Update on additives for polyolefins.

From improved stabilizers for unpainted TPO car parts to cost-saving and clarity-enhancing antiblocks for LDPE film, the recent Houston SPE polyolefins conference held important news for processors.

How do you make PP and TPO last longer in the sunlight without a protective paint covering? How can you make those materials fill molds faster without inhibiting paint or ink adhesion? Answers to these questions and others were found at SPE's International Conference on Additives for Polyolefins, held in Houston in late February. This conference was notable for its stronger emphasis on solving problems related to molded PP and TPO auto parts, while previous polyolefin meetings have focused on packaging, films, and fibers.

However, there were also new developments for non-durables presented at the meeting: improved antiblocks with low haze for PP film, an antiblock for PE films that preserves high clarity, and a phosphite that does twice the job of any previous stabilizer in maintaining color and flow rate of LLDPE and HDPE films.

Unpainted TPOs need protection

UV stability is Job One as auto makers start looking toward molded-in-color TPOs as a way to avoid the costs of painting interior and exterior trim. Ciba Specialty Chemicals reported in Houston that significantly improved uv resistance can be achieved in molded TPOs with new high-performance light-stabilizer systems that were originally developed for PP fibers. Ciba sources think these new stabilizers could also help extend use of unpainted TPOs into construction applications such as roofing, siding, and window profiles.

Ciba reported on weatherability tests of TPOs stabilized with its new alkoxyamine "NOR"-type HALS and with its new non-phenolic Fiberstab FS-042 dialkylhydroxylamine antioxidant. Among the products tested were Ciba's Tinuvin 123S, a low-molecular-weight NORHALS that is newly available in solid form, and CGL 116, a developmental high-molecular-weight, non-interactive NOR-HALS. ("Noninteractive" means inert to acid catalyst residues and pigments.)

As shown in Fig. 1, FS-042 outperforms a conventional antioxidant system (Irganox B225) in retaining gloss of a TPO that is made with a metallocene plastomer as the impact modifier. Much greater increase in gloss retention is achieved by also replacing a conventional HALS combination (Tinuvin 770 plus Chimassorb 944) with Tinuvin 123S plus a HMW noninteractive HALS like Chimassorb 119. The ultimate uv protection in this test is obtained with Tinuvin 123S plus CGL 116. All the test formulations also contain Tinuvin 328 hydroxybenzotriazole uv absorber as part of the package.

Polymer-bound HALS may be another route to improved uv stabilization of PP and TPO auto bumpers, fascias, and interior panels. Great Lakes Chemical reported on experiments showing that its new Uvasil 299LM - a lower-MW version of its polysiloxane-bound Uvasil 299HM HALS - achieves better protection against thermal oxidation and photodegradation than does standard Tinuvin 770. Uvasil's superior performance is attributed to optimized mobility in the polymer. Also, oligomeric Uvasil is said to limit losses by volatility or leaching that typically occur with low-MW HALS.

New phosphite packs punch

Although the product was first introduced in 1995, GE Specialty Chemicals, Parkersburg, W. Va., just recently began full commercial production of Ultranox 641, the first of a planned family of hydrolytically stable solid phosphites based on butyl ethyl propanediol chemistry in place of traditional pentaerythritol derivatives. At the Houston meeting, GE presented data showing superior performance in various LLDPE and HDPE film resins.

In butene and hexene LLDPE, Ultranox 641 reportedly provides better color and melt-index stability than a general-purpose TNPP widely used in LLDPE - in some cases, at half the loading level of TNPP. The new phosphite also compared favorably with Ultranox 626, a pentaerythritol diphosphite widely regarded as the highest-activity phosphite on the market.

In a bimodal HMW-HDPE film resin, Ultranox 641 showed very good color and MI stability at half the loading of Ciba's Irgafos 168, a commonly used phosphite.

Silicones have many roles

Silicone additives as flow aids and antiblocks in PP and TPO were highlighted by Dow Corning and GE Silicones. Dow Corning presented the latest data on its MB-Series masterbatches, a new family of additives based on UHMW silicones (see PT, Feb. '96, p. 21). These products contain silicones with molecular weights and viscosities several times greater than those of conventional silicone fluids. They reportedly impart improved processing, release, and lubricity at the same or lower use levels.

Spiral-flow tests [ILLUSTRATION FOR FIGURE 2 OMITTED] show the significant effect that only 0.2% of PP masterbatch MB 50-001 has on homopolymer PP. At the same temperature, silicone provides around 30% increase in flow, or the same flow can be achieved with a 90 [degrees] F reduction in molding temperature.

What's more, this additive favorably influences weld strength, printability, and paintability, Dow Coming claims. When plaques were tested without surface treatment, the UHMW silicone did not significantly reduce the printability of PP, unlike other internal lubricants such as erucamide. Similarly, there is reportedly no decrease in paint adhesion to PP with MB 50-001.

New antiblocks based on spherical silicone beads were introduced to the U.S. by GE Silicones. Called Tospearl, the products are made by Toshiba Silicones, a Japanese joint venture with GE Silicones. There are currently six commercial grades, ranging from submicron to 12-micron size.

PERFORMANCE PROPERTIES FOR 1-MIL AUTOCLAVE LDPE BLOWN FILM WITH 5000
PPM ANTIBLOCK

                            104 F Induced                See-Through
Antiblock                     Reblock, g     Haze, %      Test, in.

Optical-Grade Talc                42            6.5          16.0
Standard Talc                     43            8.2           4.2
Conventional
Diatomaceous Earth                36            9.3           4.9
Extra-Fine
Diatomaceous Earth                38            8.1           4.9

In coextruded PP film, Tospearl 120 (2 microns) shows more than twice the antiblocking effectiveness of a conventional 3-micron silica additive at the same loading of 0.2%. In monolayer PP film, less than one-third as much Tospearl 120 (0.24%) as of silica (0.09%) was required to obtain the same antiblocking properties. Moreover, the static coefficient of friction was half as large with Tospearl.

Talc won't block light

An experimental "optical-grade" talc antiblock is designed for use in high-clarity polyolefin films, where it reportedly reduces the required use level of costly fluorocarbon-elastomer processing aids. Most inorganic antiblocks can interact with or absorb processing aids, thereby reducing their effectiveness and requiring higher loadings. A solution to this problem was presented jointly by Specialty Minerals, Inc. and DuPont Dow Elastomers' Viton Business Center.

In blown-film tests, Specialty Minerals' experimental talc showed significantly lower interaction with the processing aid than did standard talc, while providing comparable antiblock performance. The accompanying table shows higher clarity and good antiblocking with 5000 ppm of this developmental antiblock in autoclave LDPE. Besides low haze, a three- to four-fold improvement in "see-through" optics was achieved relative to talc and diatomaceous earth. The see-through test is currently in development as a new method that can more accurately quantify what is perceived as high clarity by the consumer, the authors say. The new talc's effectiveness is also being evaluated in high-clarity metallocene resins.