Focus: IMMCPIM binder candidate passes the test

Research at Penn State gives the green light to a new binder.

Feedstocks play an important role in the PIM industry, so optimizing their makeup tends to be a major focus for material providers. Binders, the polymers and waxes that adhere ceramic or metal powders together, are at the heart of a feedstock. Recent research into a new binder candidate proves promising for a class of materials known as polyolefin plastomers, or POPs.Randall M. German, brush chair and director of the Center for Innovative Sintered Products at Pennsylvania State University (University Park, PA), tested the use of Affinity GA POPs (Dow Chemical) as a binder material for PIM feedstocks. The materials, introduced in July, have application for both polymer modification and hot melt adhesives. As a binder, POPs combine high flow rates and low crystallinity to produce desirable characteristics such as low ash content, low melting point, and low molecular weight.

Tests and trials done under German’s supervision compared various binder formulations with Affinity POPs substituted for paraffin wax in a standard feedstock with 35 vol-% of the binder being a low-molecular-weight polypropylene and 10 vol-% being a higher-molecular-weight polyethylene. Formulations included alumina (.7-µm size) and 17-4PH (AISI 630) stainless steel powders.

Initial mixing trials were conducted using torque rheometry to identify the solids loading compatible with injection molding. One of the first benefits was a higher solids loading possible with alumina, reaching 56 vol-%. The mixtures were tested for homogeneity and found to be uniform with only slight shear thinning. The 12-µm stainless steel was loaded to 62 vol-%.

Good Mixing, Fast Breakdown

An initial positive observation involved the miscibility of the system. The feedstocks showed no powder-binder separation, unlike more standard feedstocks. Differential thermal analysis and thermogravimetric analysis (TGA) identified temperatures in the 400°C (752°F) range at the onset of burnout. With a 450°C (842°F) burnout temperature, the trials showed a low carbon content and similar sintering behavior to that observed with the same powders and other binder systems.

Testing revealed several key characteristics for the binder candidate. It showed a low ash content (less than .01%), favorable for PIM and similar to that of polymers currently used in large production settings. TGA data show a relative stability in air or nitrogen up to 250°C (482°F) and a drastic degradation around 350°C (662°F). The progressive breakdown of binder material is very desirable, leaving only small quantities of molten polymer to form pendular bonds between the particles during burnout, and providing capillary strengthening to the powder up to the onset of sinter bonding. No reactions with metal or ceramic powders were observed.

Melting point of the material is around 70°C (158°F). In comparison, polymers used in PIM feedstocks typically melt from 140°C to 160°C (284°F to 320°F). A significantly lower melting point is useful for several reasons: Mixing can be done in a water-jacketed mixer as opposed to an oil-heated version; and the melting point is closer to lower-melt filler components in the binders, such as waxes, polyethylene glycols, and others.

This latter feature means fewer tendencies for the binder to phase-separate, also reducing the probability of surface blemishes. The lower melting temperature range might also allow molding of colder feedstock with the advantages of reduced sink marks, faster cycles, lower molding pressures, and less tool wear.

At crystallinity levels of 16% to 19%, POP materials may also help to avoid residual stresses or strains that reduce final molded component dimensional precision.