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Last modified 08/21/03

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Material Copyright © Economy's Group

[Polymer Blends] [Microelectronic Polymers] [Liquid Crystalline Polyesters] [Polyester Thermosetting Resins]

 

Background & Concepts

 

When Professor Economy tried making liquid crystalline polymer blends of polyhydroxybenzoic acid (pHBA) and polynaphthoic acid (pHNA), back in 1987, he and his researchers noticed that a random copolymer of HBA and HNA was the result (Figure 1). This couldn't be readily explained at first. Two homo-polymers of HBA and HNA turning into a random copolymer of both was not expected. As it turned out, interchain transesterification reactions (ITR) were the cause of this randomization.

 

Figure 1: Reaction Between Two Polymers Produced A Random Copolymer

 

Interchain Transesterification Reactions

Interchain transesterification reactions are intriguing. They are reactions between two ester units across a polymer chain. The ester-units swap back and forth between each chain. It's this swapping mechanism which enabled the pure homopolymers to end up as random chains. These reactions are extremely fast, happening at 460 °C.

Dr. Economy saw an opportunity: "If these bonds are being made between chains, could they be made between chains that are on different plastic parts?" He and his researchers bonded LCP coatings at various temperatures and tested them for lap-shear strengths. Their results can be seen in Figure 2. Of importance is the lap-shear strengths seen in the materials bonded at 280 °C. At this temperature, the LCP's used in the test were not molten, hence, these parts were bonded to each other in the solid-state.

Figure 2: Lap-shear of LCP bonded substrates.


This prompted Dr. Economy to make thermosetting resins which were very similar to the pHBA and pHNA LCP polyesters. These early resins are closely related to our current resin systems. They were highly aromatic in nature and had ester unit linkages between each aromatic ring, just like pHBA. Oligomer A had carboxyllic end groups, and oligomer B had acetoxy end-groups (Figure 3). Condensation of acetic acid produced an ester cross-link. Lap-shear tests between two cured resins were successful.

Figure 3: Oligomer A had B.

 

Solid-State Consolidation

ITs Reactions can be exploited to bond polyesters while in the solid-state. Both the Polyester Thermosets and the Polyester LCP research we are currently conducting utilize these unique reactions. Lap-shear testing has been done on many resins. These samples fail cohesively, not adhesively, which means the bond-strengths that can be acheived through ITR bonding are as strong as the bulk material. Lap-shear specimens utilizing our thermosetting resins are made in the following way (Figure 4). First, the oligomers are coated onto a substrate. The thermosetting resins are then fully cured as separate parts. These parts are then lapped and bonded together by using heat and pressure.

Figure 4: Solid state bonding using ITR.


Figure 5 shows the way we are utilizing the ester bond-swap of the ITR reaction to produce the bonds across the interface as seen in Figure 4 above. These bond swaps happen backwards and forwards across the bond-line. Applying pressure promotes forward bond-swapping.

Figure 5: Schematic of novel approach to bonding through ITR.

 

A Recycleable Thermoset

The unique nature of the ITR bond-swapping gives our polyesters one more interesting property not seen in other thermosets. We can recycle these thermoset resins and re-use them as oligomers either as an additive to virgin resin or as a 100 % recycled resin. Our group in the past has shown that these materials can be recycled up to three times before any degredation in properties. Figure 6 below shows the recycling process. The condensate, acetic acid, is added back to the thermoset and under heat and pressure, the system can be converted back into a mixed-endgroup oligomer.

Figure 6: Schematic of the recycling process.