Integran News Blog

Adhesion of Metals Plated onto Plastics

Posted on Thu, Apr 11, 2013

We announced yesterday a new patent covering plated plastic products that can withstand repeated, aggressive thermal cycles.  Integran can achieve these high levels of adhesion using its state of the art polymer activation processes.  Because Integran's customers are often looking for structural reinforcement of polymers, adhesion is critically important in order to effectively spread the loads between the metal exoskeleton and the polymer core. This is particularly important for our high performance applications that see significant thermal cycling, as is common is aerospace (lightweight brackets etc.), defense (housings for weight critical airborne optics, weapons and UAVs) and space applications (with heavy thermal cycle loads).  

We will go into this adhesion aspect in a bit more detail in this blog post below.  But first, here is the press release: 

TORONTO, ONTARIO--(Marketwired - April 10, 2013) - Toronto-based Integran Technologies Inc. (Integran) today announced further expansion of its "structural metal plating-on-polymer" patent portfolio (Nanovate™ NP) with the issuance on Mar 12, 2013 of US 8,394,507 which discloses lightweight metal-coated polymer articles having superior thermal cycling performance.

Differences in thermal expansion have previously limited the operating temperature range of hybrids of metals and polymers. By developing processes which can now achieve previously unattainable levels of metal-to-polymer adhesion, Integran has extended the structural applicability of its metal-polymer hybrids to much greater temperature extremes. These processes are applicable to a wide range of polymers including carbon/glass fiber reinforced polyamides (PA), polyether-imides (PEI), polyamide-imides (PAI), polyimides (PI) and polyether-ether ketones (PEEK).

Integran's President & CEO Gino Palumbo stated, "This represents a significant advance in the replacement of all-metal parts with our lightweight metal-clad polymer part technology by expanding the applicability to components which may be subjected to severe thermal cycles. Although applicable to many industrial, biomedical and consumer products, this development is particularly relevant to space, aerospace and defense applications." 

A Bit of History - Adhesion of Electroplating on Plastics 

In the early sixties, it was found that the chemical pre-treatment of ABS substrates was shown to provide adherent metallic films, with adhesion levels higher by an order of magnitude, as compared to the then-conventional mechanical roughening processes [1].  Since then, additional processes have emerged to allow plating of other polymer types, driven largely by the need for decorative finishes on plastics.

The process sequence for plating of ABS has remained very similar over the last 40 years, and consists of the following three basic steps [2]:

a)      Etching in chemical solutions containing strong acids or oxidizing agents,

b)      Activating with precious metal species

c)      Electroless deposition of continuous metal film.

Several theories have been proposed for the mechanism of adhesion of the metallic films on plastics [2], which fall broadly into two categories: a) mechanical interlocking of the metal film on to the pores in the plastic matrix, caused by etching of the surface of the plastic to create micro-roughness for mechanical keying, and; b) chemical forces such as Van der Waals attraction or valence bonds of metal with the polar surface of the plastic, resulting from exposure to etch solution.  In practice, both these mechanisms likely play a vital role in the adhesion of metallic films to plastic.

Testing of Interface Adhesion between Plated Metals and Plastics

At Integran we have engineered processes that deliver best in class adhesion levels to a wide variety of polymers to enable our structural plating platform.  A high adhesion level between the metal plating and the plastic is desired, as this has a profound effect on the integrity of the part during service.  Low adhesion results in delamination, blistering, wrinkling, etc. of the hybrid component and is especially evident in applications with high thermal cycling. The adhesion between the metallization layer and the polymer depends on a variety of factors:

a)      Resin choice (some polymers are more amenable to metallization than others)

b)      Molding parameters (which influence the surface characteristics of the polymer)

c)      Metallization process (etching, activation, electroless steps)

Adhesion testing is used to provide an objective measure of the strength of the bond between the two materials.  Integran uses these tests to then iteratively optimize the polymer selection, molding and activation processes to provide the highest adhesion levels.

