ADVANCED POLYAMIDE COMPOUNDS FOR CONNECTORS

Owing to some shortcomings of thermoplastics, a need was felt to make thermoplastics behave like thermosets. This led to the development of a material, which is easily processable like a thermoplastic and should have higher thermal properties like thermosets in application…

February 5, 2021

The highly competitive and growing market for connectors used in Electrical and Electronics (E&Es) applications is driving a trend toward smaller designs with tighter tolerances that can take higher operating temperatures and lower wall thickness. The wall thicknesses are dropping dramatically and creating demand for high performance Polyamides. Simultaneously, the OEMs are demanding lower costs, which is requiring molders to find ways to enhance both performance and productivity

Improved Polyamide 6 and Polyamide 66 (PA66) compounds have been developed recently and ramped up using new technology for the connector market that provides significant improvements by addressing the challenges, particularly in lower thickness performance, reduced plate- out, improved melt flow and enhanced part-to-part consistency. The new Formpoly and Starflam compounds that have been undergoing significant customer testing in the past few years are now widely available commercially. Results of those tests and the growing challenges in the E/E connector market are reviewed in this article.

Introduction

Thermoset plastics have been successful in Electrical & Electronic applications where they display performance at a lower cost. However, the biggest disadvantage of thermoset is their brittleness and difficulty in thin walled moldings. “To do maximum with minimum” is the main objective of Electrical and Electronics industries, which has lead to miniaturization. The challenge for the industry is that with every new development, the size shrinks in geometric proportion resulting in high heat generation. With thinner and thinner wall thickness, applications of thermoset are limited due to their inherent nature of ductility, even though they can meet the thermal requirement.

Thermoplastic is a good option for thin walled applications, but they have limitation to meet the stringent ‘in use’ fluctuations like short duration temperature shoot up and elevated high temperature due to miniaturization. There are very few thermoplastics with higher melting point of 400+ ⁰C and they are equally expensive. Hence, there was a need to make thermoplastics to behave like thermoset. This resulted in the development of a specialized material, which is easily processable as a thermoplastic and after radiation crosslinking develops higher thermal properties of thermoset in actual application during usage.

Connector Background

Plastics resins are used to provide physical structure and to hold contact points in precise position and insulate the contacts from one another.

Polyamides (nylons) widely replaced thermoset plastics that were originally used for connectors and remain the dominant plastic used in the market. Now, many plastics are used for connectors, including Polyesters (PBT), Liquid-Crystal Polymers (LCP), Polyphenylene Sulfide (PPS), Polycarbonate and other high-end engineering plastics, as well as thermoset plastics such as Phenolic and unsaturated Polyester. Resin choices are based on a combination of requirements, including cost, melt flow, moulding performance and physical properties.

Polymer compounds are often reinforced with glass fibre, to improve stiffness, tensile strength, and dimensional stability. Growing demands for high-heat performance have led to comparatively higher loadings of flame retardants at lower thickness, which is also creating greater potential for plate-out issues. Leading markets include computers, telecommunications and automotive etc.

Connector Design Issues

Many connector tools are rather complex in design to ensure the functional requirements such as contact spacing, insulation properties and physical connection style are met. There may be latches and/ or hinges designed for various connection purposes. These tools typically have side action to create specific design detail, resulting in sophisticated cavity design. The complexity can lead to mold filling and venting related issues, resulting in excessive downtime and increased molded- part cost. Designers are challenged to design the parts for functionality, and molders are challenged to produce the parts at minimal cost.

Key Performance Requirements for Connectors

Strength: High strength is required to maintain the connection in all types of environments. Latches and hinges need to maintain strength for the part’s life cycle. Pin retention and pull-out strength are critical to connector performance.
Electrical: A connector material must have excellent insulating properties and resist creation of a tracking path in various environments.
Flammability: Many connectors require UL V-2 flammability, while others require V-0 and still others require 5VA ratings of Flammability along with superior electrical properties to take care of sudden temperature spikes.
Flow: Complex connector designs and sometimes delicate mating cores and pins make flow critically important. The thinner wall sections, thin core pins and need for venting all require a material that flows well with less pressure.
Product Hygiene: Clean products are required to reduce mold deposit formation that result in downtime for mold cleaning. This will significantly increase molded-part cost..

Performance Characteristics that Make Polyamides the Material of Choice

PA6 & PA66 have many key attributes that make it a desirable material for electrical connectors:

High strength and rigidity
Excellent chemical resistance
Inherent flame resistance
Excellent resistance to tracking an electrical path
Excellent dielectric strength and volume/surface resistivity make it an ideal insulating material
Excellent thermal endurance (UL RTI) for use in higher-temperature applications

Innovative Developments in Polyamide Resin Performance

The goal of meeting the processing and design challenges in several important Polyamide markets, including connectors used for electrical and electronic applications was taken by Formulated Polymers Material and Eurostar Engineering Plastics France with the development of a new technology resulting in significant performance improvements in key areas.

