Natural rubber (NR) has been used in pressure-sensitive and solvent-based adhesives for more than 100 years and still remains one of the most important elastomers for this type of adhesive. Today, it competes with synthetic elastomers such as SBR, SBS and SIS block copolymers which have replaced natural rubber in some applications. However, natural rubber is still used on a very large scale because of its lower price, outstanding flexibility, high initial tack, and good tack retention. On the downside, natural rubber has a relative low upper service temperature which is typically in the range of 90 - 100°C when vulcanized and 60 - 70°C when unvulcanized. At higher temperatures and when exposed to UV, natural rubber rapidly degrades due to chain scission and oxidation and loses most of its strength.
NR elastomers are an excellent choice for many pressure sensitive adhesive (PSA) applications. They provide aggressive and instantaneous tack at short contact times, adhere well to a variety substrates, are of low cost and are generally considered of low toxicity.1 However, natural rubber alone has little to no tack and a low level of adhesion. To improve both properties, tackifying resins must be added.2 The blending is typically done in a Banbury (internal) mixer and two roll (open) mixer (batch process) or in an extruder (continuous process). Milling also breaks down the gel content and reduces the molecular weight, or in other words, it produces a free-flowing adhesive suitable for subsequent coating. Often, other ingredients such as plasticizers (mineral oils), pigments, and curing agents are added. The plasticizers assist in pigment dispersion, enhance tack and lower cost but also reduce peel adhesion and holding power. Pigments such as TiO2, and carbon black are added for opacity, often in combination with dyes/colorants to achieve the desired color. Other pigments such as clay or calcium carbonate are mainly added as extender to reduce cost. A third type of pigment, namely zinc oxide, is added to improve the performance. It acts as a vulcanization activator/catalyst and provides superior tack retention after aging. It also has antibacterial properties and can be added to lower cost.
Rubber-based adhesive tend to soften permanently when exposed to heat and UV light. To slow down degradation during storage and service, antioxidants are often added to the formulation. The three most common types of antioxidants are sterically hindered phenols, secondary aromatic amines, and dithiocarbamates. The most effective rubber antioxidants are secondary aromatic amines which are also very effective antiozonants. However, they cause noticeable discoloration and can only be used if discolation is not a problem as it is the case with carbon filled adhesives. For other applications, phenolic antioxidants are often used which are nonstaining and also provide some protection agaist UV and visible light.
Natural rubber-based pressure sensitive adhesives are among the cheapest adhesives on the market, but have limited applications. They are often a good choice for less expensive types of tapes used in less demanding applications. They have good initial adhesion properties, but generally do not have much creep, heat and chemical resistance unless partially crosslinked (vulcanized) which improves the holding power, as well as the solvent, oil, and heat resistance. They are perfectly adequate for applications where the adhesive is subjected to low shear stress at ambient and low temperature situations but are not recommended for high temperature and/or high shear situations. The low cost of the adhesive together with an inexpensive backing makes them ideal for short-term tape applications such as masking tapes, pressure-sensitive labels, as well as general purpose duct tapes.
Natural rubber can be used in medical applications because of its low toxicity, but recent studies have shown that some people may be allergic to natural rubber latex.3
Tack increases with increase in tackifier loading up to about 70 to 80 parts per hundred parts rubber and steadily declines with higher loading.