The level of adhesion required will vary depending on the part application. A decorative component may be able to function with a relatively low adhesion, while a structural component (like what Integran does with its Nanovate metal structural plating) will require a much higher adhesion to withstand greater stresses. Two quantitative test methods are normally used for measurement of interfacial adhesion and are specified by ASTM B533 test, “Test Method for Peel Strength of Electroplated Plastics” [5], and ASTM D4541 test, “Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers” [6], both of which are utilized by Integran and shown in the pictures below.  

It should be noted that the peel and the pull-off methods are not equivalent tests.  The pull off test measures the force required to de-laminate a unit area of the interface.  The peel test measures the interfacial fracture energy of the interface.   The pull-off test averages the force over an area, while the peel test records the force as a line scan.

Adhesion Measurement Pictures   ASTM B533 and ASTM D4541  600x265px

An additional test that may be employed for complicated parts that may not lend well to peel strength testing is a dynamic test of plate adhesion by thermal cycling, outlined in ASEP TP-201. Temperatures tend to range from -40oC up to 5.6oC below the heat-distortion temperature of the polymer substrate. One cycle consists of 1 hour at the high temperature, 30 minutes at room temperature, 1 hour at the low temperature 30 minutes at room temperature followed by an inspection for any blisters, cracks, peeling or other defects.  Typically 10 cycles are employed, and sample sizes must be large enough to insure confidence in the results [7]. 

Polymer Selection for Adhesion

Selection of a ‘platable’ polymer is a crucial step in the fabrication of the nanometal-polymer hybrids. Although several metallizable or plateable grade polymers are marketed in the ABS, mineral filled polyamide (MinlonTM) and polyetherimide (PEI) families, most engineered polymers such as PEEK, glass-filled polyamides (Nylon), PPS, polyphthalamides (PPA), etc. require more specialized activation techniques, like the ones developed at Integran, for gaining adhesion. Typical adhesion values will differ depending on the selected polymer, and customers are encouraged to get in touch to discuss the choice of polymers early in the design process.  

General Polymer Plating Steps at Integran

Once a polymer has been selected and correct molding techniques have been employed, the part is ready for activation and structural plating steps. The activation involves a chemical etch, specific to each polymer type, which roughens the surface, breaks down some of the polymer bonds, and provides mechanical interlocking sites and an active surface. 

After etching, an initial metallization occurs, which is typically an electroless plating followed by an optional electrolytic copper coating. The copper is used to enhance the conductivity of the surface for enhancing the surface conductivity which plays into the deposition speed of subsequent electroplating and is particularly important for large parts.  It also provides some levelling effects to create a smooth surface to allow a bright and decoratively attractive finish. 

Next, instead of the traditional decorative plating (which, incidentally, in many cases decrease the strength of the plastic part vs Integran's Nanovate metals) the next step is to apply the structural or functional Nanovate™ coating.  Lastly, additional top coats for asthetics can be considered (black paint for defense applications, black chrome for consumer products, etc.)

Summary

As a manufacturing process for complex geometry parts, the Nanovate™ NP structural plating process is cost effective and provides many advantages over machined aluminum or die-cast magnesium processes. Injection-molded polymers offer flexibility in the geometry and type of shapes that can easily be produced. While adhesion processes are particularly important for structural applications, Integran's state of the art activation techniques give our customers the widest range of polymer options with some of the best adhesion levels in the industry.

Interested in knowing more?  

References

[1] “Electroplating of plastics: handbook of theory and practice”, R. Weiner and J. Christoph (eds), Finishing Publications Ltd., Hampton Hill, England, 1977

[2] J. M. McCaskie, Galvanotechnik, (2006), [104], 563-574.

[3] C. A. Harper, “Handbook of Plastic Processes”, p672, 2006, Wiley Interscience.

[5] American Society of Testing Methods, ASTM B533

[6] American Society of Testing Methods, ASTM B4541

[7] American Society of Electroplated Plastics, “Standards and Guidelines for Electroplated Plastics”, 3rd Edition, G. K. Chenenko (ed), Englewood Cliffs, NJ, Prentice-Hall Inc., (1984)

Tags: Structural Plating, Plating on Plastics, Structural Plating on Plastics, Patents, Aerospace, Defense, Space