Research on Transparent Epoxy Adhesives

1. The primary factors affecting the transparency of epoxy adhesives

1.1 Epoxy Resin

As a major component of epoxy adhesives, the purity of the epoxy resin directly affects the transparency of the epoxy. Taking the commonly used bisphenol A type epoxy resin as an example, current technology can achieve low impurity content in such resins, with an appearance nearly colorless. However, the bisphenol A structure is prone to oxidation and yellowing due to light and heat exposure during use, forming carbonyl groups. Adding appropriate antioxidants and light/heat stabilizers can prevent and delay this yellowing.

1.2 Binder

Aliphatic polyamines and anhydride curing agents are the two types of curing agents with high usage. Among them, aliphatic polyamines are room temperature curing agents with a significant heat release during the curing process, leading to local temperature rise, accelerated yellowing, and reduced luster. The resulting products are brittle, with poor thermal and acidic resistance, and have strong irritancy and high toxicity. Additionally, they are prone to produce whitening, further reducing the transparency of the epoxy cured materials. To address this, modification of the raw material amines is necessary. For instance, substitution of hydroxylalkyl groups on the N atom of the amine can effectively reduce toxicity and improve its own color stability, to some extent inhibiting or delaying yellowing during the curing process, and enhancing the transparency of the epoxy adhesive. Moreover, there are many modification methods, including Michael addition, amidomethylation reaction, polyamine condensation, condensation with thiourea, addition with epoxides, reaction with carbonyl compounds, reaction with organic acids, reaction with anhydrides of organic acids, eutectic mixing of polyamines, and more.

Anhydride-based curing agents are commonly used in heat-curing epoxy adhesives. To accelerate the reaction between anhydrides and epoxy resins, it is often necessary to add appropriate accelerators such as tertiary amines. To achieve a transparent, colorless epoxy/anhydride curing system with good light transmission, it is recommended to choose anhydrides with good weather resistance and a saturated cyclic structure, avoiding discoloration during the heat curing process and long-term thermal use.

1.3 Other Additives

Other additives include accelerators, toughening agents, diluents, stabilizers, plasticizers, fillers, reinforcing agents, coupling agents, flame retardants, and defoamers. Besides the color and transparency of the additives themselves affecting the transparency of the adhesive, color changes during use should not be overlooked. For example, under conditions of heat and oxygen, as well as ultraviolet radiation, tertiary amines and nonylphenol accelerators are prone to yellowing, which reduces the transparency of the adhesive. Therefore, appropriate amounts of antioxidants and ultraviolet absorbers need to be added.

2. Colorless transparent epoxy resin

Current production technologies can produce epoxy resins with low impurity content and near-transparent purity. The transparency of epoxy resins can be evaluated using the Gardner colorimetric method or the platinum-cobalt colorimetric method (also known as the Hazen method). The color measured by the Gardner method deepens with increasing color number, with pure water's Gardner value being 1. The platinum-cobalt method also deepens with increasing color value, with color numbers 1-30 being colorless and 30-60 being nearly colorless. As early as the 1980s, the color of epoxy resins in developed countries could be stabilized below Gardner 1, with all products on the market having values less than 1. By the 1990s, the colorimetry of foreign epoxy resins was significantly less than G1, approaching colorless and transparent states below Gardner 0.2. This article considers epoxy resins with Gardner values ≤1 or platinum-cobalt values ≤60 as colorless and transparent.