to produce tougher HDPE resins that have higher tie-molecule concentrations.

POP AS HDPE TOUGHENER

One way to exploit this phenomenon is to modify HDPE with low-density POPs. Dow has experimented with a number of its Affinity resins and found the best toughener so far to be Affinity SM 1300, which has 30 MI and 0.902 g/cc density. Blended at a 15% level with a standard 60-MI, 0.952-g/cc HDPE, this POP has been shown to provide nearly twice the impact resistance with only 15% reduction in container topload strength and slight reduction in processability, Faulkner reports. He says the topload strength remains acceptable for packaging applications.

Faulkner also notes that some lower melt index POPs, such as Affinity PT-1409 (6 MI, 0.911 g/cc), process well and improve impact resistance when they are blended with lower-melt-index HDPEs that are used in production of thicker-walled products such as buckets.

NEW TOUGH/STIFF HDPE RESIN

Although blending POPs into HDPE offers excellent impact resistance and adequate stiffness, there are the drawbacks of slightly less processability and slightly lower modulus or topload strength. In addition, processors suffer the inconvenience of blending two materials. Using Insite catalyst technology, Dow has developed enhanced HDPE resins that Faulkner says can meet all the requirements of thin-wall injection molding without impact modification.

Developmental grade XUR4 is a 60-MI, 0.950-g/cc HDPE that is said to provide both process-ability and modulus comparable to a standard 60- MI, 0.950-density HDPE together with much higher impact resistance than would normally be expected. Dow compared the two resins in 28-oz containers that had wall thicknesses of 27 mils. Drop impact tests were performed by filling the containers with ice water and recording the 50% failure height. The new resin showed more than threefold toughness improvement and 11-12% reduction in topload strength (see Table 1).

TABLE 1 - ENHANCED POLYETHYLENE FOR THIN-WALL CONTAINERS

                           Conventional             Enhanced
                               HDPE                   HDPE

Melt Index, g/10 min            60                     60
Density, g/cc                 0.952                   0.950
Drop Impact, ft               2.59                    8.20
Topload, lb                    126                     112
TABLE 2 - CUSTOMER VALIDATIONS ON DOWNGAUGED CONTAINERS

                                 Traditional          Enhanced
                                     HDPE                HDPE

Wall Thickness, mil                   27                  24
Melt Index, g/10 min                  65                  60
Density, g/cc                        0.952               0.950
Dart Impact, lb                       13                  13
Top Load, lb                          46                  46
Drop Test, ft                        5.5                 6.0

The same container was down-gauged to 19 mils for recent testing by a customer. In this case, the enhanced HDPE showed no sacrifice of impact or topload strength as compared with a conventional 65-MI HDPE. The result of the customer test are shown in Table 2.