Reduced Plate-Out

Plate-out, defined as the release of compounds that foul the tool, is generally a major problem for injection moulders. At typical PA66 processing temperatures (280 to 320 ⁰C), there is a gradual build-up of volatiles that precipitate out of the polymer melt and condense on the relatively cold mold surface. The volatiles include monomers and oligomers in material inherent in PA66, residual material from glass sizing, lubricants and other additives. They can plug vents, reduce the speed of air evacuation from the mould cavity and impede the moulding process. Cycle times and part quality are affected. Excessive plate-out leads to operational inefficiencies including vent plugging, burning, increased mould maintenance and even increased cycle times.

One expert in mould design and maintenance says that more labour-hours are spent cleaning mould plates and tooling than any other tooling repair stage. It is necessary to keep mould plates and tooling clean to maintain product quality and extend tool life. Most moulds are cleaned manually, adding to the expense. Downtime, however, is the biggest expense, particularly for high-cavitation, expensive tools. In some moulding shops, tools are removed and staged for cleaning, often creating a backlog of maintenance work. Formulated Polymers Material targeted this problem with development of a proprietary compound and process.

Improved Melt Flow

Polyamide grades have been optimized grades for improved flow using proprietary compounding technology during processing while maintaining high flammability performance in final component. Improved flow offers advantages in processing (including reduced pressures) and increased design flexibility, including increased length over thickness (L/t) flow ratio. L/t is an experimental index that indicates the distance the leading edge of the plastic melt flow can reach in a fixed thickness and at a fixed pressure.

Advantages of Formpoly Cross-linkable Grades

No Penetration of Pin at 90 ⁰C i.e., above melting point showed that it has become thermoset by total cross link, which ensures performance compliance during sudden temperature surges/variations.

No more melting of the part. It only chars at higher temperature
Improvement in properties-mechanical (tensile, flexural), thermal (HDT), chemical (reduction in solubility & hydrolysis) , electrical (CTI)
Increased resistance to stress cracking, reduced crack propagation
Greater resistance to frictional heat damage
Higher dimensional stability
Improvement in fiber-to-matrix adhesion and strength in glass reinforced
Reduction of cold flow (creep), Improvement in tensile set and compression set
Improved heat resistance- Higher limit values for working temperatures
Higher hot wire resistance
Higher ageing resistance

Need for the Innovation

The challenge for the industry is with every new development, the size shrinks in geometric proportion resulting in high heat generation. With thinner and thinner wall thickness, applications of thermoset are limited due to their inherent nature of ductility, even though they can meet the thermal requirement.

Thermoplastic is a good option for thin walled applications, but they have limitation to meet the short duration temperature shoot up and elevated high temperature due to miniaturization. Of course there are very few thermoplastics with higher melting point of 400+ ⁰C equally they are very expensive. Hence need was felt to make thermoplastics behave like thermoset. It creates the need to develop a material which can be easily processable as a thermoplastics and should have higher thermal properties as thermoset in application.

Starflam RX12414 PA66 X-linkable Flame retardant survives the lead-free soldering process and has snap fit and hinge effect capability. The compounds are available in any colour including laser markable with both Improved resistance to creep under heat ( fixings …) and improved resistance to over-heating (seals, connections …). These Polyamide grades are used to replace more expensive materials ( LCP )and over-designed materials (selected for the high heat capabilities – whereas only a short term high heat behaviour is required).

Reduced Shot-to-Shot Variability

Compounds with improved melt rheology and tighter viscosity specifications than current industry standards provide a larger processing window that leaves the moulder with more options to fine-tune the moulding process and reduce cycle time. This will translate into improved shot-to- shot control and consistent cavity pressure. Robust process control is critical to producing connectors with uniform physical properties. The end result is more consistent part weight and physical properties from shot to shot. The lower injection pressures required to mould these special grades will reduce fatigue of mould components, particularly small core pins.

Evaluations in Lab at 295⁰C in air ventilated oven

without crosslinked polyamide deformation in 20 min
with crosslinked Polyamide the distortion is after 2hrs Radiated component can withstand high temperature for more than one & a half hour.

Summary

High performance Polyamides PA6, PA66 grades having flame retardant additives and glass fibre have been converted to thermoset after injection moulding by radiation. Significant increase in physical properties can be achieved. Beta or gamma radiation of 50 -100kGy for the cross-linking of polyamide is highly effective in this two-step process – where polyamide is injection moulded into desired shape and then the component is exposed to radiation.

This innovation in Polyamide has been patented and is a good alternative for thermoset material, where thin walled mouldings and high productivity is required and can be a cost-effective alternate for high-cost, high-performance thermoplastics.

This innovation is first of its kind in India and Asia – and a significant increase in physical properties can be achieved through it. FPL has these speciality PA6 & PA66 grades, which are readily available.

Dr. Rashmi Bhardwaj, The author is the Vice President (Development) at Formulated Polymers Limited, India. He holds a PhD degree in Polymer Technology as well as he is an MBA.