Understanding Composite Materials: Classifications

Composites can be a tricky material to classify and standardize because by their nature they are not very standard at all. This can be a challenge for sourcing teams, product development groups, and R&D staff when they are working with flexible composite materials. Complexities exist within the broad material category of flexible composites that make classification and standardization a challenge.  For example, one subset of composite materials has an adhesive coating that changes from an un-tacky coating to a tacky adhesive coating when exposed to a given set of environmental conditions.  So, what is it?  How do you classify it, discuss it, and find suppliers of it? We start with a simple definition of what exactly a composite material is. 

A composite is a combination of two or more distinct materials having a recognizable interface.

Then we use three simple Composite Type classifications to differentiate composites and standardize our language and communication about them.

The Framework Think About Composites

Before diving into composite types it is important to understand the broad categorization of advanced flexible materials. This term is specifically referencing nine material types: composites, extruded nets, film, foams, foils, membranes, nonwovens, technical papers, and technical textiles. The composite types below all include at least one of the other eight advanced flexible material types as a substrate or component of the composite.  This is helpful to understand because in many ways it is the core material that provides the basis for functionality in the final composite.      

Below is a simple outline of the three main composite types of material which will help you search, compare, and select the appropriate raw material for your project whether you work with wound care dressings or biopharmaceuticals. The three main composite types outlined below are tapes, laminates, and coated materials.

Tapes

A tape is categorized as a substrate that is coated on one or two sides with an adhesive.  The substrate of a tape is one of the material types listed above, like flexible foam, which provides the basis for functionality in the final tape. There is one exception to this rule where a composite subtype within tapes is identified as a transfer tape. This subtype is an unsupported adhesive product that does not have any supporting web of material, a substrate, and is usually provided on one, or between two liners. Although it does not have the same structure as the other materials in the tape category it serves the same functions.

Once a desired material type is identified as a substrate, the next step in selecting a tape is understanding the composition of adhesive and how it must work with the substrate. Adhesive selection is a critical step in the material sourcing process for tapes because it contributes significantly to the overall functionality of the material, and will directly impact the end use application. It is important to be aware of the specifications, performance characteristics, and regulations that must be met or followed when selecting a substrate and an adhesive in technical markets. These requirements are especially important in regulated markets such as medical device and life sciences. An adhesive being used in these industries may be in contact with a patient’s skin, another component, or product being implanted or injected into a patient, presenting significant risk considerations.

Laminates

Laminate material is categorized as two or more webs of similar or dissimilar material, which are then bonded together by physical, chemical, or thermal means. Common lamination processes include heat, pressure, and adhesives. The lamination process is intended to be a permanent modification to the materials and should invoke properties that cannot be achieved by each material independently. Laminates are the broadest composite type and are unified by the multi-layer structure of the material.

Laminates are commonly constructed with two or three materials and can be any combination of the core material types, excluding composites, which they are categorized under. The most common material type used in laminates are films because they can provide a variety of technical attributes including stability, strength, and barrier properties. Many manufacturers that produce laminate materials are able to provide customized products for specific applications.  As a result, the available laminate material subtypes are merely a starting point for development work, and have far reaching technical capabilities and applications.

Coated Materials

A coated material has had a surface modification where an additional layer, usually a liquid chemistry, has been applied to one or both sides of the material. The purpose of the finish is to enhance the function of the material and usually introduces functionality such as waterproofing, anti-static, flame retardant, antimicrobial, thermal insulation, etc.

Common types of coating processes include spray, bath, spin, chemical vapor deposition, and physical vapor deposition. Coated materials allow for a large amount of customization. Like laminate materials, a manufacturer’s portfolio is a representation of the companies capabilities and serves as a starting point for engagement. Coated materials are a good composite type to search through for core material types that require an exotic performance capability that is facilitated by the coating.

Conclusion

Considering these subtypes, in the example presented in the introduction, a material that is commercialized with a tacky adhesive would be considered a tape and categorized under the tape Composite Type. Conversely, if a material is commercialized as an un-tacky adhesive coating that requires an additional process to alter the material to become adhesive it would then be considered a coated material and categorized under the coated material Composite Type.

The use of multiple components in composite materials allows them to achieve a range of functionality that could not have been attained singularly by one part of the composite. As a result, composites are the most diverse material type, and also the most widely used in technical applications. As material technologies continue to advanced the benefits of multiple materials or coatings working together are growing.  

A sound foundation in the identification and classification of these materials can streamline the process to engaging in meaningful development work with them. Categorizing composites into three Composite Types; tapes, laminates, and coated materials, and then further characterizing them by subtype allows sourcing teams, product development groups, and R&D personnel to correctly identify the materials and effectively communicate about them.