ORIGINAL PAPER Preparation of poly (urea-formaldehyde) microcapsules for use in capsular adhesive Mohammad Bagheri Kashani 1 & Ali Salimi 1 & Mohammad Jalal Zohuriaan-Mehr 1 & Ahad Hanifpour 2 Received: 7 September 2019 /Accepted: 30 October 2019 # The Polymer Society, Taipei 2019 Abstract A novel capsular adhesive was developed using an acrylic binder and poly (urea-formaldehyde) (PUF) microcapsules containing benzoyl peroxide (BPO) as a core material. The synthesis of the PUF microcapsules was optimized through balancing different parameters including the microcapsule core-shell ratio, release percentage (storage stability), filling content, and the reaction yield. The morphological studies of the microcapsules were conducted using optical microscopy and scanning electron micros- copy. The FTIR spectra and the microscopic images confirmed the successful formation of microcapsules containing BPO solution. The filling content of the microcapsules was measured using thermo gravimetric analysis (TGA). The optimum core- shell ratio of the PUF microcapsules was selected based on the results obtained from filling content test, release percentage and the reaction yield. Different mixing speeds of the mechanical stirrer up to 800 rpm were examined to obtain the optimum mean diameter of the microcapsules and optimum filling content. The optimal microcapsule sample was obtained at a core-shell ratio of 90/10 and a mixing speed of 450 rpm. A characteristic spherical morphology with an average diameter of 120 μm with no shell wrinkles and no agglomeration was obtained in this sample. In the next step, the capsular adhesive was prepared through loading different PUF microcapsules in acrylic binder. The adhesive was then pre-coated on metallic bolt and set after microcapsule breakage during fixing nuts. The strength of the capsular adhesive at 15 wt.% of microcapsule loading showed a maximum torque of 8.5 N.m. Keywords Microcapsule . Poly (urea–formaldehyde) . Morphological . Adhesive . Torque Introduction Microencapsulation is a technique in which solid, liquid or gaseous active core ingredients are encapsulated in a polymer- ic shell [1]. Microencapsulation has been widely used in a various range of applications such as self-healing or anti cor- rosive coatings [2–4], wound healing [4], drug delivery sys- tems [5] and latent curing agents [6]. The microencapsulation of active agents of adhesive formulation such as resins [7], initiator or even curing agent is of specific importance for the encapsulation of a formulation part of an adhesive or a self-healing agent [8, 9]. Microcapsules would release their core materials through a controlled release procedure or when they are affected by a stimulus such as mechanical shear or force [10, 11]. The capsular adhesive was introduced as a group of hi-tech adhesives which are consisted of a binder and microencapsu- lated initiator, monomer or resin. The microcapsules breakage leads to release of the active core mainly the initiator ingredi- ent into binder and hence, initiation of the polymerization reactions [12]. The poly (urea-formaldehyde) (PUF) as a promising shell material of microcapsules, has accomplished the requirements for the encapsulation of liquid core materials through in-situ polymerization [3, 13]. It should be noted that the peroxide solutions are mostly known to have thermal and chemical sensitivity; hence, the microencapsulation method should be done considering several prerequisites to avoid ad- verse effects on the chemical activity of the compound [9]. Zhang and coworkers prepared two types of microcapsules with melamine formaldehyde shell, benzoyl peroxide solution in toluene and styrene as core materials through in-situ poly- merization [ 14]. After embedment of aforementioned * Ali Salimi a.salimi@ippi.ac.ir 1 Adhesive and Resin Department, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965-115, Tehran, Iran 2 Polymerization Engineering Department, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965-115, Tehran, Iran Journal of Polymer Research (2019) 26:270 https://doi.org/10.1007/s10965-019-1965-4